Mutant Cosplay

You probably recognize these costumed comic fans in Figure 5.8.1 as two of the four Teenage Mutant Ninja Turtles. Can a mutation really turn a reptile into an anthropomorphic superhero? Of course not — but mutations can often result in other drastic (but more realistic) changes in living things.

Mutations are random changes in the sequence of bases in DNA or RNA. The word mutation may make you think of the Ninja Turtles, but that's a misrepresentation of how most mutations work. First of all, everyone has mutations. In fact, most people have dozens (or even hundreds!) of mutations in their DNA. Secondly, from an evolutionary perspective, mutations are essential. They are needed for evolution to occur because they are the ultimate source of all new genetic variation in any species.

Mutations have many possible causes. Some mutations seem to happen spontaneously, without any outside influence. They occur when errors are made during DNA replication or during the transcription phase of protein synthesis. Other mutations are caused by environmental factors. Anything in the environment that can cause a mutation is known as a mutagenno post. Examples of mutagens are shown in the figure below.

Types of Mutations

Mutations come in a variety of types. Two major categories of mutations are germline mutations and somatic mutations.

• Germline mutations occur in gametes (the sex cells), such as eggs and sperm. These mutations are especially significant because they can be transmitted to offspring, causing every cell in the offspring to carry those mutations.
• Somatic mutations occur in other cells of the body. These mutations may have little effect on the organism, because they are confined to just one cell and its daughter cells. Somatic mutations cannot be passed on to offspring.

Mutations also differ in the way that the genetic material is changed. Mutations may change an entire chromosome, or they may alter just one or a few nucleotides.

Chromosomal Alterations

Chromosomal alterations are mutations that change chromosome structure. They occur when a section of a chromosome breaks off and rejoins incorrectly, or otherwise does not rejoin at all. Possible ways in which these mutations can occur are illustrated in the figure below. Chromosomal alterations are very serious. They often result in the death of the organism in which they occur. If the organism survives, it may be affected in multiple ways. An example of a human disease caused by a chromosomal duplication is Charcot-Marie-Tooth disease type 1 (CMT1). It is characterized by muscle weakness, as well as loss of muscle tissue and sensation. The most common cause of CMT1 is a duplication of part of chromosome 17.

A point mutation is a change in a single nucleotide in DNA. This type of mutation is usually less serious than a chromosomal alteration. An example of a point mutation is a mutation that changes the codon UUU to the codon UCU. Point mutations can be silent, missense, or nonsense mutations, as described in Table 5.8.1. The effects of point mutations depend on how they change the genetic code.

Frameshift Mutations

A frameshift mutation is a deletion or insertion of one or more nucleotides, changing the reading frame of the base sequence. Deletions remove nucleotides, and insertions add nucleotides. Consider the following sequence of bases in RNA:

AUG-AAU-ACG-GCU = start-asparagine-threonine-alanine

Now, assume that an insertion occurs in this sequence. Let’s say an A nucleotide is inserted after the start codon AUG. The sequence of bases becomes:

AUG-AAA-UAC-GGC-U = start-lysine-tyrosine-glycine

Even though the rest of the sequence is unchanged, this insertion changes the reading frame and, therefore, all of the codons that follow it. As this example shows, a frameshift mutation can dramatically change how the codons in mRNA are read. This can have a drastic effect on the protein product.

The majority of mutations have neither negative nor positive effects on the organism in which they occur. These mutations are called neutral mutations. Examples include silent point mutations, which are neutral because they do not change the amino acids in the proteins they encode.

Many other mutations have no effects on the organism because they are repaired before protein synthesis occurs. Cells have multiple repair mechanisms to fix mutations in DNA.

Beneficial Mutations

Some mutations — known as beneficial mutations — have a positive effect on the organism in which they occur. They generally code for new versions of proteins that help organisms adapt to their environment. If they increase an organism’s chances of surviving or reproducing, the mutations are likely to become more common over time. There are several well-known examples of beneficial mutations. Here are two such examples:

1. Mutations have occurred in bacteria that allow the bacteria to survive in the presence of antibiotic drugs, leading to the evolution of antibiotic-resistant strains of bacteria.
2. A unique mutation is found in people in Limone,  a small town in Italy. The mutation protects them from developing atherosclerosis, which is the dangerous buildup of fatty materials in blood vessels despite a high-fat diet. The individual in which this mutation first appeared has even been identified and many of his descendants carry this gene.

Harmful Mutations

Imagine making a random change in a complicated machine, such as a car engine. There is a chance that the random change would result in a car that does not run well — or perhaps does not run at all. By the same token, a random change in a gene's DNA may result in the production of a protein that does not function normally... or may not function at all. Such mutations are likely to be harmful. Harmful mutations may cause genetic disorders or cancer.

• A genetic disorder is a disease, syndrome, or other abnormal condition caused by a mutation in one or more genes, or by a chromosomal alteration. An example of a genetic disorder is cystic fibrosis. A mutation in a single gene causes the body to produce thick, sticky mucus that clogs the lungs and blocks ducts in digestive organs.
• Cancer is a disease in which cells grow out of control and form abnormal masses of cells (called tumors). It is generally caused by mutations in genes that regulate the cell cycle. Because of the mutations, cells with damaged DNA are allowed to divide without restriction.

Inherited mutations are thought to play a role in roughly five to ten per cent of all cancers. Specific mutations that cause many of the known hereditary cancers have been identified. Most of the mutations occur in genes that control the growth of cells or the repair of damaged DNA.

Genetic testing can be done to determine whether individuals have inherited specific cancer-causing mutations. Some of the most common inherited cancers for which genetic testing is available include hereditary breast and ovarian cancer, caused by mutations in genes called BRCA1 and BRCA2. Besides breast and ovarian cancers, mutations in these genes may also cause pancreatic and prostate cancers. Genetic testing is generally done on a small sample of body fluid or tissue, such as blood, saliva, or skin cells. The sample is analyzed by a lab that specializes in genetic testing, and it usually takes at least a few weeks to get the test results.

Should you get genetic testing to find out whether you have inherited a cancer-causing mutation? Such testing is not done routinely just to screen patients for risk of cancer. Instead, the tests are generally done only when the following three criteria are met:

1. The test can determine definitively whether a specific gene mutation is present. This is the case with the BRCA1 and BRCA2 gene mutations, for example.
2. The test results would be useful to help guide future medical care. For example, if you found out you had a mutation in the BRCA1 or BRCA2 gene, you might get more frequent breast and ovarian cancer screenings than are generally recommended.
3. You have a personal or family history that suggests you are at risk of an inherited cancer.

Criterion number 3 is based, in turn, on such factors as:

• Diagnosis of cancer at an unusually young age.
• Several different cancers occurring independently in the same individual.
• Several close genetic relatives having the same type of cancer (such as a maternal grandmother, mother, and sister all having breast cancer).
• Cancer occurring in both organs in a set of paired organs (such as both kidneys or both breasts).

If you meet the criteria for genetic testing and are advised to undergo it, genetic counseling is highly recommended. A genetic counselor can help you understand what the results mean and how to make use of them to reduce your risk of developing cancer. For example, a positive test result that shows the presence of a mutation may not necessarily mean that you will develop cancer. It may depend on whether the gene is located on an autosome or sex chromosome, and whether the mutation is dominant or recessive. Lifestyle factors may also play a role in cancer risk even for hereditary cancers. Early detection can often be life saving if cancer does develop. Genetic counseling can also help you assess the chances that any children you may have will inherit the mutation.

Attributions

Figure 5.8.1

Ninja Turtles by Pat Loika on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Figure 5.8.2

• Beauty treatment face mask by no-longer-here on Pixabay is used under the Pixabay License (https://pixabay.com/service/license/).
• HPV by AJC1 on Flickr is used under a CC BY-NC 2.0 (https://creativecommons.org/licenses/by-nc/2.0/) license.
• H Pylori by AJC1 on Flickr is used under a CC BY-NC 2.0 (https://creativecommons.org/licenses/by-nc/2.0/) license. 
• Vape and Cigarette by Vaping360 on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.
• Hand X-Ray by Hellerhoff on Wikimedia Commons - CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en) license.
• Hot dogs by unknown on PxFuel is used under the Pxfuel Terms (https://www.pxfuel.com/terms-of-use).
• Sunshine face is clipart.

Figure 5.8.3

Scheme of possible chromosome mutations/ Chromosomenmutationen by unknown on Wikimedia Commons is adapted from NIH's Talking Glossary of Genetics. [Changes as described by de:user:Dietzel65]. Further use and adapation (text translated to English) by Christine Miller as image is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

References

Seeker. (2016, April 23). How radiation changes your DNA. YouTube. https://www.youtube.com/watch?v=PQjL4ZDuq2o&feature=youtu.be

TED. (2019, June 5). What you should know about vaping and e-cigarettes | Suchitra Krishnan-Sarin. YouTube. https://www.youtube.com/watch?v=a63t8r70QN0&feature=youtu.be

TED-Ed. (2014, September 22). Where do genes come from? - Carl Zimmer. YouTube. https://www.youtube.com/watch?v=z9HIYjRRaDE&feature=youtu.be

Wikipedia contributors. (2020, July 6). Breast cancer. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Breast_cancer&oldid=966366739

Wikipedia contributors. (2020, July 9). Charcot–Marie–Tooth disease. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Charcot%E2%80%93Marie%E2%80%93Tooth_disease&oldid=966912915

Wikipedia contributors. (2020, July 7). Cystic fibrosis. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Cystic_fibrosis&oldid=966566921

Wikipedia contributors. (2020, June 4). Limone sul Garda. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Limone_sul_Garda&oldid=960771991

Wikipedia contributors. (2020, June 23). Ovarian cancer. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Ovarian_cancer&oldid=964157192

Wikipedia contributors. (2020, May 7). BRCA mutation. In Wikipedia. https://en.wikipedia.org/w/index.php?title=BRCA_mutation&oldid=955463902

Wikipedia contributors. (2020, July 10). Teenage Mutant Ninja Turtles. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Teenage_Mutant_Ninja_Turtles&oldid=967030468

The breakdown of larger molecules into smaller ones.

A chameleon on a branch, surrounded by foliage. The chameleon is camouflaged to blend into its surroundings.

Image shows a Lego (TM) representation of Gregor Mendel with his plants.

Created by: CK-12/Adapted by Christine Miller

Mutant Cosplay

You probably recognize these costumed comic fans in Figure 5.8.1 as two of the four Teenage Mutant Ninja Turtles. Can a mutation really turn a reptile into an anthropomorphic superhero? Of course not — but mutations can often result in other drastic (but more realistic) changes in living things.

Mutations are random changes in the sequence of bases in DNA or RNA. The word mutation may make you think of the Ninja Turtles, but that's a misrepresentation of how most mutations work. First of all, everyone has mutations. In fact, most people have dozens (or even hundreds!) of mutations in their DNA. Secondly, from an evolutionary perspective, mutations are essential. They are needed for evolution to occur because they are the ultimate source of all new genetic variation in any species.

Mutations have many possible causes. Some mutations seem to happen spontaneously, without any outside influence. They occur when errors are made during DNA replication or during the transcription phase of protein synthesis. Other mutations are caused by environmental factors. Anything in the environment that can cause a mutation is known as a mutagenno post. Examples of mutagens are shown in the figure below.

Types of Mutations

Mutations come in a variety of types. Two major categories of mutations are germline mutations and somatic mutations.

• Germline mutations occur in gametes (the sex cells), such as eggs and sperm. These mutations are especially significant because they can be transmitted to offspring, causing every cell in the offspring to carry those mutations.
• Somatic mutations occur in other cells of the body. These mutations may have little effect on the organism, because they are confined to just one cell and its daughter cells. Somatic mutations cannot be passed on to offspring.

Mutations also differ in the way that the genetic material is changed. Mutations may change an entire chromosome, or they may alter just one or a few nucleotides.

Chromosomal Alterations

Chromosomal alterations are mutations that change chromosome structure. They occur when a section of a chromosome breaks off and rejoins incorrectly, or otherwise does not rejoin at all. Possible ways in which these mutations can occur are illustrated in the figure below. Chromosomal alterations are very serious. They often result in the death of the organism in which they occur. If the organism survives, it may be affected in multiple ways. An example of a human disease caused by a chromosomal duplication is Charcot-Marie-Tooth disease type 1 (CMT1). It is characterized by muscle weakness, as well as loss of muscle tissue and sensation. The most common cause of CMT1 is a duplication of part of chromosome 17.

A point mutation is a change in a single nucleotide in DNA. This type of mutation is usually less serious than a chromosomal alteration. An example of a point mutation is a mutation that changes the codon UUU to the codon UCU. Point mutations can be silent, missense, or nonsense mutations, as described in Table 5.8.1. The effects of point mutations depend on how they change the genetic code.

Frameshift Mutations

A frameshift mutation is a deletion or insertion of one or more nucleotides, changing the reading frame of the base sequence. Deletions remove nucleotides, and insertions add nucleotides. Consider the following sequence of bases in RNA:

AUG-AAU-ACG-GCU = start-asparagine-threonine-alanine

Now, assume that an insertion occurs in this sequence. Let’s say an A nucleotide is inserted after the start codon AUG. The sequence of bases becomes:

AUG-AAA-UAC-GGC-U = start-lysine-tyrosine-glycine

Even though the rest of the sequence is unchanged, this insertion changes the reading frame and, therefore, all of the codons that follow it. As this example shows, a frameshift mutation can dramatically change how the codons in mRNA are read. This can have a drastic effect on the protein product.

The majority of mutations have neither negative nor positive effects on the organism in which they occur. These mutations are called neutral mutations. Examples include silent point mutations, which are neutral because they do not change the amino acids in the proteins they encode.

Many other mutations have no effects on the organism because they are repaired before protein synthesis occurs. Cells have multiple repair mechanisms to fix mutations in DNA.

Beneficial Mutations

Some mutations — known as beneficial mutations — have a positive effect on the organism in which they occur. They generally code for new versions of proteins that help organisms adapt to their environment. If they increase an organism’s chances of surviving or reproducing, the mutations are likely to become more common over time. There are several well-known examples of beneficial mutations. Here are two such examples:

1. Mutations have occurred in bacteria that allow the bacteria to survive in the presence of antibiotic drugs, leading to the evolution of antibiotic-resistant strains of bacteria.
2. A unique mutation is found in people in Limone,  a small town in Italy. The mutation protects them from developing atherosclerosis, which is the dangerous buildup of fatty materials in blood vessels despite a high-fat diet. The individual in which this mutation first appeared has even been identified and many of his descendants carry this gene.

Harmful Mutations

Imagine making a random change in a complicated machine, such as a car engine. There is a chance that the random change would result in a car that does not run well — or perhaps does not run at all. By the same token, a random change in a gene's DNA may result in the production of a protein that does not function normally... or may not function at all. Such mutations are likely to be harmful. Harmful mutations may cause genetic disorders or cancer.

• A genetic disorder is a disease, syndrome, or other abnormal condition caused by a mutation in one or more genes, or by a chromosomal alteration. An example of a genetic disorder is cystic fibrosis. A mutation in a single gene causes the body to produce thick, sticky mucus that clogs the lungs and blocks ducts in digestive organs.
• Cancer is a disease in which cells grow out of control and form abnormal masses of cells (called tumors). It is generally caused by mutations in genes that regulate the cell cycleno post. Because of the mutations, cells with damaged DNA are allowed to divide without restriction.

Inherited mutations are thought to play a role in roughly five to ten per cent of all cancers. Specific mutations that cause many of the known hereditary cancers have been identified. Most of the mutations occur in genes that control the growth of cells or the repair of damaged DNA.

Genetic testing can be done to determine whether individuals have inherited specific cancer-causing mutations. Some of the most common inherited cancers for which genetic testing is available include hereditary breast and ovarian cancer, caused by mutations in genes called BRCA1 and BRCA2. Besides breast and ovarian cancers, mutations in these genes may also cause pancreatic and prostate cancers. Genetic testing is generally done on a small sample of body fluid or tissue, such as blood, saliva, or skin cells. The sample is analyzed by a lab that specializes in genetic testing, and it usually takes at least a few weeks to get the test results.

Should you get genetic testing to find out whether you have inherited a cancer-causing mutation? Such testing is not done routinely just to screen patients for risk of cancer. Instead, the tests are generally done only when the following three criteria are met:

1. The test can determine definitively whether a specific gene mutation is present. This is the case with the BRCA1 and BRCA2 gene mutations, for example.
2. The test results would be useful to help guide future medical care. For example, if you found out you had a mutation in the BRCA1 or BRCA2 gene, you might get more frequent breast and ovarian cancer screenings than are generally recommended.
3. You have a personal or family history that suggests you are at risk of an inherited cancer.

Criterion number 3 is based, in turn, on such factors as:

• Diagnosis of cancer at an unusually young age.
• Several different cancers occurring independently in the same individual.
• Several close genetic relatives having the same type of cancer (such as a maternal grandmother, mother, and sister all having breast cancer).
• Cancer occurring in both organs in a set of paired organs (such as both kidneys or both breasts).

If you meet the criteria for genetic testing and are advised to undergo it, genetic counseling is highly recommended. A genetic counselor can help you understand what the results mean and how to make use of them to reduce your risk of developing cancer. For example, a positive test result that shows the presence of a mutation may not necessarily mean that you will develop cancer. It may depend on whether the gene is located on an autosome or sex chromosome, and whether the mutation is dominant or recessive. Lifestyle factors may also play a role in cancer risk even for hereditary cancers. Early detection can often be life saving if cancer does develop. Genetic counseling can also help you assess the chances that any children you may have will inherit the mutation.

Attributions

Figure 5.8.1

Ninja Turtles by Pat Loika on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Figure 5.8.2

• Beauty treatment face mask by no-longer-here on Pixabay is used under the Pixabay License (https://pixabay.com/service/license/).
• HPV by AJC1 on Flickr is used under a CC BY-NC 2.0 (https://creativecommons.org/licenses/by-nc/2.0/) license.
• H Pylori by AJC1 on Flickr is used under a CC BY-NC 2.0 (https://creativecommons.org/licenses/by-nc/2.0/) license. 
• Vape and Cigarette by Vaping360 on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.
• Hand X-Ray by Hellerhoff on Wikimedia Commons - CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en) license.
• Hot dogs by unknown on PxFuel is used under the Pxfuel Terms (https://www.pxfuel.com/terms-of-use).
• Sunshine face is clipart.

Figure 5.8.3

Scheme of possible chromosome mutations/ Chromosomenmutationen by unknown on Wikimedia Commons is adapted from NIH's Talking Glossary of Genetics. [Changes as described by de:user:Dietzel65]. Further use and adapation (text translated to English) by Christine Miller as image is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

References

Seeker. (2016, April 23). How radiation changes your DNA. YouTube. https://www.youtube.com/watch?v=PQjL4ZDuq2o&feature=youtu.be

TED. (2019, June 5). What you should know about vaping and e-cigarettes | Suchitra Krishnan-Sarin. YouTube. https://www.youtube.com/watch?v=a63t8r70QN0&feature=youtu.be

TED-Ed. (2014, September 22). Where do genes come from? - Carl Zimmer. YouTube. https://www.youtube.com/watch?v=z9HIYjRRaDE&feature=youtu.be

Wikipedia contributors. (2020, July 6). Breast cancer. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Breast_cancer&oldid=966366739

Wikipedia contributors. (2020, July 9). Charcot–Marie–Tooth disease. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Charcot%E2%80%93Marie%E2%80%93Tooth_disease&oldid=966912915

Wikipedia contributors. (2020, July 7). Cystic fibrosis. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Cystic_fibrosis&oldid=966566921

Wikipedia contributors. (2020, June 4). Limone sul Garda. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Limone_sul_Garda&oldid=960771991

Wikipedia contributors. (2020, June 23). Ovarian cancer. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Ovarian_cancer&oldid=964157192

Wikipedia contributors. (2020, May 7). BRCA mutation. In Wikipedia. https://en.wikipedia.org/w/index.php?title=BRCA_mutation&oldid=955463902

Wikipedia contributors. (2020, July 10). Teenage Mutant Ninja Turtles. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Teenage_Mutant_Ninja_Turtles&oldid=967030468

A stem cell can become any type of body cell based on gene regulation. Types of cells a stem cell can become include, but are not limited to: Sex cells, muscle cells, fat cells, immune cells, bone cells, epithelial cells, nervous cells, and blood cells.

Image shows that Human Adult Height. Like many other polygenic traits, adult height has a bell-shaped distribution. The average height for adult females is 5 feet 5 inches, from ranges from 4 feet 7 inches all the way up to 6 feet 5 inches. In males, the same normal curve is observed, but with slightly higher values.

A very large molecule, such as protein, commonly created by the polymerization of smaller subunits (monomers).

A biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1. Complex carbohydrates are polymers made from monomers of simple carbohydrates, also termed monosaccharides.

A class of biological molecule consisting of linked monomers of amino acids and which are the most versatile macromolecules in living systems and serve crucial functions in essentially all biological processes.

A substance that is insoluble in water. Examples include fats, oils and cholesterol. Lipids are made from monomers such as glycerol and fatty acids.

A complex organic substance present in living cells, especially DNA or RNA, whose molecules consist of many nucleotides linked in a long chain.

Amino acids are organic compounds that combine to form proteins.

Long chains of hydrocarbons with a carboxyl group and a methyl group at opposite ends. Can be either saturated, containing mostly single bonds between adjacent carbons, or unsaturated, containing many double bonds between adjacent carbons.

Image shows the process of crossing over. A pair of homologous chromosomes are side by side. They then overlap the ends of their structures and trades these sections, attaching some maternal DNA onto the paternal chromosome, and some paternal DNA onto the maternal chromosome. The result is that, within the homologous pair of chromosomes, each of the four pieces of DNA, destined to enter 4 separate cells, is now consists of a unique mix of genes.

Biological molecules that lower amount the energy required for a reaction to occur.

Gland such as a sweat gland, salivary gland, or mammary gland that secretes a substance into a duct that carries the secretion to the outside of the body.

Image shows the inheritance pattern of blossom colour in pea plants. In the parental generation, a purple blossomed plant was crossed with a white blossomed plant. In the next generation (F1), all blossoms were purple. When the F1 generation self-pollinated, 75% of the resulting F2 generation had purple blossoms while the remaining 25% had white blossoms.

Image illustrates how independent alignment greatly increases the genetic diversity among gametes produced. In a single cell undergoing Meiosis, independent alignment means that whether the paternal or maternal chromosome for each homologous pair ends up on the left or right is totally random, and is random for each pair of homologous chromosomes. In a cell which has 3 pairs of homologous chromosomes, there are 8 possible random alignments, which would each result in a different set of gametes being produced.

Image shows a photograph of Gregor Mendel.

Image shows a diagram of Killer T Cell funtion. An infected cell displays a pathogen antigen on an MHC. The Killer T Cell interacts with the MHC and in response produces perforin ( a protein that pokes holes in cell membranes) and granzymes (proteins that instruct a cell to carry out programmed cell death). The infected cell dies from the combination of these substances, and as it dies, so does the pathogen inside the infected cell. The Killer T Cell is free to move on and find and destroy other infected cells.

Polysaccharides are carbohydrate molecules composed of long chains of monosaccharide units bound together. They range in structure from linear to highly branched.

Glucose (also called dextrose) is a simple sugar with the molecular formula C6H12O6. Glucose is the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using energy from sunlight.

A stored form of glucose used by plants.

Created by: CK-12/Adapted by Christine Miller

People tend to carry similar traits to their biological parents, as illustrated by the family tree. Beyond just appearance, you can also inherit traits from your parents that you can’t see.

Rebecca becomes very aware of this fact when she visits her new doctor for a physical exam. Her doctor asks several questions about her family medical history, including whether Rebecca has or had relatives with cancer. Rebecca tells her that her grandmother, aunt, and uncle — who have all passed away — had cancer. They all had breast cancer, including her uncle, and her aunt also had ovarian cancer. Her doctor asks how old they were when they were diagnosed with cancer. Rebecca is not sure exactly, but she knows that her grandmother was fairly young at the time, probably in her forties.

Rebecca’s doctor explains that while the vast majority of cancers are not due to inherited factors, a cluster of cancers within a family may indicate that there are mutations in certain genes that increase the risk of getting certain types of cancer, particularly breast and ovarian cancer. Some signs that cancers may be due to these genetic factors are present in Rebecca’s family, such as cancer with an early age of onset (e.g., breast cancer before age 50), breast cancer in men, and breast cancer and ovarian cancer within the same person or family.

Based on her family medical history, Rebecca’s doctor recommends that she see a genetic counselor, because these professionals can help determine whether the high incidence of cancers in her family could be due to inherited mutations in their genes. If so, they can test Rebecca to find out whether she has the particular variations of these genes that would increase her risk of getting cancer.

When Rebecca sees the genetic counselor, he asks how her grandmother, aunt, and uncle with cancer are related to her. She says that these relatives are all on her mother’s side — they are her mother’s mother and siblings. The genetic counselor records this information in the form of a specific type of family tree, called a pedigree, indicating which relatives had which type of cancer, and how they are related to each other and to Rebecca.

He also asks her ethnicity. Rebecca says that her family on both sides are Ashkenazi Jews (Jews whose ancestors came from central and eastern Europe). “But what does that have to do with anything?” she asks. The counselor tells Rebecca that mutations in two tumor-suppressor genes called BRCA1 and BRCA2, located on chromosome 17 and 13, respectively, are particularly prevalent in people of Ashkenazi Jewish descent and greatly increase the risk of getting cancer. About one in 40 Ashkenazi Jewish people have one of these mutations, compared to about one in 800 in the general population. Her ethnicity, along with the types of cancer, age of onset, and the specific relationships between her family members who had cancer, indicate to the counselor that she is a good candidate for genetic testing for the presence of these mutations.

Rebecca says that her 72-year-old mother never had cancer, nor had many other relatives on that side of the family. How could the cancers be genetic? The genetic counselor explains that the mutations in the BRCA1 and BRCA2 genes, while dominant, are not inherited by everyone in a family. Also, even people with mutations in these genes do not necessarily get cancer — the mutations simply increase their risk of getting cancer. For instance, 55 to 65 per cent of women with a harmful mutation in the BRCA1 gene will get breast cancer before age 70, compared to 12 per cent of women in the general population who will get breast cancer sometime over the course of their lives.

Rebecca is not sure she wants to know whether she has a higher risk of cancer. The genetic counselor understands her apprehension, but explains that if she knows that she has harmful mutations in either of these genes, her doctor will screen her for cancer more often and at earlier ages. Therefore, any cancers she may develop are likely to be caught earlier when they are often much more treatable. Rebecca decides to go through with the testing, which involves taking a blood sample, and nervously waits for her results.

Attributions

Figure 5.1.1

Family Tree [all individual face images] from Clker.com used and adapted by Christine Miller under a CC0 1.0 public domain dedication license (https://creativecommons.org/publicdomain/zero/1.0/).

Figure 5.1.2

Rebecca by Kyle Broad on Unsplash is used under the Unsplash License (https://unsplash.com/license).

References

Wikipedia contributors. (2020, June 27). Ashkenazi Jews. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Ashkenazi_Jews&oldid=964691647

Wikipedia contributors. (2020, June 22). BRCA1. In Wikipedia. https://en.wikipedia.org/w/index.php?title=BRCA1&oldid=963868423

Wikipedia contributors. (2020, May 25). BRCA2. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=BRCA2&oldid=958722957

Image shows a hand with a tan line where a watch had been worn.

Image shows a sperm fertilizing an egg. The Sperm is much smaller than the egg.

Image shows the difference in morphology between a sickle cell and a normal red blood cell. The normal red blood cells are shaped like danishes, while the sickle cells are shaped like bananas

Figure 5.14.1 Collage of Diverse Faces.

This collage shows some of the variation in human skin colour, which can range from very light to very dark, with every possible gradation in between.  As you might expect, the skin color trait has a more complex genetic basis than just one gene with two alleles, which is the type of simple trait that Mendel studied in pea plants. Like skin color, many other human traits have more complicated modes of inheritance than Mendelian traits. Such modes of inheritance are called non-Mendelian inheritance, and they include inheritance of multiple allele traits, traits with codominance or incomplete dominance, and polygenic traits, among others. All of these modes are described below.

The majority of human genes are thought to have more than two normal versions, or alleles. Traits controlled by a single gene with more than two alleles are called multiple allele traits. An example is ABO blood type. Your blood type refers to which of certain proteins called antigens are found on your red blood cells. There are three common alleles for this trait, which are represented by the letters A, B, and O.

As shown in the table there are six possible ABO genotypes, because the three alleles, taken two at a time, result in six possible combinations. The A and B alleles are dominant to the O allele. As a result, both AA and AO genotypes have the same phenotype, with the A antigen in their blood (type A blood). Similarly, both BB and BO genotypes have the same phenotype, with the B antigen in their blood (type B blood). No antigen is associated with the O allele, so people with the OO genotype have no antigens for ABO blood type in their blood (type O blood).

Look at the genotype AB in the ABO blood group table. Alleles A and B for ABO blood type are neither dominant nor recessive to one another. Instead, they are codominant. Codominance occurs when two alleles for a gene are expressed equally in the phenotype of heterozygotes. In the case of ABO blood type, AB heterozygotes have a unique phenotype, with both A and B antigens in their blood (type AB blood).

Another relationship that may occur between alleles for the same gene is incomplete dominance. This occurs when the dominant allele is not completely dominant. In this case, an intermediate phenotype results in heterozygotes who inherit both alleles. Generally, this happens when the two alleles for a given gene both produce proteins, but one protein is not functional. As a result, the heterozygote individual produces only half the amount of normal protein as is produced by an individual who is homozygous for the normal allele.

An example of incomplete dominance in humans is Tay Sachs disease. The normal allele for the gene in this case produces an enzyme that is responsible for breaking down lipids. A defective allele for the gene results in the production of a nonfunctional enzyme. Heterozygotes who have one normal and one defective allele produce half as much functional enzyme as the normal homozygote, and this is enough for normal development. Homozygotes who have only defective allele, however, produce only nonfunctional enzyme. This leads to the accumulation of lipids in the brain starting in utero, which causes significant brain damage. Most individuals with Tay Sachs disease die at a young age, typically by the age of five years.

Another good example of incomplete dominance in humans is hair type.  There are genes for straight and curly hair, and if an individual is heterozygous, they will typically have the phenotype of wavy hair.

Many human traits are controlled by more than one gene. These traits are called polygenic traits. The alleles of each gene have a minor additive effect on the phenotype. There are many possible combinations of alleles, especially if each gene has multiple alleles. Therefore, a whole continuum of phenotypes is possible.

An example of a human polygenic trait is adult height. Several genes, each with more than one allele, contribute to this trait, so there are many possible adult heights. One adult’s height might be 1.655 m (5.430 feet), and another adult’s height might be 1.656 m (5.433 feet). Adult height ranges from less than 5 feet to more than 6 feet, with males, on average, being somewhat taller than females. The majority of people fall near the middle of the range of heights for their sex, as shown in Figure 5.14.4.

Many traits are affected by the environment, as well as by genes. This may be especially true for polygenic traits. Adult height, for example, might be negatively impacted by poor diet or childhood illness. Skin color is another polygenic trait. There is a wide range of skin colors in people worldwide. In addition to differences in genes, differences in exposure to ultraviolet (UV) light cause some variation. As shown in Figure 5.14.5, exposure to UV light darkens the skin.

Some genes affect more than one phenotypic trait. This is called pleiotropy. There are numerous examples of pleiotropy in humans. They generally involve important proteins that are needed for the normal development or functioning of more than one organ system. An example of pleiotropy in humans occurs with the gene that codes for the main protein in collagen, a substance that helps form bones. This protein is also important in the ears and eyes. Mutations in the gene result in problems not only in bones, but also in these sensory organs, which is how the gene's pleiotropic effects were discovered.

Another example of pleiotropy occurs with sickle cell anemia. This recessive genetic disorder occurs when there is a mutation in the gene that normally encodes the red blood cell protein called hemoglobin. People with the disorder have two alleles for sickle cell hemoglobin, so named for the sickle shape (pictured in Figure 5.14.6) that their red blood cells take on under certain conditions (like physical exertion). The sickle-shaped red blood cells clog small blood vessels, causing multiple phenotypic effects, including stunting of physical growth, certain bone deformities, kidney failure, and strokes.

Some genes affect the expression of other genes. This is called epistasis. Epistasis is similar to dominance, except that it occurs between different genes, rather than between different alleles for the same gene.

Albinism is an example of epistasis. A person with albinism has virtually no pigment in the skin. The condition occurs due to an entirely different gene than the genes that encode skin color. Albinism occurs because a protein called tyrosinase, which is needed for the production of normal skin pigment, is not produced, due to a gene mutation. If an individual has the albinism mutation, he or she will not have any skin pigment, regardless of the skin color genes that were inherited.

Do you know your ABO blood type? In an emergency, knowing this valuable piece of information could possibly save your life. If you ever need a blood transfusion, it is vital that you receive blood that matches your own blood type. Why? If the blood transfused into your body contains an antigen that your own blood does not contain, antibodies in your blood plasma (the liquid part of your blood) will recognize the antigen as foreign to your body and cause a reaction called agglutination. In this reaction, the transfused red blood cells will clump together. The agglutination reaction is serious and potentially fatal.

Knowing the antigens and antibodies present in each of the ABO blood types will help you understand which type(s) of blood you can safely receive if you ever need a transfusion. This information is shown in Figure 5.14.7 for all of the ABO blood types. If you have blood type A, this means that your red blood cells have the A antigen and that your blood plasma contains anti-B antibodies. If you were to receive a transfusion of type B or type AB blood, both of which have the B antigen, your anti-B antibodies would attack the transfused red blood cells, causing agglutination.

You may have heard that people with blood type O are called "universal donors," and that people with blood type AB are called universal recipients. People with type O blood have neither A nor B antigens in their blood, so if their blood is transfused into someone with a different ABO blood type, it causes no immune reaction, meaning they can donate blood to anyone. On the other hand, people with type AB blood have no anti-A or anti-B antibodies in their blood, so they can receive a transfusion of blood from anyone. Which blood type(s) can safely receive a transfusion of type AB blood, and which blood type(s) can be safely received by those with type O blood?

Attributes

Figure 5.14.1

• Woman's Face from Iran by Omid Armin on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Woman Wearing Black Coat by Anastasiya Pavlova on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Dark haired man, Queretaro, México by Leonel Hernandez Arteaga on Unsplash is used under the Unsplash License (https://unsplash.com/license). <not found on Unsplash>
• Man in White V-Neck T-Shirt (self) by Joseph Gonzalez on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Natural Redhead in Brazil by Gabriel Silvério on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Dark-Skinned Woman with Large White Rose by Oladimeji Oduns on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 5.14.2

ABO Blood Types Per Genotype by Christine Miller is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 5.14.3

Three Phenotypes of Hair Based on Inheritance/ Incomplete Dominance Hair by Christine Miller is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 5.14.4

Average height /Human Adult Height by CK-12 Foundation is used under a CC BY 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

©CK-12 Foundation Licensed under  • Terms of Use • Attribution

Figure 5.14.5

Tan lines by katiebordner on Flickr is used under a  CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Figure 5.14.6

Sickle cell anemia by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) ©

Figure 5.14.7

ABO_blood_type.svg by InvictaHOG on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

References

Amoeba Sisters. (2015, May 25). Incomplete dominance, codominance, polygenic traits, and epistasis! YouTube. https://www.youtube.com/watch?v=YJHGfbW55l0

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 18.9 Sickle cells [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/18-3-erythrocytes

Brainard, J/ CK-12 Foundation. (2016). Figure 2 Human adult height [digital image]. In CK-12 College Human Biology (Section 5.13) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-college-human-biology/section/5.13/

Mayo Clinic Staff. (n.d.). Tay-Sachs disease [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/tay-sachs-disease/symptoms-causes/syc-20378190

Mayo Clinic Staff. (n.d.). Sickle cell anemia [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/sickle-cell-anemia/symptoms-causes/syc-20355876

Teacher's Pet. (2015, January 25). Non-mendelian genetics. YouTube. https://www.youtube.com/watch?v=-4vsio8TZrU

Created by: CK-12/Adapted by Christine Miller

Figure 5.14.1 Collage of Diverse Faces.

This collage shows some of the variation in human skin colour, which can range from very light to very dark, with every possible gradation in between.  As you might expect, the skin color trait has a more complex genetic basis than just one gene with two alleles, which is the type of simple trait that Mendel studied in pea plants. Like skin color, many other human traits have more complicated modes of inheritance than Mendelian traits. Such modes of inheritance are called non-Mendelian inheritance, and they include inheritance of multiple allele traits, traits with codominance or incomplete dominance, and polygenic traits, among others. All of these modes are described below.

The majority of human genes are thought to have more than two normal versions, or alleles. Traits controlled by a single gene with more than two alleles are called multiple allele traits. An example is ABO blood type. Your blood type refers to which of certain proteins called antigens are found on your red blood cells. There are three common alleles for this trait, which are represented by the letters A, B, and O.

As shown in the table there are six possible ABO genotypes, because the three alleles, taken two at a time, result in six possible combinations. The A and B alleles are dominant to the O allele. As a result, both AA and AO genotypes have the same phenotype, with the A antigen in their blood (type A blood). Similarly, both BB and BO genotypes have the same phenotype, with the B antigen in their blood (type B blood). No antigen is associated with the O allele, so people with the OO genotype have no antigens for ABO blood type in their blood (type O blood).

Look at the genotype AB in the ABO blood group table. Alleles A and B for ABO blood type are neither dominant nor recessive to one another. Instead, they are codominant. Codominance occurs when two alleles for a gene are expressed equally in the phenotype of heterozygotes. In the case of ABO blood type, AB heterozygotes have a unique phenotype, with both A and B antigens in their blood (type AB blood).

Another relationship that may occur between alleles for the same gene is incomplete dominance. This occurs when the dominant allele is not completely dominant. In this case, an intermediate phenotype results in heterozygotes who inherit both alleles. Generally, this happens when the two alleles for a given gene both produce proteins, but one protein is not functional. As a result, the heterozygote individual produces only half the amount of normal protein as is produced by an individual who is homozygous for the normal allele.

An example of incomplete dominance in humans is Tay Sachs disease. The normal allele for the gene in this case produces an enzyme that is responsible for breaking down lipids. A defective allele for the gene results in the production of a nonfunctional enzyme. Heterozygotes who have one normal and one defective allele produce half as much functional enzyme as the normal homozygote, and this is enough for normal development. Homozygotes who have only defective allele, however, produce only nonfunctional enzyme. This leads to the accumulation of lipids in the brain starting in utero, which causes significant brain damage. Most individuals with Tay Sachs disease die at a young age, typically by the age of five years.

Another good example of incomplete dominance in humans is hair type.  There are genes for straight and curly hair, and if an individual is heterozygous, they will typically have the phenotype of wavy hair.

Many human traits are controlled by more than one gene. These traits are called polygenic traits. The alleles of each gene have a minor additive effect on the phenotype. There are many possible combinations of alleles, especially if each gene has multiple alleles. Therefore, a whole continuum of phenotypes is possible.

An example of a human polygenic trait is adult height. Several genes, each with more than one allele, contribute to this trait, so there are many possible adult heights. One adult’s height might be 1.655 m (5.430 feet), and another adult’s height might be 1.656 m (5.433 feet). Adult height ranges from less than 5 feet to more than 6 feet, with males, on average, being somewhat taller than females. The majority of people fall near the middle of the range of heights for their sex, as shown in Figure 5.14.4.

Many traits are affected by the environment, as well as by genes. This may be especially true for polygenic traits. Adult height, for example, might be negatively impacted by poor diet or childhood illness. Skin color is another polygenic trait. There is a wide range of skin colors in people worldwide. In addition to differences in genes, differences in exposure to ultraviolet (UV) light cause some variation. As shown in Figure 5.14.5, exposure to UV light darkens the skin.

Some genes affect more than one phenotypic trait. This is called pleiotropy. There are numerous examples of pleiotropy in humans. They generally involve important proteins that are needed for the normal development or functioning of more than one organ system. An example of pleiotropy in humans occurs with the gene that codes for the main protein in collagen, a substance that helps form bones. This protein is also important in the ears and eyes. Mutations in the gene result in problems not only in bones, but also in these sensory organs, which is how the gene's pleiotropic effects were discovered.

Another example of pleiotropy occurs with sickle cell anemia. This recessive genetic disorder occurs when there is a mutation in the gene that normally encodes the red blood cell protein called hemoglobin. People with the disorder have two alleles for sickle cell hemoglobin, so named for the sickle shape (pictured in Figure 5.14.6) that their red blood cells take on under certain conditions (like physical exertion). The sickle-shaped red blood cells clog small blood vessels, causing multiple phenotypic effects, including stunting of physical growth, certain bone deformities, kidney failure, and strokes.

Some genes affect the expression of other genes. This is called epistasis. Epistasis is similar to dominance, except that it occurs between different genes, rather than between different alleles for the same gene.

Albinism is an example of epistasis. A person with albinism has virtually no pigment in the skin. The condition occurs due to an entirely different gene than the genes that encode skin color. Albinism occurs because a protein called tyrosinase, which is needed for the production of normal skin pigment, is not produced, due to a gene mutation. If an individual has the albinism mutation, he or she will not have any skin pigment, regardless of the skin color genes that were inherited.

Do you know your ABO blood type? In an emergency, knowing this valuable piece of information could possibly save your life. If you ever need a blood transfusion, it is vital that you receive blood that matches your own blood type. Why? If the blood transfused into your body contains an antigen that your own blood does not contain, antibodies in your blood plasma (the liquid part of your blood) will recognize the antigen as foreign to your body and cause a reaction called agglutination. In this reaction, the transfused red blood cells will clump together. The agglutination reaction is serious and potentially fatal.

Knowing the antigens and antibodies present in each of the ABO blood types will help you understand which type(s) of blood you can safely receive if you ever need a transfusion. This information is shown in Figure 5.14.7 for all of the ABO blood types. If you have blood type A, this means that your red blood cells have the A antigen and that your blood plasma contains anti-B antibodies. If you were to receive a transfusion of type B or type AB blood, both of which have the B antigen, your anti-B antibodies would attack the transfused red blood cells, causing agglutination.

You may have heard that people with blood type O are called "universal donors," and that people with blood type AB are called universal recipients. People with type O blood have neither A nor B antigens in their blood, so if their blood is transfused into someone with a different ABO blood type, it causes no immune reaction, meaning they can donate blood to anyone. On the other hand, people with type AB blood have no anti-A or anti-B antibodies in their blood, so they can receive a transfusion of blood from anyone. Which blood type(s) can safely receive a transfusion of type AB blood, and which blood type(s) can be safely received by those with type O blood?

Attributes

Figure 5.14.1

• Woman's Face from Iran by Omid Armin on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Woman Wearing Black Coat by Anastasiya Pavlova on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Dark haired man, Queretaro, México by Leonel Hernandez Arteaga on Unsplash is used under the Unsplash License (https://unsplash.com/license). <not found on Unsplash>
• Man in White V-Neck T-Shirt (self) by Joseph Gonzalez on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Natural Redhead in Brazil by Gabriel Silvério on Unsplash is used under the Unsplash License (https://unsplash.com/license).
• Dark-Skinned Woman with Large White Rose by Oladimeji Oduns on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 5.14.2

ABO Blood Types Per Genotype by Christine Miller is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 5.14.3

Three Phenotypes of Hair Based on Inheritance/ Incomplete Dominance Hair by Christine Miller is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 5.14.4

Average height /Human Adult Height by CK-12 Foundation is used under a CC BY 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

©CK-12 Foundation Licensed under  • Terms of Use • Attribution

Figure 5.14.5

Tan lines by katiebordner on Flickr is used under a  CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Figure 5.14.6

Sickle cell anemia by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) ©

Figure 5.14.7

ABO_blood_type.svg by InvictaHOG on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

References

Amoeba Sisters. (2015, May 25). Incomplete dominance, codominance, polygenic traits, and epistasis! YouTube. https://www.youtube.com/watch?v=YJHGfbW55l0

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 18.9 Sickle cells [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/18-3-erythrocytes

Brainard, J/ CK-12 Foundation. (2016). Figure 2 Human adult height [digital image]. In CK-12 College Human Biology (Section 5.13) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-college-human-biology/section/5.13/

Mayo Clinic Staff. (n.d.). Tay-Sachs disease [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/tay-sachs-disease/symptoms-causes/syc-20378190

Mayo Clinic Staff. (n.d.). Sickle cell anemia [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/sickle-cell-anemia/symptoms-causes/syc-20355876

Teacher's Pet. (2015, January 25). Non-mendelian genetics. YouTube. https://www.youtube.com/watch?v=-4vsio8TZrU

Image shows a photo of a young child exhibiting Polydactyly- a condition in which a person is born with extra fingers or toes. In this photo, the child has an extra pinky finger on each hand.

Created by CK-12 Foundation/Adapted by Christine Miller

The swimmer in the Figure 13.3.1 photo is doing the butterfly stroke, a swimming style that requires the swimmer to carefully control his breathing so it is coordinated with his swimming movements. Breathing is the process of moving air into and out of the lungs, which are the organs in which gas exchange takes place between the atmosphere and the body. Breathing is also called ventilation, and it is one of two parts of the life-sustaining process of respiration. The other part is gas exchange. Before you can understand how breathing is controlled, you need to know how breathing occurs.

Breathing is a two-step process that includes drawing air into the lungs, or inhaling, and letting air out of the lungs, or exhaling. Both processes are illustrated in Figure 13.3.2.

Inhaling

Inhaling is an active process that results mainly from contraction of a muscle called the diaphragm, shown in Figure 13.3.2. The diaphragm is a large, dome-shaped muscle below the lungs that separates the thoracic (chest) and abdominal cavities. When the diaphragm contracts it moves down causing the thoracic cavity to expand, and the contents of the abdomen to be pushed downward. Other muscles — such as intercostal muscles between the ribs — also contribute to the process of inhalation, especially when inhalation is forced, as when taking a deep breath. These muscles help increase thoracic volume by expanding the ribs outward. The increase in thoracic volume creates a decrease in thoracic air pressure.  With the chest expanded, there is lower air pressure inside the lungs than outside the body, so outside air flows into the lungs via the respiratory tract according the the pressure gradient (high pressure flows to lower pressure).

Exhaling

Exhaling involves the opposite series of events. The diaphragm relaxes, so it moves upward and decreases the volume of the thorax. Air pressure inside the lungs increases, so it is higher than the air pressure outside the lungs. Exhalation, unlike inhalation, is typically a passive process that occurs mainly due to the elasticity of the lungs. With the change in air pressure, the lungs contract to their pre-inflated size, forcing out the air they contain in the process. Air flows out of the lungs, similar to the way air rushes out of a balloon when it is released. If exhalation is forced, internal intercostal and abdominal muscles may help move the air out of the lungs.

Breathing is one of the few vital bodily functions that can be controlled consciously, as well as unconsciously. Think about using your breath to blow up a balloon. You take a long, deep breath, and then you exhale the air as forcibly as you can into the balloon. Both the inhalation and exhalation are consciously controlled.

Conscious Control of Breathing

You can control your breathing by holding your breath, slowing your breathing, or hyperventilating, which is breathing more quickly and shallowly than necessary. You can also exhale or inhale more forcefully or deeply than usual. Conscious control of breathing is common in many activities besides blowing up balloons, including swimming, speech training, singing, playing many different musical instruments (Figure 13.3.3), and doing yoga, to name just a few.

There are limits on the conscious control of breathing. For example, it is not possible for a healthy person to voluntarily stop breathing indefinitely. Before long, there is an irrepressible urge to breathe. If you were able to stop breathing for a long enough time, you would lose consciousness. The same thing would happen if you were to hyperventilate for too long. Once you lose consciousness so you can no longer exert conscious control over your breathing, involuntary control of breathing takes over.

Unconscious Control of Breathing

Unconscious breathing is controlled by respiratory centers in the medulla and pons of the brainstem (see Figure 13.3.4). The respiratory centers automatically and continuously regulate the rate of breathing based on the body’s needs. These are determined mainly by blood acidity, or pH. When you exercise, for example, carbon dioxide levels increase in the blood, because of increased cellular respiration by muscle cells. The carbon dioxide reacts with water in the blood to produce carbonic acid, making the blood more acidic, so pH falls. The drop in pH is detected by chemoreceptors in the medulla. Blood levels of oxygen and carbon dioxide, in addition to pH, are also detected by chemoreceptors in major arteries, which send the “data” to the respiratory centers. The latter respond by sending nerve impulses to the diaphragm, “telling” it to contract more quickly so the rate of breathing speeds up. With faster breathing, more carbon dioxide is released into the air from the blood, and blood pH returns to the normal range.

The opposite events occur when the level of carbon dioxide in the blood becomes too low and blood pH rises. This may occur with involuntary hyperventilation, which can happen in panic attacks, episodes of severe pain, asthma attacks, and many other situations. When you hyperventilate, you blow off a lot of carbon dioxide, leading to a drop in blood levels of carbon dioxide. The blood becomes more basic (alkaline), causing its pH to rise.

Nasal breathing is breathing through the nose rather than the mouth, and it is generally considered to be superior to mouth breathing. The hair-lined nasal passages do a better job of filtering particles out of the air before it moves deeper into the respiratory tract. The nasal passages are also better at warming and moistening the air, so nasal breathing is especially advantageous in the winter when the air is cold and dry. In addition, the smaller diameter of the nasal passages creates greater pressure in the lungs during exhalation. This slows the emptying of the lungs, giving them more time to extract oxygen from the air.

Drowning is defined as respiratory impairment from being in or under a liquid. It is further classified according to its outcome into: death, ongoing health problems, or no ongoing health problems (full recovery). Four hundred Canadians die annually from drowning, and drowning is one of the leading causes of death in children under the age of five. There are some potentially dangerous myths about drowning, and knowing what they are might save your life or the life of a loved one, especially a child.

Attributions

Figure 13.3.1

US_Marines_butterfly_stroke by Cpl. Jasper Schwartz from U.S. Marine Corps on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 13.3.2

Inhale Exhale/Breathing cycle by Siyavula Education on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Figure 13.3.3

Trumpet/ Frenchmen Street [photo] by Morgan Petroski on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 13.3.4

Respiratory_Centers_of_the_Brain by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 13.3.5

Lily & Ava in the Kiddie Pool by mob mob on Flickr is used under a CC BY-NC 2.0 (https://creativecommons.org/licenses/by-nc/2.0/) license.

References

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 22.20 Respiratory centers of the brain [digital image].  In Anatomy and Physiology (Section 22.3). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/22-3-the-process-of-breathing

Kai Simon. (2015, January 11). The ultimate relaxation technique: How to practice diaphragmatic breathing for beginners. YouTube. https://www.youtube.com/watch?v=Vca6DyFqt4c&feature=youtu.be

TED. (2010, January 19). How I held my breath for 17 minutes | David Blaine. YouTube. https://www.youtube.com/watch?v=XFnGhrC_3Gs&feature=youtu.be

TED-Ed. (2012, October 4). How breathing works - Nirvair Kaur. YouTube. https://www.youtube.com/watch?v=Kl4cU9sG_08&feature=youtu.be

TED-Ed. (2020, May 21). How do ventilators work? - Alex Gendler. YouTube. https://www.youtube.com/watch?v=yDtKBXOEsoM&feature=youtu.be

Image shows a medical professional blotting a blood sample from a small prick on an infant's heel onto special filter paper for the purposes of screening for PKU.

A community of livings things interrelated with their physical and chemical environment.

The body system which acts as a chemical messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs. In humans, the major endocrine glands are the thyroid gland and the adrenal glands.

One of two main divisions of the nervous system that includes the brain and spinal cord.

Image shows components of DNA regulating transcription. One a section of DNA regulating a specific gene, upstream, there is an enhancer, promoter sequences, and the TATA box. These preceded the coding strand section of DNA which includes introns and exons.

Structures containing neuronal cell bodies in the peripheral nervous system.

Image shows a table with illustrations showing the variation that exists within pea plans. The peas can either be smooth or wrinkled. The peas can either be green or yellow. The flowers could either be white or purple. The pods could either be smooth or constricted. The pods could either be yellow or green. The plants could either be short or tall. The plants could either end with flowers or end with foliage.

Polly Who?

Each hand in the Figure 5.15.1 photo has an extra pinky finger. This is a condition called polydactyly, which literally means "many digits." People with polydactyly may have extra fingers and/or toes, and the condition may affect just one hand or foot, or both hands and feet. Polydactyly is often genetic in origin and may be part of a genetic disorder associated with other abnormalities.

Genetic disorders are diseases, syndromes, or other abnormal conditions caused by mutations in one or more genes, or by chromosomal alterations. Genetic disorders are typically present at birth, but they should not be confused with congenital disorders, a category that includes any disorder present at birth, regardless of cause. Some congenital disorders are not caused by genetic mutations or chromosomal alterations. Instead, they are caused by problems that arise during embryonic or fetal development, or during the process of birth. An example of a nongenetic congenital disorder is fetal alcohol syndrome. This is a collection of birth defects, including facial anomalies and intellectual disability, caused by maternal alcohol consumption during pregnancy.

Table 5.15.1 lists several genetic disorders caused by mutations in just one gene. Some of the disorders are caused by mutations in autosomal genes, others by mutations in X-linked genes. Which disorders would you expect to be more common in males than females?

Very few genetic disorders are controlled by dominant mutant [pHypophosphatemicb_glossary id="2119"]alleles[/pb_glossary]. A dominant allele is expressed in every individual who inherits even one copy of it. If it causes a serious disorder, affected people may die young and fail to reproduce. Therefore, the mutant dominant allele is likely to die out of a population.

A recessive mutant allele — such as the allele that causes sickle cell anemia or cystic fibrosis — is not expressed in people who inherit just one copy of it. These people are called carriers. They do not have the disorder themselves, but they carry the mutant allele and their offspring can inherit it. Thus, the allele is likely to pass on to the next generation, rather than die out.

Mistakes may occur during meiosis that result in nondisjunction. This is the failure of replicated chromosomes to separate properly during meiosis. Some of the resulting gametes will be missing all or part of a chromosome, while others will have an extra copy of all or part of the chromosome. If such gametes are fertilized and form zygotes, they usually do not survive. If they do survive, the individuals are likely to have serious genetic disorders.

Table 5.15.2 lists several genetic disorders that are caused by abnormal numbers of chromosomes. Most chromosomal disorders involve the X chromosome. The X and Y chromosomes are the only chromosome pair in which the two chromosomes are very different in size. This explains why nondisjunction tends to occur more frequently in sex chromosomes than in autosomes.

A karyotype is a picture of a cell's chromosomes. In Figure 5.15.3, note the extra chromosome 21. In Figure 5.15.2, a young man with Down syndrome exhibits the characteristic facial appearance.

Diagnosing and Treating Genetic Disorders

A genetic disorder that is caused by a mutation can be inherited. Therefore, people with a genetic disorder in their family may be concerned about having children with the disorder. A genetic counselor can help them understand the risks of their children being affected. If they decide to have children, they may be advised to have prenatal (“before birth”) testing to see if the fetus has any genetic abnormalities. One method of prenatal testing is amniocentesis. In this procedure, a few fetal cells are extracted from the fluid surrounding the fetus in utero, and the fetal chromosomes are examined. Down syndrome and other chromosomal alterations can be detected in this way.

The symptoms of genetic disorders can sometimes be treated or prevented. In the genetic disorder called phenylketonuria (PKU), for example, the amino acid phenylalanine builds up in the body to harmful levels. PKU is caused by a mutation in a gene that normally codes for an enzyme needed to break down phenylalanine. When a person with PKU consumes foods high in phenylalanine (including many high-protein foods), the buildup of PKU can lead to serious health problems. In infants and young children, the build-up of phenylalanine can cause intellectual disability and delayed development, along with other serious problems. All babies in Canada and the United States and many other countries are screened for PKU soon after birth.  As shown in Figure 5.15.3, the PKU test involves collecting a small amount of blood from the infant, typically from the heel using a small lancet.  The blood is collected on a special type of filter paper and then brought to a laboratory for analysis. If PKU is diagnosed, the infant can be fed a low-phenylalanine diet, which prevents the buildup of phenylalanine and the health problems associated with it, including intellectual disability. As long as a low-phenylalanine diet is followed throughout life, most symptoms of the disorder can be prevented.

Cures for genetic disorders are still in the early stages of development. One potential cure is gene therapy. Gene therapy is an experimental technique that uses genes to treat or prevent disease. In gene therapy, normal genes are introduced into cells to compensate for abnormal genes. If a mutated gene causes a necessary protein to be nonfunctional or missing, gene therapy may be able to introduce a normal copy of the gene to produce the needed functional protein.

A gene inserted directly into a cell usually does not function, so a carrier called a vector is genetically engineered to deliver the gene (see Figure 5.15.4 illustration). Certain viruses, such as adenoviruses, are often used as vectors. They can deliver the new gene by infecting cells. The viruses are modified so they do not cause disease when used in people. If the treatment is successful, the new gene delivered by the vector will allow the synthesis of a functioning protein. Researchers still must overcome many technical challenges before gene therapy will be a practical approach to curing genetic disorders.

Down syndrome is the most common genetic cause of intellectual disability. It occurs in about one in every 700 live births, and it currently affects nearly half a million Americans. Until recently, scientists thought that the changes leading to intellectual disability in people with Down syndrome all happen before birth.

Even more recently, researchers discovered a genetic abnormality that affects brain development in people with Down syndrome throughout childhood and into adulthood. The newly discovered genetic abnormality changes communication between nerve cells in the brain, resulting in slower transmission of nerve impulses. This finding may eventually allow the development of strategies to promote brain functioning in Down syndrome patients, and it may also be applicable to other development disabilities, such as autism. The results of this promising study were published in the March 16, 2016 issue of the scientific journal Neuron.

Attributions

Figure 5.15.1

Polydactyly_ECS by Baujat G, Le Merrer M. on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.

Figure 5.15.2

Downs/ All the Family [photo] by Nathan Anderson on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 5.15.3

Phenylketonuria_testing by U.S. Air Force photo/Staff Sgt Eric T. Sheler in the US Air Force National Archives on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 5.15.4

Gene_therapy by National Institutes of Health (NIH) on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 5.15.5

Boy_with_Down_Syndrome by Vanellus Foto on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en) license.

References

Baujat, G., Le Merrer, M. (2007, January 23). Ellis-Van Creveld syndrome. Orphanet Journal of Rare Diseases, 2, 27. https://doi.org/10.1186/1750-1172-2-27

Hecht, M. (2019, June 26). What is polydactyly? [online article]. Healthline. https://www.healthline.com/health/polydactyly

Genetic and Rare Diseases Information Center (GARD). (2016). Hypophosphatemic rickets (previously called vitamin D-resistant rickets) [online article]. NIH. https://rarediseases.info.nih.gov/diseases/6735/hypophosphatemic-rickets [last updated 7/1/2020]

Mayo Clinic Staff. (n.d.). Cystic fibrosis [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/cystic-fibrosis/symptoms-causes/syc-20353700

Mayo Clinic Staff. (n.d.). Down syndrome [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/down-syndrome/diagnosis-treatment/drc-20355983

Mayo Clinic Staff. (n.d.). Hemophilia [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/hemophilia/symptoms-causes/syc-20373327

Mayo Clinic Staff. (n.d.). Klinefelter syndrome [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/klinefelter-syndrome/symptoms-causes/syc-20353949

Mayo Clinic Staff. (n.d.). Marfan syndrome [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/marfan-syndrome/symptoms-causes/syc-20350782

Mayo Clinic Staff. (n.d.). Phenylketonuria (PKU) [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/phenylketonuria/symptoms-causes/syc-20376302

Mayo Clinic Staff. (n.d.). Sickle cell anemia [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/sickle-cell-anemia/symptoms-causes/syc-20355876

Mayo Clinic Staff. (n.d.). Turner syndrome [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/turner-syndrome/symptoms-causes/syc-20360782

Mayo Clinic Staff. (n.d.). Triple X syndrome [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/triple-x-syndrome/symptoms-causes/syc-20350977

National Center on Birth Defects and Developmental Disabilities. (2020). Fetal alcohol spectrum disorders (FASDs): Basics about FASDs [webpage]. Centers for Disease Control and Prevention (CDC). https://www.cdc.gov/ncbddd/fasd/facts.html

TED-Ed. (2019, January 24). How CRISPR lets you edit DNA - Andrea M. Henle. YouTube. https://www.youtube.com/watch?v=6tw_JVz_IEc

TED. (2016, October 24). What you need to know about CRISPR | Ellen Jorgensen. YouTube. https://www.youtube.com/watch?v=1BXYSGepx7Q&feature=youtu.be

TED. (2017, February 10). The ethical dilemma of designer babies | Paul Knoepfler. YouTube. https://www.youtube.com/watch?v=nOHbn8Q1fBM&t=3s

Created by: CK-12/Adapted by Christine Miller

Polly Who?

Each hand in the Figure 5.15.1 photo has an extra pinky finger. This is a condition called polydactyly, which literally means "many digits." People with polydactyly may have extra fingers and/or toes, and the condition may affect just one hand or foot, or both hands and feet. Polydactyly is often genetic in origin and may be part of a genetic disorder associated with other abnormalities.

Genetic disorders are diseases, syndromes, or other abnormal conditions caused by mutations in one or more genes, or by chromosomal alterations. Genetic disorders are typically present at birth, but they should not be confused with congenital disorders, a category that includes any disorder present at birth, regardless of cause. Some congenital disorders are not caused by genetic mutations or chromosomal alterations. Instead, they are caused by problems that arise during embryonic or fetal development, or during the process of birth. An example of a nongenetic congenital disorder is fetal alcohol syndrome. This is a collection of birth defects, including facial anomalies and intellectual disability, caused by maternal alcohol consumption during pregnancy.

Table 5.15.1 lists several genetic disorders caused by mutations in just one gene. Some of the disorders are caused by mutations in autosomal genes, others by mutations in X-linked genes. Which disorders would you expect to be more common in males than females?

Very few genetic disorders are controlled by dominant mutant [pHypophosphatemicb_glossary id="2119"]alleles[/pb_glossary]. A dominant allele is expressed in every individual who inherits even one copy of it. If it causes a serious disorder, affected people may die young and fail to reproduce. Therefore, the mutant dominant allele is likely to die out of a population.

A recessive mutant allele — such as the allele that causes sickle cell anemia or cystic fibrosis — is not expressed in people who inherit just one copy of it. These people are called carriers. They do not have the disorder themselves, but they carry the mutant allele and their offspring can inherit it. Thus, the allele is likely to pass on to the next generation, rather than die out.

Mistakes may occur during meiosis that result in nondisjunction. This is the failure of replicated chromosomes to separate properly during meiosis. Some of the resulting gametes will be missing all or part of a chromosome, while others will have an extra copy of all or part of the chromosome. If such gametes are fertilized and form zygotes, they usually do not survive. If they do survive, the individuals are likely to have serious genetic disorders.

Table 5.15.2 lists several genetic disorders that are caused by abnormal numbers of chromosomes. Most chromosomal disorders involve the X chromosome. The X and Y chromosomes are the only chromosome pair in which the two chromosomes are very different in size. This explains why nondisjunction tends to occur more frequently in sex chromosomes than in autosomes.

A karyotype is a picture of a cell's chromosomes. In Figure 5.15.3, note the extra chromosome 21. In Figure 5.15.2, a young man with Down syndrome exhibits the characteristic facial appearance.

Diagnosing and Treating Genetic Disorders

A genetic disorder that is caused by a mutation can be inherited. Therefore, people with a genetic disorder in their family may be concerned about having children with the disorder. A genetic counselor can help them understand the risks of their children being affected. If they decide to have children, they may be advised to have prenatal (“before birth”) testing to see if the fetus has any genetic abnormalities. One method of prenatal testing is amniocentesis. In this procedure, a few fetal cells are extracted from the fluid surrounding the fetus in utero, and the fetal chromosomes are examined. Down syndrome and other chromosomal alterations can be detected in this way.

The symptoms of genetic disorders can sometimes be treated or prevented. In the genetic disorder called phenylketonuria (PKU), for example, the amino acid phenylalanine builds up in the body to harmful levels. PKU is caused by a mutation in a gene that normally codes for an enzyme needed to break down phenylalanine. When a person with PKU consumes foods high in phenylalanine (including many high-protein foods), the buildup of PKU can lead to serious health problems. In infants and young children, the build-up of phenylalanine can cause intellectual disability and delayed development, along with other serious problems. All babies in Canada and the United States and many other countries are screened for PKU soon after birth.  As shown in Figure 5.15.3, the PKU test involves collecting a small amount of blood from the infant, typically from the heel using a small lancet.  The blood is collected on a special type of filter paper and then brought to a laboratory for analysis. If PKU is diagnosed, the infant can be fed a low-phenylalanine diet, which prevents the buildup of phenylalanine and the health problems associated with it, including intellectual disability. As long as a low-phenylalanine diet is followed throughout life, most symptoms of the disorder can be prevented.

Cures for genetic disorders are still in the early stages of development. One potential cure is gene therapy. Gene therapy is an experimental technique that uses genes to treat or prevent disease. In gene therapy, normal genes are introduced into cells to compensate for abnormal genes. If a mutated gene causes a necessary protein to be nonfunctional or missing, gene therapy may be able to introduce a normal copy of the gene to produce the needed functional protein.

A gene inserted directly into a cell usually does not function, so a carrier called a vector is genetically engineered to deliver the gene (see Figure 5.15.4 illustration). Certain viruses, such as adenoviruses, are often used as vectors. They can deliver the new gene by infecting cells. The viruses are modified so they do not cause disease when used in people. If the treatment is successful, the new gene delivered by the vector will allow the synthesis of a functioning protein. Researchers still must overcome many technical challenges before gene therapy will be a practical approach to curing genetic disorders.

Down syndrome is the most common genetic cause of intellectual disability. It occurs in about one in every 700 live births, and it currently affects nearly half a million Americans. Until recently, scientists thought that the changes leading to intellectual disability in people with Down syndrome all happen before birth.

Even more recently, researchers discovered a genetic abnormality that affects brain development in people with Down syndrome throughout childhood and into adulthood. The newly discovered genetic abnormality changes communication between nerve cells in the brain, resulting in slower transmission of nerve impulses. This finding may eventually allow the development of strategies to promote brain functioning in Down syndrome patients, and it may also be applicable to other development disabilities, such as autism. The results of this promising study were published in the March 16, 2016 issue of the scientific journal Neuron.

Attributions

Figure 5.15.1

Polydactyly_ECS by Baujat G, Le Merrer M. on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.

Figure 5.15.2

Downs/ All the Family [photo] by Nathan Anderson on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 5.15.3

Phenylketonuria_testing by U.S. Air Force photo/Staff Sgt Eric T. Sheler in the US Air Force National Archives on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 5.15.4

Gene_therapy by National Institutes of Health (NIH) on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 5.15.5

Boy_with_Down_Syndrome by Vanellus Foto on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en) license.

References

Baujat, G., Le Merrer, M. (2007, January 23). Ellis-Van Creveld syndrome. Orphanet Journal of Rare Diseases, 2, 27. https://doi.org/10.1186/1750-1172-2-27

Hecht, M. (2019, June 26). What is polydactyly? [online article]. Healthline. https://www.healthline.com/health/polydactyly

Genetic and Rare Diseases Information Center (GARD). (2016). Hypophosphatemic rickets (previously called vitamin D-resistant rickets) [online article]. NIH. https://rarediseases.info.nih.gov/diseases/6735/hypophosphatemic-rickets [last updated 7/1/2020]

Mayo Clinic Staff. (n.d.). Cystic fibrosis [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/cystic-fibrosis/symptoms-causes/syc-20353700

Mayo Clinic Staff. (n.d.). Down syndrome [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/down-syndrome/diagnosis-treatment/drc-20355983

Mayo Clinic Staff. (n.d.). Hemophilia [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/hemophilia/symptoms-causes/syc-20373327

Mayo Clinic Staff. (n.d.). Klinefelter syndrome [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/klinefelter-syndrome/symptoms-causes/syc-20353949

Mayo Clinic Staff. (n.d.). Marfan syndrome [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/marfan-syndrome/symptoms-causes/syc-20350782

Mayo Clinic Staff. (n.d.). Phenylketonuria (PKU) [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/phenylketonuria/symptoms-causes/syc-20376302

Mayo Clinic Staff. (n.d.). Sickle cell anemia [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/sickle-cell-anemia/symptoms-causes/syc-20355876

Mayo Clinic Staff. (n.d.). Turner syndrome [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/turner-syndrome/symptoms-causes/syc-20360782

Mayo Clinic Staff. (n.d.). Triple X syndrome [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/triple-x-syndrome/symptoms-causes/syc-20350977

National Center on Birth Defects and Developmental Disabilities. (2020). Fetal alcohol spectrum disorders (FASDs): Basics about FASDs [webpage]. Centers for Disease Control and Prevention (CDC). https://www.cdc.gov/ncbddd/fasd/facts.html

TED-Ed. (2019, January 24). How CRISPR lets you edit DNA - Andrea M. Henle. YouTube. https://www.youtube.com/watch?v=6tw_JVz_IEc

TED. (2016, October 24). What you need to know about CRISPR | Ellen Jorgensen. YouTube. https://www.youtube.com/watch?v=1BXYSGepx7Q&feature=youtu.be

TED. (2017, February 10). The ethical dilemma of designer babies | Paul Knoepfler. YouTube. https://www.youtube.com/watch?v=nOHbn8Q1fBM&t=3s

Image shows a diagram of the three stages of Translation. In Initiation, mRNA, a ribosome and tRNA carry methionine form a complex. In elongation, the ribosome moves along the mRNA, as tRNA with matching anticodons bring and then drop off the corresponding amino acids. In termination, a stop codon is reached, and the entire complex disassembles, and releases the newly synthesize polypeptide.

Image shows an example of a Punnett Square. In this example, a mother and father who are both heterozygous for eye colour result in four children of whom: 25% have blue eyes, 75% have brown eyes, 25% are homozygous dominant (with brown eyes being the dominant trait), 25% are homozygous recessive, and 50% are heterozygous

The smallest type of blood vessel that connects arterioles and venules and that transfers substances between blood and tissues.

Image shows a diagram of polymerase chain reaction, which occurs in three steps: 1) Denaturing, which separates the strands of DNA 2) Annealing, in which primers bind to the template DNA strands 3) Extension, in which Taq polymerase synthesizes new DNA strands from the original, now separated strands.