Case Study Conclusion: Your Genes May Help You Save a Life<\/h1>\r\n<\/div>\r\nAs you have learned in this chapter, humans are much more genetically similar to each other than they are different. Any two people on Earth are 99.9 per cent genetically identical to each other \u2014 but the mere 0.1 per cent that is different can be very important, as in the case of bone marrow donation to treat diseases such as leukemia. A good match must exist between a bone marrow donor and recipient in genes that encode for human leukocyte antigen (HLA) proteins. As you have learned, antigens are molecules \u2014 often on the surface of cells \u2014 that the immune system uses to identify foreign invaders. If a patient receives a bone marrow transplant from a donor that has different types of HLAs than the patient does, antibodies in their immune system will identify the antigens as nonself and will launch an attack on the transplanted cells. Also, since bone marrow produces immune cells, antibodies in the transplanted tissue can actually attack the patient\u2019s own cells using the same mechanism.\r\n\r\nAs you have also learned, a good HLA match is often difficult to find, even between full siblings.\u00a0 Finding a match in the registries is particularly hard for non-Caucasian people \u2014 and even harder for people from multiethnic backgrounds, such as seven-year-old Mateo, who you read about in the beginning of this chapter. Mateo is of African, Japanese, and Caucasian descent \u2014 a relatively rare combination. Because HLA matches are more likely to occur between people of the same ethnicity, the donor registries would ideally have sufficient potential donors from every ethnicity and ethnic combination. Unfortunately, some ethnicities are not sufficiently represented in the donor registry. According to the U.S. National Marrow Donor Program, while 97 per cent of Caucasian patients find a match, the match rate drops to 83 per cent for Hispanic or Latino patients and 76 per cent for African American or black patients. Multi-ethnic patients generally have an even harder time finding a match because the relative rareness of their particular ethnic combination in the general population makes it less likely that enough people of their same ancestry are registered donors.\r\n\r\nAs you learned in this chapter, human variation has historically been classified in several different ways, some of which resulted from or have contributed to racism. Most biological traits in humans exist on a continuum, and attempting to create biological categories of race based on discrete categories using a handful of traits is generally arbitrary and inaccurate. Gene flow through migration and mating between populations, genetic drift, and natural selection results in a gradual, clinal distribution of many human traits, rather than discrete categories. Mateo, for example, cannot be neatly placed into one racial category or another. Race and ethnic identity, however, remain important social and cultural concepts.\r\n\r\nMateo\u2019s ancestry does<\/em> play a role in determining his specific types of HLA proteins, and he is more likely to find a bone marrow match with a donor of an ethnic background similar to his own. Although there is much more genetic variation within<\/em>\u00a0races than\u00a0between <\/em>races, HLA types tend to correlate with ethnicity more than some other traits. As you have seen throughout this chapter, some environmental factors in different geographic regions have provided strong natural selection pressures, resulting in the development of genetic differences between people whose ancestors came from different areas. For example, adaptations to differing UV levels, diseases, altitudes, and climates all likely led to the evolution of human variations in skin colour, blood cells, and body morphology. This type of association between race and ethnicity and genetic variation is similar to the link between ethnicity and HLA type.\r\n\r\nMateo\u2019s family was not able to find a match for him in the bone marrow registries, unlike the little boy pictured in Figure 6.9.1, but they are not giving up hope. His parents have started working with organizations to host bone marrow drives, where potential donors can provide cheek swabs to add themselves to the donor registry. His parents have contacted the news media with Mateo\u2019s story, and family and friends are getting the word out on social media that more donors are needed, particular those with Mateo\u2019s specific combination of ethnicities. They hope that even if they are unable to find a match for Mateo, bringing awareness to the issue may increase the ethnic diversity of the donor registry to save other lives.\r\nYou Can Help!<\/h1>\r\nAccording to the Canadian Blood Services<\/a>, more donors are needed to join the bone marrow registry.\u00a0 Currently, there is a need for more young male donors: male stem cell donors are more likely to be matched with recipients because they offer better patient outcomes after transplant.\u00a0 There is also a need for donors with diverse ethnic backgrounds, particularly Aboriginal, Hispanic, African-Canadian, Filipino, and more.\u00a0 DIverse donors are needed to acheive the closest possible match for HLA between the donor and the recipient.\r\n\r\nLeukemia is not the only disease in which treatment involves bone marrow transplant \u2014 this course of action is often taken for conditions such as:\r\n\r\n \t- Aplastic anemia<\/li>\r\n \t
- Inherited immune system disorders<\/li>\r\n \t
- Inherited metabolic disorders<\/li>\r\n \t
- Bone marrow diseases<\/li>\r\n \t
- Lymphomas<\/li>\r\n<\/ul>\r\nAre you registered? If not, it is a relatively simple process that could save someone\u2019s life. A cheek swab is all that is initially needed. Only about one in 430 potential donors will actually be matched with a patient, and if you are chosen, it means that you are one of the only people on Earth who can donate to this patient because of your genetic similarity! If you decide to donate, bone marrow will either be surgically removed from the back of your pelvic bone, or blood-forming cells will be removed non-surgically from your bloodstream. Most donors are able to return to their normal activities one to seven days after donation \u2014 a small price to pay for potentially saving someone\u2019s life!\r\n\r\n \r\n\r\nhttps:\/\/www.youtube.com\/watch?v=FyriQibRhLA\r\n
Stem cell donation: Step by step, hemaquebec1998, 2015.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=rLO0Usg8vcY&feature=emb_logo\r\n
Marrow donors talk about donating and the donation process, Be The Match, 2012.<\/p>\r\n \r\n
\r\nChapter 6\u00a0Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nIn this chapter, you learned about human variation and its origins. Specifically, you learned that:\r\n\r\n \t- No two human individuals are genetically identical (except for monozygotic twins), but the human species as a whole exhibits relatively little genetic diversity relative to other mammalian species. Genetically, two people chosen at random are likely to be 99.9 per cent identical.<\/li>\r\n \t
- Of the total genetic variation in humans, about 90 per cent occurs between people within continental populations, and only about 10 per cent occurs between people from different continents. Older, larger populations tend to have greater genetic variation, because there's been more time and there are more people in which to accumulate mutations.<\/li>\r\n \t
- [pb_glossary id=\"2635\"]Single nucleotide polymorphisms[\/pb_glossary]\u00a0account for most human genetic differences. Allele frequencies for polymorphic genes generally have a clinal, rather than discrete, distribution.\u00a0A minority of alleles seem to cluster in particular geographic areas, such as the allele for no antigen in the Duffy blood group. Such alleles may be useful as genetic markers to establish the ancestry of individuals.<\/li>\r\n \t
- Knowledge of genetic variation can help us understand our similarities and differences. It can also help us reconstruct our evolutionary origins and history as a species. For example, the distribution of modern human genetic variation is consistent with the out-of-Africa hypothesis for the origin of modern humans.<\/li>\r\n \t
- An important benefit of studying human genetic variation is learning more about the genetic basis of human diseases. This should help us find more effective treatments and cures.<\/li>\r\n \t
- Humans seem to have a need to classify and label people based on their similarities and differences. Three approaches to classifying human variation\u00a0are\u00a0typological, populational, and clinal approaches.<\/li>\r\n<\/ul>\r\n
\r\n \t- \r\n
\r\n \t- The typological approach involves creating a system of discrete categories, or races (no longer used). This approach was widely used by scientists until the early 20th century. Racial categories are based on observable phenotypic traits (such as skin colour), but other traits and behaviors are often mistakenly assumed to apply to racial groups, as well. The use of racial classifications often leads to racism.<\/li>\r\n \t
- By the mid-20th century, scientists started advocating a population approach (no longer used). This assumes that the breeding population, which is the unit of evolution, is the only biologically meaningful group. While this approach makes sense in theory, in reality, it can rarely be applied to actual human populations. With few exceptions, most human populations are not closed breeding populations.<\/li>\r\n \t
- By the 1960s, scientists began to use a clinal approach to classify human variation. This approach maps variation in the frequency of traits or alleles over geographic regions or worldwide. Clinal maps for many genetic traits show variation that changes gradually from one geographic area to another. [pb_glossary id=\"6033\"]Gene flow[\/pb_glossary] and\/or [pb_glossary id=\"2633\"]natural selection[\/pb_glossary] can cause this type of distribution.<\/li>\r\n<\/ul>\r\n<\/li>\r\n \t
- Humans may respond to environmental stress in four different ways: adaptation, developmental adjustment, acclimatization, and cultural responses.<\/li>\r\n<\/ul>\r\n
\r\n \t- \r\n
\r\n \t- An [pb_glossary id=\"5863\"]adaptation[\/pb_glossary] is a genetically based trait that has evolved because it helps living things survive and reproduce in a given environment. Adaptations evolve by natural selection in populations over a relatively long period to time. Examples of adaptations include sickle cell trait as an adaptation to endemic malaria and the ability to taste bitter compounds as an adaptation to bitter-tasting toxins in plants.<\/li>\r\n \t
- A developmental adjustment is a nongenetic response to stress that occurs during infancy or childhood. It may persist into adulthood and may be irreversible. Developmental adjustment is possible because humans have a high degree of phenotypic plasticity. It may be the result of environmental stresses, such as inadequate food \u2014 which may stunt growth \u2014 or cultural practices, such as orthodontic treatments, which re-align the teeth and jaws.<\/li>\r\n \t
- [pb_glossary id=\"5861\"]Acclimatization[\/pb_glossary] is the development of reversible changes to environmental stress that develop over a relatively short period of time. The changes revert to the normal baseline state after the stress is removed. Examples of acclimatization include tanning of the skin and physiological changes (such as increased sweating) that occur with heat acclimatization.<\/li>\r\n \t
- Cultural responses consist of learned behaviors and technology that allow us to change our environment to control stress, rather than changing our bodies genetically or physiologically to cope with stress. Examples include using shelter, fire, and clothing to cope with a cold climate.<\/li>\r\n<\/ul>\r\n<\/li>\r\n<\/ul>\r\n
\r\n \t- Blood type is a genetic characteristic associated with the presence or absence of [pb_glossary id=\"2704\"]antigens[\/pb_glossary]\u00a0on the surface of red blood cells. A blood group system refers to all of the [pb_glossary id=\"5521\"]gene[\/pb_glossary](s), alleles, and possible [pb_glossary id=\"6039\"]genotypes[\/pb_glossary]\u00a0and [pb_glossary id=\"2477\"]phenotypes[\/pb_glossary]\u00a0that exist for a particular set of blood type antigens.<\/li>\r\n<\/ul>\r\n
\r\n \t- \r\n
\r\n \t- Antigens are molecules that the immune system identifies as either self or nonself. If antigens are identified as nonself, the immune system responds by forming [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] that are specific to the nonself antigens, leading to the destruction of cells bearing the antigens.<\/li>\r\n<\/ul>\r\n<\/li>\r\n \t
- The ABO blood group system is a system of red blood cell antigens controlled by a single gene with three common alleles on chromosome 9. There are four possible ABO blood types: A, B, AB, and O. The ABO system is the most important blood group system in blood transfusions. People with type O blood are universal donors, and people with type AB blood are universal recipients.<\/li>\r\n \t
- The frequencies of ABO blood type alleles and blood groups vary around the world. The allele for the B antigen is least common, and blood type O is the most common. Evolutionary forces of founder effect, genetic drift, and natural selection are responsible for the geographic distribution of ABO alleles and blood types. For example, people with type O blood may be somewhat resistant to malaria, possibly giving them a selective advantage where malaria is endemic.<\/li>\r\n \t
- The Rhesus blood group system is a system of red blood cell antigens controlled by two genes with many alleles on chromosome 1. There are five common Rhesus antigens, of which antigen D is the most significant. Individuals who have antigen D are called Rh+, and individuals who lack antigen D are called Rh-. Rh- mothers of Rh+ fetuses may produce antibodies against the D antigen in the fetal blood, causing hemolytic disease of the newborn (HDN).<\/li>\r\n \t
- The majority of people worldwide are Rh+, but there is regional variation in this blood group system. This variation may be explained by natural selection that favors [pb_glossary id=\"2474\"]heterozygotes[\/pb_glossary]\u00a0for the D antigen, because this genotype seems to be protected against some of the neurological\u00a0consequences\u00a0of the common parasitic infection toxoplasmosis.<\/li>\r\n<\/ul>\r\n
\r\n \t- At high altitudes, humans face the stress of [pb_glossary id=\"2725\"]hypoxia[\/pb_glossary], or a lack of oxygen. Hypoxia occurs at high altitude because there is less oxygen in each breath of air and lower air pressure, which prevents adequate absorption of oxygen from the lungs.\r\n
\r\n \t- Initial responses to hypoxia include hyperventilation and elevated heart rate, but these responses are stressful to the body. Continued exposure to high altitude may cause high altitude sickness, with symptoms such as fatigue, shortness of breath, and loss of appetite. At higher altitudes, there is greater risk of serious illness.<\/li>\r\n \t
- After several days at high altitude, [pb_glossary id=\"5861\"]acclimatization[\/pb_glossary] starts to occur in someone from a lowland population. More red blood cells and capillaries form,\u00a0along with other changes. Full acclimatization may take several weeks. Returning to low altitude causes a reversal of the changes to the pre-high altitude state in a matter of weeks.<\/li>\r\n \t
- Well over 100 million people live at altitudes higher than 2,500 metres above sea level. Some indigenous populations of Tibet, Peru, and Ethiopia have been living above 2,500 metres for thousands of years, and have evolved genetic adaptations to high altitude hypoxia. Different high altitude populations have evolved different adaptations to the same hypoxic stress. Tibetan highlanders, for example, have a faster rate of breathing and wider arteries, whereas Peruvian highlanders have larger red blood cells and a greater concentration of the oxygen-carrying protein hemoglobin.<\/li>\r\n<\/ul>\r\n<\/li>\r\n \t
- Both hot and cold temperatures are serious environmental stresses on the human body. In the cold, there is risk of hypothermia, which is a dangerous decrease in core body temperature. In the heat, there is risk of hyperthermia, which is a dangerous increase in core body temperature.<\/li>\r\n \t
- According to [pb_glossary id=\"2748\"]Bergmann\u2019s rule[\/pb_glossary], body size tends to be negatively correlated with temperature, because larger body size increases heat production and decreases heat loss. The opposite holds true for small body size. Bergmann\u2019s rule applies to many human populations that are hot or cold adapted.<\/li>\r\n \t
- According [pb_glossary id=\"5625\"]Allen\u2019s rule[\/pb_glossary], the length of body extremities is positively correlated with temperature, because longer extremities are better at dissipating excess body heat. The opposite applies to shorter extremities. Allen\u2019s rule applies to relative limb lengths in many human populations that have adapted to heat or cold.<\/li>\r\n \t
- Sweating is the primary way humans lose body heat. The evaporation of sweat from the skin cools the body. This only works well when the relative humidity is fairly low. At high relative humidity, sweat does not readily evaporate to cool us down. The heat index (HI) indicates how hot it feels due to the humidity.<\/li>\r\n \t
- Gradually working longer and harder in the heat can bring about heat acclimatization, in which the body has improved responses to heat stress. Sweating starts earlier, sweat contains less salt, and vasodilation brings heat to the surface to help cool the body. Full acclimatization takes up to 14 days and reverses just as quickly when the heat stress is removed.<\/li>\r\n \t
- The human body can respond to cold by producing more heat (by shivering or increasing the basal metabolic rate) or by conserving heat (by vasoconstriction at the body surface or a layer of fat-insulating internal organs).<\/li>\r\n \t
- At temperatures below freezing, the hunting response occurs to prevent cold injury (such as frostbite). This is a process of alternating vasoconstriction and vasodilation in extremities that are exposed to dangerous cold. Where temperatures are repeatedly cold but rarely below freezing, the hunting response may not occur, and the skin may remain cold due to vasoconstriction alone.<\/li>\r\n \t
- Milk contains the sugar lactose, a disaccharide. Lactose must be broken down into its two component sugars to be absorbed by the small intestine, and the enzyme lactase is needed for this process.\r\n
\r\n \t- In about 60 per cent of people worldwide, the ability to synthesize lactase and digest lactose declines after the first two years of life. These people become lactose intolerant, and cannot consume much milk without suffering symptoms such as bloating, cramps, and diarrhea.<\/li>\r\n \t
- In populations that herded milking animals for thousands of years, lactase persistence evolved. People who were able to synthesize lactase and digest lactose throughout life were strongly favored by natural selection. People who descended from these early herders generally still have lactase persistence. That includes many Europeans and European-Americans.<\/li>\r\n<\/ul>\r\n<\/li>\r\n \t
- Human populations may vary in how efficiently they use calories in food. Some people (especially South Pacific Islanders, Native Americans, and sub-Saharan Africans) may be able to get by on fewer calories than would be adequate for others, so they tend to easily gain weight, become obese, and develop diseases such as diabetes.<\/li>\r\n \t
- The [pb_glossary id=\"2770\"]thrifty gene hypothesis[\/pb_glossary] proposes that \u201cthrifty genes\u201d were selected for because they allowed people to use calories efficiently and store body fat when food was plentiful so they had a reserve to use when food was scarce. According to the hypothesis, thrifty genes become detrimental and lead to obesity and diabetes when food is plentiful all of the time.<\/li>\r\n \t
- Several assumptions underlying the thrifty gene hypothesis have been called into question, and genetic research has been unable to actually identify thrifty genes. Alternate hypotheses to the thrifty gene hypothesis have been proposed, including the drifty gene hypothesis. The latter hypothesis explains variation in the tendency to become obese by genetic drift on neutral genes.<\/li>\r\n<\/ul>\r\nIn this chapter, you learned about how genetic variation can lead to differences in human characteristics. Genes encode for proteins, which carry out our bodies\u2019 life processes. In the next chapter, you will learn about how proteins and other molecules make up the cells, tissues, and organs of the human body, and how these units work together in interacting systems to allow us to function.\r\n\r\n<\/div>\r\n<\/div>\r\n
\r\nChapter 6\u00a0Review<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- Explain why an evolutionarily older population is likely to have more genetic variation than a similarly-sized younger population.<\/li>\r\n \t
- The genetic difference between any two people on Earth is only about 0.1 percent. Based on our evolutionary history, describe one reason why humans are relatively homogeneous genetically.<\/li>\r\n \t
- What aspect(s) of human skin colour are due to adaptation? Be sure to define adaptation in your answer. What aspect(s) of human skin colour are due to acclimatization? Be sure to define acclimatization in your answer.<\/li>\r\n \t
- For each of the following human responses to the environment, list whether it can be best described as an example of adaptation, acclimatization, or developmental adjustment:\r\n
\r\n \t- Reduction in height due to lack of food in childhood<\/li>\r\n \t
- Resistance to malaria<\/li>\r\n \t
- Shivering in the cold<\/li>\r\n \t
- Changes in body size and dimensions to better tolerate heat or cold<\/li>\r\n<\/ol>\r\n<\/li>\r\n \t
- Give an example of a human response to environmental stress that involves changes in behavior, instead of changes in physiology.<\/li>\r\n \t
- What are two types of environmental stresses that caused genetic changes related to hemoglobin in some populations of humans?<\/li>\r\n \t
- The ability of an organism to change their phenotype in response to the environment is called phenotypic __________ .<\/li>\r\n \t
- List three natural selection pressures that differ geographically across the world and contributed to the evolution of human genetic variation in different regions.<\/li>\r\n \t
- [h5p id=\"529\"]<\/li>\r\n \t
- You may have noticed that when a sudden hot day occurs during a cool period, it can feel even more uncomfortable than higher temperatures during a hot period \u2014 even with the same humidity levels. Using what you learned in this chapter, explain why you think that happens.<\/li>\r\n \t
- Out of all mammals, why are humans the only ones that evolved lactase persistence?<\/li>\r\n \t
- If the Inuit people who live in the Arctic were not able to get enough vitamin D from their diet, what do you think might happen to their skin colour over a long period of time? Explain your answer.<\/li>\r\n \t
- Explain why malaria has been such a strong force of natural selection on human populations.<\/li>\r\n \t
- Give one example of \u201cheterozygote advantage\u201d (i.e. when the heterozygous genotype has higher relative fitness than the dominant or recessive homozygous genotype) in humans.<\/li>\r\n \t
- What is one way in which humans have evolved genetic adaptations in response to their food sources?<\/li>\r\n \t
- Do you think adaptation to high altitude evolved once or several times? Explain your reasoning.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\u00a0Attribution<\/strong><\/h2>\r\nFigure 6.9.1<\/strong>\r\n\r\nGive Life - Donner la vie<\/a> by Andrew Scheer<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en) license.\r\nReferences<\/h2>\r\n
Be The Match. (2012, December 19). Marrow donors talk about donating and the donation process. YouTube. https:\/\/www.youtube.com\/watch?v=rLO0Usg8vcY&feature=youtu.be<\/p>\r\n
Canadian Blood Services. (n.d.). There is an immediate need for blood as demand is rising.\u00a0 https:\/\/www.blood.ca\/en<\/p>\r\n
hemaquebec1998. (2015, August 27).Stem cell donation: Step by step. YouTube. https:\/\/www.youtube.com\/watch?v=FyriQibRhLA&feature=youtu.be<\/p>","rendered":"
<\/p>\n![\"\"](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Give_Life_-_Donner_la_vie_37394425121-2.jpg\")
Figure 6.9.1 Becoming a bone marrow donor can save the life of another.<\/em><\/figcaption><\/figure>\n\nCase Study Conclusion: Your Genes May Help You Save a Life<\/h1>\n<\/div>\n
As you have learned in this chapter, humans are much more genetically similar to each other than they are different. Any two people on Earth are 99.9 per cent genetically identical to each other \u2014 but the mere 0.1 per cent that is different can be very important, as in the case of bone marrow donation to treat diseases such as leukemia. A good match must exist between a bone marrow donor and recipient in genes that encode for human leukocyte antigen (HLA) proteins. As you have learned, antigens are molecules \u2014 often on the surface of cells \u2014 that the immune system uses to identify foreign invaders. If a patient receives a bone marrow transplant from a donor that has different types of HLAs than the patient does, antibodies in their immune system will identify the antigens as nonself and will launch an attack on the transplanted cells. Also, since bone marrow produces immune cells, antibodies in the transplanted tissue can actually attack the patient\u2019s own cells using the same mechanism.<\/p>\n
As you have also learned, a good HLA match is often difficult to find, even between full siblings.\u00a0 Finding a match in the registries is particularly hard for non-Caucasian people \u2014 and even harder for people from multiethnic backgrounds, such as seven-year-old Mateo, who you read about in the beginning of this chapter. Mateo is of African, Japanese, and Caucasian descent \u2014 a relatively rare combination. Because HLA matches are more likely to occur between people of the same ethnicity, the donor registries would ideally have sufficient potential donors from every ethnicity and ethnic combination. Unfortunately, some ethnicities are not sufficiently represented in the donor registry. According to the U.S. National Marrow Donor Program, while 97 per cent of Caucasian patients find a match, the match rate drops to 83 per cent for Hispanic or Latino patients and 76 per cent for African American or black patients. Multi-ethnic patients generally have an even harder time finding a match because the relative rareness of their particular ethnic combination in the general population makes it less likely that enough people of their same ancestry are registered donors.<\/p>\n
As you learned in this chapter, human variation has historically been classified in several different ways, some of which resulted from or have contributed to racism. Most biological traits in humans exist on a continuum, and attempting to create biological categories of race based on discrete categories using a handful of traits is generally arbitrary and inaccurate. Gene flow through migration and mating between populations, genetic drift, and natural selection results in a gradual, clinal distribution of many human traits, rather than discrete categories. Mateo, for example, cannot be neatly placed into one racial category or another. Race and ethnic identity, however, remain important social and cultural concepts.<\/p>\n
Mateo\u2019s ancestry does<\/em> play a role in determining his specific types of HLA proteins, and he is more likely to find a bone marrow match with a donor of an ethnic background similar to his own. Although there is much more genetic variation within<\/em>\u00a0races than\u00a0between <\/em>races, HLA types tend to correlate with ethnicity more than some other traits. As you have seen throughout this chapter, some environmental factors in different geographic regions have provided strong natural selection pressures, resulting in the development of genetic differences between people whose ancestors came from different areas. For example, adaptations to differing UV levels, diseases, altitudes, and climates all likely led to the evolution of human variations in skin colour, blood cells, and body morphology. This type of association between race and ethnicity and genetic variation is similar to the link between ethnicity and HLA type.<\/p>\nMateo\u2019s family was not able to find a match for him in the bone marrow registries, unlike the little boy pictured in Figure 6.9.1, but they are not giving up hope. His parents have started working with organizations to host bone marrow drives, where potential donors can provide cheek swabs to add themselves to the donor registry. His parents have contacted the news media with Mateo\u2019s story, and family and friends are getting the word out on social media that more donors are needed, particular those with Mateo\u2019s specific combination of ethnicities. They hope that even if they are unable to find a match for Mateo, bringing awareness to the issue may increase the ethnic diversity of the donor registry to save other lives.<\/p>\n
You Can Help!<\/h1>\n
- \r\n \t
- Aplastic anemia<\/li>\r\n \t
- Inherited immune system disorders<\/li>\r\n \t
- Inherited metabolic disorders<\/li>\r\n \t
- Bone marrow diseases<\/li>\r\n \t
- Lymphomas<\/li>\r\n<\/ul>\r\nAre you registered? If not, it is a relatively simple process that could save someone\u2019s life. A cheek swab is all that is initially needed. Only about one in 430 potential donors will actually be matched with a patient, and if you are chosen, it means that you are one of the only people on Earth who can donate to this patient because of your genetic similarity! If you decide to donate, bone marrow will either be surgically removed from the back of your pelvic bone, or blood-forming cells will be removed non-surgically from your bloodstream. Most donors are able to return to their normal activities one to seven days after donation \u2014 a small price to pay for potentially saving someone\u2019s life!\r\n\r\n \r\n\r\nhttps:\/\/www.youtube.com\/watch?v=FyriQibRhLA\r\n
Stem cell donation: Step by step, hemaquebec1998, 2015.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=rLO0Usg8vcY&feature=emb_logo\r\n
Marrow donors talk about donating and the donation process, Be The Match, 2012.<\/p>\r\n \r\n
\r\n Chapter 6\u00a0Summary<\/span><\/h1>\r\n<\/header>\r\n
\r\n\r\nIn this chapter, you learned about human variation and its origins. Specifically, you learned that:\r\n- \r\n \t
- No two human individuals are genetically identical (except for monozygotic twins), but the human species as a whole exhibits relatively little genetic diversity relative to other mammalian species. Genetically, two people chosen at random are likely to be 99.9 per cent identical.<\/li>\r\n \t
- Of the total genetic variation in humans, about 90 per cent occurs between people within continental populations, and only about 10 per cent occurs between people from different continents. Older, larger populations tend to have greater genetic variation, because there's been more time and there are more people in which to accumulate mutations.<\/li>\r\n \t
- [pb_glossary id=\"2635\"]Single nucleotide polymorphisms[\/pb_glossary]\u00a0account for most human genetic differences. Allele frequencies for polymorphic genes generally have a clinal, rather than discrete, distribution.\u00a0A minority of alleles seem to cluster in particular geographic areas, such as the allele for no antigen in the Duffy blood group. Such alleles may be useful as genetic markers to establish the ancestry of individuals.<\/li>\r\n \t
- Knowledge of genetic variation can help us understand our similarities and differences. It can also help us reconstruct our evolutionary origins and history as a species. For example, the distribution of modern human genetic variation is consistent with the out-of-Africa hypothesis for the origin of modern humans.<\/li>\r\n \t
- An important benefit of studying human genetic variation is learning more about the genetic basis of human diseases. This should help us find more effective treatments and cures.<\/li>\r\n \t
- Humans seem to have a need to classify and label people based on their similarities and differences. Three approaches to classifying human variation\u00a0are\u00a0typological, populational, and clinal approaches.<\/li>\r\n<\/ul>\r\n
- \r\n \t
- \r\n
- \r\n \t
- The typological approach involves creating a system of discrete categories, or races (no longer used). This approach was widely used by scientists until the early 20th century. Racial categories are based on observable phenotypic traits (such as skin colour), but other traits and behaviors are often mistakenly assumed to apply to racial groups, as well. The use of racial classifications often leads to racism.<\/li>\r\n \t
- By the mid-20th century, scientists started advocating a population approach (no longer used). This assumes that the breeding population, which is the unit of evolution, is the only biologically meaningful group. While this approach makes sense in theory, in reality, it can rarely be applied to actual human populations. With few exceptions, most human populations are not closed breeding populations.<\/li>\r\n \t
- By the 1960s, scientists began to use a clinal approach to classify human variation. This approach maps variation in the frequency of traits or alleles over geographic regions or worldwide. Clinal maps for many genetic traits show variation that changes gradually from one geographic area to another. [pb_glossary id=\"6033\"]Gene flow[\/pb_glossary] and\/or [pb_glossary id=\"2633\"]natural selection[\/pb_glossary] can cause this type of distribution.<\/li>\r\n<\/ul>\r\n<\/li>\r\n \t
- Humans may respond to environmental stress in four different ways: adaptation, developmental adjustment, acclimatization, and cultural responses.<\/li>\r\n<\/ul>\r\n
- \r\n \t
- \r\n
- \r\n \t
- An [pb_glossary id=\"5863\"]adaptation[\/pb_glossary] is a genetically based trait that has evolved because it helps living things survive and reproduce in a given environment. Adaptations evolve by natural selection in populations over a relatively long period to time. Examples of adaptations include sickle cell trait as an adaptation to endemic malaria and the ability to taste bitter compounds as an adaptation to bitter-tasting toxins in plants.<\/li>\r\n \t
- A developmental adjustment is a nongenetic response to stress that occurs during infancy or childhood. It may persist into adulthood and may be irreversible. Developmental adjustment is possible because humans have a high degree of phenotypic plasticity. It may be the result of environmental stresses, such as inadequate food \u2014 which may stunt growth \u2014 or cultural practices, such as orthodontic treatments, which re-align the teeth and jaws.<\/li>\r\n \t
- [pb_glossary id=\"5861\"]Acclimatization[\/pb_glossary] is the development of reversible changes to environmental stress that develop over a relatively short period of time. The changes revert to the normal baseline state after the stress is removed. Examples of acclimatization include tanning of the skin and physiological changes (such as increased sweating) that occur with heat acclimatization.<\/li>\r\n \t
- Cultural responses consist of learned behaviors and technology that allow us to change our environment to control stress, rather than changing our bodies genetically or physiologically to cope with stress. Examples include using shelter, fire, and clothing to cope with a cold climate.<\/li>\r\n<\/ul>\r\n<\/li>\r\n<\/ul>\r\n
- \r\n \t
- Blood type is a genetic characteristic associated with the presence or absence of [pb_glossary id=\"2704\"]antigens[\/pb_glossary]\u00a0on the surface of red blood cells. A blood group system refers to all of the [pb_glossary id=\"5521\"]gene[\/pb_glossary](s), alleles, and possible [pb_glossary id=\"6039\"]genotypes[\/pb_glossary]\u00a0and [pb_glossary id=\"2477\"]phenotypes[\/pb_glossary]\u00a0that exist for a particular set of blood type antigens.<\/li>\r\n<\/ul>\r\n
- \r\n \t
- \r\n
- \r\n \t
- Antigens are molecules that the immune system identifies as either self or nonself. If antigens are identified as nonself, the immune system responds by forming [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] that are specific to the nonself antigens, leading to the destruction of cells bearing the antigens.<\/li>\r\n<\/ul>\r\n<\/li>\r\n \t
- The ABO blood group system is a system of red blood cell antigens controlled by a single gene with three common alleles on chromosome 9. There are four possible ABO blood types: A, B, AB, and O. The ABO system is the most important blood group system in blood transfusions. People with type O blood are universal donors, and people with type AB blood are universal recipients.<\/li>\r\n \t
- The frequencies of ABO blood type alleles and blood groups vary around the world. The allele for the B antigen is least common, and blood type O is the most common. Evolutionary forces of founder effect, genetic drift, and natural selection are responsible for the geographic distribution of ABO alleles and blood types. For example, people with type O blood may be somewhat resistant to malaria, possibly giving them a selective advantage where malaria is endemic.<\/li>\r\n \t
- The Rhesus blood group system is a system of red blood cell antigens controlled by two genes with many alleles on chromosome 1. There are five common Rhesus antigens, of which antigen D is the most significant. Individuals who have antigen D are called Rh+, and individuals who lack antigen D are called Rh-. Rh- mothers of Rh+ fetuses may produce antibodies against the D antigen in the fetal blood, causing hemolytic disease of the newborn (HDN).<\/li>\r\n \t
- The majority of people worldwide are Rh+, but there is regional variation in this blood group system. This variation may be explained by natural selection that favors [pb_glossary id=\"2474\"]heterozygotes[\/pb_glossary]\u00a0for the D antigen, because this genotype seems to be protected against some of the neurological\u00a0consequences\u00a0of the common parasitic infection toxoplasmosis.<\/li>\r\n<\/ul>\r\n
- \r\n \t
- At high altitudes, humans face the stress of [pb_glossary id=\"2725\"]hypoxia[\/pb_glossary], or a lack of oxygen. Hypoxia occurs at high altitude because there is less oxygen in each breath of air and lower air pressure, which prevents adequate absorption of oxygen from the lungs.\r\n
- \r\n \t
- Initial responses to hypoxia include hyperventilation and elevated heart rate, but these responses are stressful to the body. Continued exposure to high altitude may cause high altitude sickness, with symptoms such as fatigue, shortness of breath, and loss of appetite. At higher altitudes, there is greater risk of serious illness.<\/li>\r\n \t
- After several days at high altitude, [pb_glossary id=\"5861\"]acclimatization[\/pb_glossary] starts to occur in someone from a lowland population. More red blood cells and capillaries form,\u00a0along with other changes. Full acclimatization may take several weeks. Returning to low altitude causes a reversal of the changes to the pre-high altitude state in a matter of weeks.<\/li>\r\n \t
- Well over 100 million people live at altitudes higher than 2,500 metres above sea level. Some indigenous populations of Tibet, Peru, and Ethiopia have been living above 2,500 metres for thousands of years, and have evolved genetic adaptations to high altitude hypoxia. Different high altitude populations have evolved different adaptations to the same hypoxic stress. Tibetan highlanders, for example, have a faster rate of breathing and wider arteries, whereas Peruvian highlanders have larger red blood cells and a greater concentration of the oxygen-carrying protein hemoglobin.<\/li>\r\n<\/ul>\r\n<\/li>\r\n \t
- Both hot and cold temperatures are serious environmental stresses on the human body. In the cold, there is risk of hypothermia, which is a dangerous decrease in core body temperature. In the heat, there is risk of hyperthermia, which is a dangerous increase in core body temperature.<\/li>\r\n \t
- According to [pb_glossary id=\"2748\"]Bergmann\u2019s rule[\/pb_glossary], body size tends to be negatively correlated with temperature, because larger body size increases heat production and decreases heat loss. The opposite holds true for small body size. Bergmann\u2019s rule applies to many human populations that are hot or cold adapted.<\/li>\r\n \t
- According [pb_glossary id=\"5625\"]Allen\u2019s rule[\/pb_glossary], the length of body extremities is positively correlated with temperature, because longer extremities are better at dissipating excess body heat. The opposite applies to shorter extremities. Allen\u2019s rule applies to relative limb lengths in many human populations that have adapted to heat or cold.<\/li>\r\n \t
- Sweating is the primary way humans lose body heat. The evaporation of sweat from the skin cools the body. This only works well when the relative humidity is fairly low. At high relative humidity, sweat does not readily evaporate to cool us down. The heat index (HI) indicates how hot it feels due to the humidity.<\/li>\r\n \t
- Gradually working longer and harder in the heat can bring about heat acclimatization, in which the body has improved responses to heat stress. Sweating starts earlier, sweat contains less salt, and vasodilation brings heat to the surface to help cool the body. Full acclimatization takes up to 14 days and reverses just as quickly when the heat stress is removed.<\/li>\r\n \t
- The human body can respond to cold by producing more heat (by shivering or increasing the basal metabolic rate) or by conserving heat (by vasoconstriction at the body surface or a layer of fat-insulating internal organs).<\/li>\r\n \t
- At temperatures below freezing, the hunting response occurs to prevent cold injury (such as frostbite). This is a process of alternating vasoconstriction and vasodilation in extremities that are exposed to dangerous cold. Where temperatures are repeatedly cold but rarely below freezing, the hunting response may not occur, and the skin may remain cold due to vasoconstriction alone.<\/li>\r\n \t
- Milk contains the sugar lactose, a disaccharide. Lactose must be broken down into its two component sugars to be absorbed by the small intestine, and the enzyme lactase is needed for this process.\r\n
- \r\n \t
- In about 60 per cent of people worldwide, the ability to synthesize lactase and digest lactose declines after the first two years of life. These people become lactose intolerant, and cannot consume much milk without suffering symptoms such as bloating, cramps, and diarrhea.<\/li>\r\n \t
- In populations that herded milking animals for thousands of years, lactase persistence evolved. People who were able to synthesize lactase and digest lactose throughout life were strongly favored by natural selection. People who descended from these early herders generally still have lactase persistence. That includes many Europeans and European-Americans.<\/li>\r\n<\/ul>\r\n<\/li>\r\n \t
- Human populations may vary in how efficiently they use calories in food. Some people (especially South Pacific Islanders, Native Americans, and sub-Saharan Africans) may be able to get by on fewer calories than would be adequate for others, so they tend to easily gain weight, become obese, and develop diseases such as diabetes.<\/li>\r\n \t
- The [pb_glossary id=\"2770\"]thrifty gene hypothesis[\/pb_glossary] proposes that \u201cthrifty genes\u201d were selected for because they allowed people to use calories efficiently and store body fat when food was plentiful so they had a reserve to use when food was scarce. According to the hypothesis, thrifty genes become detrimental and lead to obesity and diabetes when food is plentiful all of the time.<\/li>\r\n \t
- Several assumptions underlying the thrifty gene hypothesis have been called into question, and genetic research has been unable to actually identify thrifty genes. Alternate hypotheses to the thrifty gene hypothesis have been proposed, including the drifty gene hypothesis. The latter hypothesis explains variation in the tendency to become obese by genetic drift on neutral genes.<\/li>\r\n<\/ul>\r\nIn this chapter, you learned about how genetic variation can lead to differences in human characteristics. Genes encode for proteins, which carry out our bodies\u2019 life processes. In the next chapter, you will learn about how proteins and other molecules make up the cells, tissues, and organs of the human body, and how these units work together in interacting systems to allow us to function.\r\n\r\n<\/div>\r\n<\/div>\r\n
\r\n Chapter 6\u00a0Review<\/span><\/h1>\r\n<\/header>\r\n
\r\n- \r\n \t
- Explain why an evolutionarily older population is likely to have more genetic variation than a similarly-sized younger population.<\/li>\r\n \t
- The genetic difference between any two people on Earth is only about 0.1 percent. Based on our evolutionary history, describe one reason why humans are relatively homogeneous genetically.<\/li>\r\n \t
- What aspect(s) of human skin colour are due to adaptation? Be sure to define adaptation in your answer. What aspect(s) of human skin colour are due to acclimatization? Be sure to define acclimatization in your answer.<\/li>\r\n \t
- For each of the following human responses to the environment, list whether it can be best described as an example of adaptation, acclimatization, or developmental adjustment:\r\n
- \r\n \t
- Reduction in height due to lack of food in childhood<\/li>\r\n \t
- Resistance to malaria<\/li>\r\n \t
- Shivering in the cold<\/li>\r\n \t
- Changes in body size and dimensions to better tolerate heat or cold<\/li>\r\n<\/ol>\r\n<\/li>\r\n \t
- Give an example of a human response to environmental stress that involves changes in behavior, instead of changes in physiology.<\/li>\r\n \t
- What are two types of environmental stresses that caused genetic changes related to hemoglobin in some populations of humans?<\/li>\r\n \t
- The ability of an organism to change their phenotype in response to the environment is called phenotypic __________ .<\/li>\r\n \t
- List three natural selection pressures that differ geographically across the world and contributed to the evolution of human genetic variation in different regions.<\/li>\r\n \t
- [h5p id=\"529\"]<\/li>\r\n \t
- You may have noticed that when a sudden hot day occurs during a cool period, it can feel even more uncomfortable than higher temperatures during a hot period \u2014 even with the same humidity levels. Using what you learned in this chapter, explain why you think that happens.<\/li>\r\n \t
- Out of all mammals, why are humans the only ones that evolved lactase persistence?<\/li>\r\n \t
- If the Inuit people who live in the Arctic were not able to get enough vitamin D from their diet, what do you think might happen to their skin colour over a long period of time? Explain your answer.<\/li>\r\n \t
- Explain why malaria has been such a strong force of natural selection on human populations.<\/li>\r\n \t
- Give one example of \u201cheterozygote advantage\u201d (i.e. when the heterozygous genotype has higher relative fitness than the dominant or recessive homozygous genotype) in humans.<\/li>\r\n \t
- What is one way in which humans have evolved genetic adaptations in response to their food sources?<\/li>\r\n \t
- Do you think adaptation to high altitude evolved once or several times? Explain your reasoning.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\u00a0Attribution<\/strong><\/h2>\r\nFigure 6.9.1<\/strong>\r\n\r\nGive Life - Donner la vie<\/a> by Andrew Scheer<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en) license.\r\n
References<\/h2>\r\n
Be The Match. (2012, December 19). Marrow donors talk about donating and the donation process. YouTube. https:\/\/www.youtube.com\/watch?v=rLO0Usg8vcY&feature=youtu.be<\/p>\r\n
Canadian Blood Services. (n.d.). There is an immediate need for blood as demand is rising.\u00a0 https:\/\/www.blood.ca\/en<\/p>\r\n
hemaquebec1998. (2015, August 27).Stem cell donation: Step by step. YouTube. https:\/\/www.youtube.com\/watch?v=FyriQibRhLA&feature=youtu.be<\/p>","rendered":"
<\/p>\n
Figure 6.9.1 Becoming a bone marrow donor can save the life of another.<\/em><\/figcaption><\/figure>\n \nCase Study Conclusion: Your Genes May Help You Save a Life<\/h1>\n<\/div>\n
As you have learned in this chapter, humans are much more genetically similar to each other than they are different. Any two people on Earth are 99.9 per cent genetically identical to each other \u2014 but the mere 0.1 per cent that is different can be very important, as in the case of bone marrow donation to treat diseases such as leukemia. A good match must exist between a bone marrow donor and recipient in genes that encode for human leukocyte antigen (HLA) proteins. As you have learned, antigens are molecules \u2014 often on the surface of cells \u2014 that the immune system uses to identify foreign invaders. If a patient receives a bone marrow transplant from a donor that has different types of HLAs than the patient does, antibodies in their immune system will identify the antigens as nonself and will launch an attack on the transplanted cells. Also, since bone marrow produces immune cells, antibodies in the transplanted tissue can actually attack the patient\u2019s own cells using the same mechanism.<\/p>\n
As you have also learned, a good HLA match is often difficult to find, even between full siblings.\u00a0 Finding a match in the registries is particularly hard for non-Caucasian people \u2014 and even harder for people from multiethnic backgrounds, such as seven-year-old Mateo, who you read about in the beginning of this chapter. Mateo is of African, Japanese, and Caucasian descent \u2014 a relatively rare combination. Because HLA matches are more likely to occur between people of the same ethnicity, the donor registries would ideally have sufficient potential donors from every ethnicity and ethnic combination. Unfortunately, some ethnicities are not sufficiently represented in the donor registry. According to the U.S. National Marrow Donor Program, while 97 per cent of Caucasian patients find a match, the match rate drops to 83 per cent for Hispanic or Latino patients and 76 per cent for African American or black patients. Multi-ethnic patients generally have an even harder time finding a match because the relative rareness of their particular ethnic combination in the general population makes it less likely that enough people of their same ancestry are registered donors.<\/p>\n
As you learned in this chapter, human variation has historically been classified in several different ways, some of which resulted from or have contributed to racism. Most biological traits in humans exist on a continuum, and attempting to create biological categories of race based on discrete categories using a handful of traits is generally arbitrary and inaccurate. Gene flow through migration and mating between populations, genetic drift, and natural selection results in a gradual, clinal distribution of many human traits, rather than discrete categories. Mateo, for example, cannot be neatly placed into one racial category or another. Race and ethnic identity, however, remain important social and cultural concepts.<\/p>\n
Mateo\u2019s ancestry does<\/em> play a role in determining his specific types of HLA proteins, and he is more likely to find a bone marrow match with a donor of an ethnic background similar to his own. Although there is much more genetic variation within<\/em>\u00a0races than\u00a0between <\/em>races, HLA types tend to correlate with ethnicity more than some other traits. As you have seen throughout this chapter, some environmental factors in different geographic regions have provided strong natural selection pressures, resulting in the development of genetic differences between people whose ancestors came from different areas. For example, adaptations to differing UV levels, diseases, altitudes, and climates all likely led to the evolution of human variations in skin colour, blood cells, and body morphology. This type of association between race and ethnicity and genetic variation is similar to the link between ethnicity and HLA type.<\/p>\n
Mateo\u2019s family was not able to find a match for him in the bone marrow registries, unlike the little boy pictured in Figure 6.9.1, but they are not giving up hope. His parents have started working with organizations to host bone marrow drives, where potential donors can provide cheek swabs to add themselves to the donor registry. His parents have contacted the news media with Mateo\u2019s story, and family and friends are getting the word out on social media that more donors are needed, particular those with Mateo\u2019s specific combination of ethnicities. They hope that even if they are unable to find a match for Mateo, bringing awareness to the issue may increase the ethnic diversity of the donor registry to save other lives.<\/p>\n
You Can Help!<\/h1>\n
- At high altitudes, humans face the stress of [pb_glossary id=\"2725\"]hypoxia[\/pb_glossary], or a lack of oxygen. Hypoxia occurs at high altitude because there is less oxygen in each breath of air and lower air pressure, which prevents adequate absorption of oxygen from the lungs.\r\n
- \r\n
- Blood type is a genetic characteristic associated with the presence or absence of [pb_glossary id=\"2704\"]antigens[\/pb_glossary]\u00a0on the surface of red blood cells. A blood group system refers to all of the [pb_glossary id=\"5521\"]gene[\/pb_glossary](s), alleles, and possible [pb_glossary id=\"6039\"]genotypes[\/pb_glossary]\u00a0and [pb_glossary id=\"2477\"]phenotypes[\/pb_glossary]\u00a0that exist for a particular set of blood type antigens.<\/li>\r\n<\/ul>\r\n
- \r\n
- \r\n
![Give Life - Donner la vie<\/a> by](\"https:\/\/commons.wikimedia.org\/wiki\/File:Give_Life_-_Donner_la_vie_(37394425121).jpg\"){kind=link}