Like Father, Like Son

This father-son duo share some similarities.  The shape of their faces and their facial features look very similar. If you saw them together, you might well guess that they are father and son. People have long known that the characteristics of living things are similar between parents and their offspring. However, it wasn’t until the experiments of Gregor Mendel that scientists understood how those traits are inherited.

Mendel did experiments with pea plants to show how traits such as seed shape and flower colour are inherited. Based on his research, he developed his two well known laws of inheritance: the law of segregation and the law of independent assortment. When Mendel died in 1884, his work was still virtually unknown. In 1900, three other researchers working independently came to the same conclusions that Mendel had drawn almost half a century earlier. Only then was Mendel’s work rediscovered.

Mendel knew nothing about genes, because they were discovered after his death. He did think, however, that some type of “factors” controlled traits, and that those “factors” were passed from parents to offspring. We now call these “factors” genes. Mendel's laws of inheritance, now expressed in terms of genes, form the basis of genetics, the science of heredity. For this reason, Mendel is often called the father of genetics.

Today, we know that traits of organisms are controlled by genes on chromosomes. To talk about inheritance in terms of genes and chromosomes, you need to know the language of genetics. The terms below serve as a good starting point. They are illustrated in the figure that follows.

• A gene is the part of a chromosome that contains the genetic code for a given protein. For example, in pea plants, a given gene might code for flower colour.
• The position of a given gene on a chromosome is called its locus (plural, loci). A gene might be located near the center, or at one end or the other of a chromosome.
• A given gene may have different normal versions, which are called alleles. For example, in pea plants, there is a purple-flower allele (B) and a white-flower allele (b) for the flower-colour gene. Different alleles account for much of the variation in the traits of organisms, including people.
• In sexually reproducing organisms, each individual has two copies of each type of chromosome. Paired chromosomes of the same type are called homologous chromosomes. They are about the same size and shape, and they have all the same genes at the same loci.

When sexual reproduction occurs, sex cells (called gametes) unite during fertilization to form a single cell called a zygote. The zygote inherits two of each type of chromosome, with one chromosome of each type coming from the father, and the other coming from the mother. Because homologous chromosomes have the same genes at the same loci, each individual also inherits two copies of each gene. The two copies may be the same allele or different alleles. The alleles an individual inherits for a given gene make up the individual’s genotype.  As shown in Table 5.11.1, an organism with two of the same allele (for example, BB or bb) is called a homozygote. An organism with two different alleles (in this example, Bb) is called a heterozygote.

Table 5.11.1 

Allele Combinations Associated With the Terms Homozygous and Heterozygous

Phenotype

The expression of an organism’s genotype is referred to as its phenotype, and it refers to the organism’s traits, such as purple or white flowers in pea plants. As you can see from Table 5.11.1, different genotypes may produce the same phenotype. In this example, both BB and Bb genotypes produce plants with the same phenotype, purple flowers. Why does this happen? In a Bb heterozygote, only the B allele is expressed, so the b allele doesn’t influence the phenotype. In general, when only one of two alleles is expressed in the phenotype, the expressed allele is called dominant, and the allele that isn’t expressed is called recessive.

The terms dominant and recessive may also be used to refer to phenotypic traits. For example, purple flower colour in pea plants is a dominant trait. It shows up in the phenotype whenever a plant inherits even one dominant allele for the trait. Similarly, white flower colour is a recessive trait. Like other recessive traits, it shows up in the phenotype only when a plant inherits two recessive alleles for the trait.

Attributions

Figure 5.11.1

Father holding his baby boy with matching haircut [photo] by Kelly Sikkema on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 5.11.2

Chromosome, Gene, Locus, and Allele by CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

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

Table 5.11.1

Allele Combinations Associated With the Terms Homozygous and Heterozygous by Christine Miller is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

References

Amoeba Sisters. (2018, February 1). Alleles and genes. YouTube. https://www.youtube.com/watch?v=pv3Kj0UjiLE&feature=youtu.be

Bozeman Science. (2011, August 4). Genotypes and phenotypes. YouTube. https://www.youtube.com/watch?v=OaovnS7BAoc&feature=youtu.be

Brainard, J/ CK-12 Foundation. (2016). Figure 2 Chromosome, gene, locus, and allele [digital image]. In CK-12 College Human Biology (Section 5.10) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-human-biology/section/5.9/