When learning about genetic crosses and the utility of Punnett squares, we benefit from the fact that some alleles are dominant, and so are expressed even if only one copy is present, and that some alleles are recessive, meaning that the individual must be homozygous recessive for that particular trait to express the recessive allele. There’s the familiar 3:1 ratio of offspring genotypes if we cross two heterozygous parents, and then variations depending on the genotypes of the parents. But why are some alleles dominant and recessive? And how about Pleiotropy?1
Before you read onwards, it might be useful to have a grasp of a few terms listed here – do look them up if they are unfamiliar to you:
Gene, allele, dominant, recessive, genotype, phenotype, homozygous, heterozygous, F0 generation, F1 and F2 generations
Dominant vs Recessive Alleles
Genes code for the synthesis of a protein by determining the series of amino acids in a polypeptide. Different versions of a gene, i.e. alleles, therefore determine the amount of a certain protein that is produced, and therefore the subsequent trait that the protein helps determine. For example, melanin, the pigment that determines eye color amongst many other traits, is a protein (for interest, it is a polymer derived from the tyrosine amino acid). If the gene that controls the production of melanin is ‘turned-off,’ then you won’t synthesize any melanin, and your eyes will just refract light similar to the sky does – and we see that as blue eyes. On the other hand, if we have the ‘turned-on,’ variety of the gene, then melanin will be synthesized and we will have brown eyes (simplification here as there are multiple genes controlling eye color which explain the other eye colors, but this will do for now).
You can see why the allele for brown eyes is therefore dominant – if you only have 1 copy of the allele, melanin will still be synthesized, and your eyes will appear brown. However, the allele for blue eyes must be recessive, because you will only have blue eyes if neither of your eye color alleles code for melanin production.
Another brilliant example is red hair color, involving a protein called MC1R – interestingly a receptor protein that activates the synthesis of eumelanin (a subgroup of the melanin pigment) – which when functional means the individual does not have red hair. However, a genetic variation in the MC1R gene means the receptor cannot stimulate eumelanin production, and instead synthesizes a different type of melanin called pheomelanin, which leads to red hair, freckles and unfortunately a tendency to get sunburns. This is very much the same as with brown and blue eyes – the allele leading to red hair must be recessive, because any eumelanin production will prevent red hair from forming. By extension, the allele leading to other hair colors, for example, black and brown, must be dominant. Blonde hair is the result of a different gene entirely, and its intricacies are left out of this article for sake of simplicity.
Pleiotropy is where the alleles and genes affect more than one phenotypic trait. This usually occurs when the gene is responsible for a protein involved in multiple organs or different tissues. For example, the gene responsible for the production of collagen fibres, which affects the formation of bone structure, but also your skin by providing strength and structure. This means collagen is also crucial in preventing the formation of cataracts in your eyes and hearing loss in the ears (collagen is necessary for cartilage in the middle and inner ear).
Another example would be beta-globin gene, which when mutated leads to sickle-cell anemia. We learn that the allele for sickle-cell anemia is recessive, even though some of the red blood cells will be malformed in a sickle shape. Note how it is not the same as the eye color allele, where one copy of the recessive allele will not be noticed in the phenotype – in this case some of the red blood cells will be malformed even if the individual is heterozygous. The interesting aspect, however, are the other phenotypic effects of having sickle-shaped red blood cells. Because the red blood cells clog up smaller blood vessels, not only does one suffer from anemia, but physical growth is also stunted, bone deformities are possible, and there is also a risk of kidney failure.
Therefore, you can observe just how complicated the initial genetic crosses become – links between different alleles, some alleles not being completely dominant or recessive, epistasis (which I have not touched upon) where genes affect the expression of different genes rather than other alleles for the same genes, the list goes on. For now, just enjoy the Punnett Squares and brief insights into the complexity beyond our syllabi!