Reality Rears Its Head
There are Many Shades of Brown
Genetic inheritance is often presented with straightforward examples involving only two alleles, one of which is dominant over the other. This makes sense, in that it makes the inheritance patterns easy to see, and because it follows the history of genetics reasonably well. Mendel, after all, figured out the rules of inheritance precisely because the traits he examined showed clear-cut inheritance patterns.
But, how many traits in living organisms actually show clear-cut dominance, with only two alleles? As we have learned more and more about genetics, and as we have begun to analyze the DNA sequences of the genes themselves, we have found that there are often hundreds of alleles for any particular gene. At an intuitive level, just from our innate instinct for "making sense of the world," we probably know this already. As we look around us at other people, we see nearly infinite variation. Part of the challenge in understanding genetic inheritance is matching the examples we study in the classroom with the reality of the world around us.
For example, a simple, two-allele model of inheritance seems incapable of explaining the variation in human hair color. With a 2-allele model, we would imagine a dominant allele, B, responsible for brown hair, and a recessive allele, b, responsible for blonde hair. Looking back at My Family, and the diagram illustrating Aunt Jess's family, we would consider Tage to have two b alleles (genotype bb), and Jess to have one b and one B (genotype Bb). Now certainly neither Tage nor Jess can carry more than 2 alleles of a single gene, so this simple genetic model is satisfactory for analyzing this small family. But this model fails when we consider all of the different shades of brown that we see around us. How can we understand real-world inheritance, where we have honey-blonde and Swedish-blonde, pecan-brown, mahogany-brown, and ebony-brown (i.e. black)?
There are too many shades of brown to account for all of them with only two alleles. In fact, we cannot even use B and b to represent the alleles in our diagrams; we need more symbols than two.
There is a simple solution to this problem. We can name the gene (perhaps brown), and then refer to different alleles of that gene by adding specific indentifiers, such as brown-ebony, brown-mahogany, etc. Once we recognize that there may be many alleles of a gene, it becomes much easier to relate classroom-genetics to real-world-genetics. Consider, for example, the pedigree below:
At first, this looks rather messy and complicated (even though we have limited it to only 5 shades of brown). However, even with this complicated-looking pedigree, we can make sense of it with a simple model of genetic inheritance.
- Different alleles of the gene change the DNA sequence of the gene slightly.
- Therefore, the enzymes encoded by different alleles are slightly different from each other.
- Some alleles, such as brown-ebony, produce enzymes with very high activity, which in turn produce a large quantity of the brown pigment.
- Some alleles, such as brown-swedish-blonde, produce enzymes with very little activity, which in turn produce only a small amount of the brown pigment.
- All alleles fall somewhere on a continuum, from no activity to very high activity--which geneticists refer to as an allelic series.
- In this example, alleles with more activity are dominant to alleles with less activity. More activity —› more pigment —› hair color phenotype.
- This is a common (but not absolute) pattern for the relationships of different alleles. Usually, alleles with more activity are dominant to alleles with less activity.
In this pedigree, swedish-blonde is recessive to all of the other alleles. The honey allele is dominant to swedish-blonde, but recessive to the other alleles. The ebony allele is dominant to all of the other alleles.
Even this complex pedigree makes sense genetically. Each individual inherits one allele of the brown gene from each parent. The activities of the two alleles determine the individual's hair color.
Is Rule #6 in the above really, strictly true, or is it a simplification? It is, in fact, another simplification.
Both alleles contribute to the phenotype.
When an allele is "dominant" over another, it does NOT somehow, magically, prevent the other allele from working. (It does not "dominate" the other allele.) Both alleles function, and produce the protein encoded by the gene.* Therefore, the total enzyme activity reflects the enzyme activity of one allele plus the enzyme activity of the other allele.
* [Unless an allele is a DNA sequence change that prevents the protein from being made at all, such as a deletion of most of the DNA.]
For traits in which both alleles contribute equally to the phenotype, what we see in heterozygotes is very much like an average between the phenotypes of each allele when it is homozygous. This type of interaction between alleles is often referred to as blending, or as incomplete dominance. The more-active allele contributes more to the phenotype of the heterozygote, but the less-active allele "dilutes" the phenotype somewhat. The two illustrations below compare strict dominance (or "complete" dominance), in which the dominant allele fully determines the phenotype, to additive contributions of both alleles.
Additivity looks somewhat like dominance of one allele over the other (dark + light —› dark), but with the dominance being somehow "incomplete" (dark/light is not as dark as dark/dark). Thus, the term "incomplete dominance" was created to refer to this kind of relationship. Yet, in terms of production of the enzyme, both alleles are fully functional and produce their product--which is the definition of "codominance." Should this type of allele interaction be called incomplete dominance, codominance, blending, or something else? Now that we understand the biochemical and molecular-genetic basis for this type of allele interaction (both alleles contribute to the phenotype), there is probably no good reason to continue using the older terms (codominance, partial dominance, incomplete dominance, blending, etc.). It is preferable simply to refer to "the heterozygous phenotype."
Additive Contributions of Both Alleles
|In these two tables, we have used color to indicate the phenotypes--as might be appropriate for flower color, hair or eye color, skin color, or any other trait that depends on the production of a pigment. The numbers with percentages indicate the total enzyme activity in individuals who are homozygous for a particular allele (left column and top row), or who are heterozygous for two different alleles.|
As we look around at different characteristics in different types of plants and animals, we see that strict dominance is relatively uncommon. Complete and perfect additivity is also fairly uncommon. Usually, the reality is somewhere in-between these two extremes.
The data--i. e. what we see in the world around us--reveal the following:
- Most genes have multiple alleles.
- In heterozygotes, the allele with "more activity" contributes more to the organism's phenotype than does the allele with "less activity."
- Strict dominance, and strict additivity are rare; usually, heterozygotes have a phenotype somewhere in-between full dominance and "blending" inheritance.
When we consider genetic inheritance involving a particular dominant allele in combination with a particular recessive allele, we are considering one example out of many different possible examples. It is important to keep this firmly in mind.