MadSci Network: Genetics |
Great scott! I must have screwed up in March when I tried to defer this question to another mad scientist because my schedule was too tight. My sincerest apologies, Neil. Now to the question. I'm going to divide my answer into three sections: a quick review of dominance in Mendelian traits, a review of what "dominant" means in context, and finally, whether there is a meaningful answer to your question. Dominance is classically defined for a set of alleles at a gene by looking at the phenotype of heterozygous individuals and comparing with the phenotype of homozygous individuals. An example you should be aware of is the ABO blood group system. At the ABO locus there are three alleles: A, B and O. If you look at the phenotype of an AO heterozygote, it is indistiguishable from an AA homozygote, so A is said to be dominant over O and O is said to be recesive to A. The same relationship holds for B and O: B is dominant over O and O is recessive to B. However, an AB heterozygote is phenotypically different from both the AA and the BB homozygotes. In this case, the two alleles are said to be codominant. In most basic bio classes we are taught the classic Mendelian Dominant/ recessive model, and sent on our way with a few examples such as dark hair and eye color is dominant over light, tongue rolling is dominant over non- rolling, attached earlobes are dominant over separated, Phenyl thio carbamide (PTC) tasting is dominant over non-tasting, etc. Of these four examples, only PTC tasting holds up to scrutiny. There are some rare families in which red hair is a dominant trait, but in most families it is clear that the hair color of the progeny is somewhere in between the parents suggesting a codominant inheritance. Tongue rolling is an unfortunate piece of misinformation which has been propagated for decades. The original author of the tongue rolling paper, Sturtevant, went on to have a brilliant career in Drosophila geneitcs, but nobody ever listened to him when he tried to retract it. As for the attached earlobe thing, I've never been able to clearly differentiate between classes: there is clearly a large amount of variation in how closely attached the lobes are, so it's a lousy example. As it turns out, there are extremely few traits in normal human beings which can be split into trait present/ trait absent categories and therefore discussed in terms of dominance. For example, most of the parameters we use to describe a person are quantitative: there is no clear separation of categories for height, weight, hair color, eye color, nose length, armspan, etc. Just to add to the fun, a single allele can contribute to multiple phenotypes, with different modes of inheritance for each phenotype. For example, the sickle cell mutation in beta globin contributes to at least two phenotypes: malaria resistance and sickle cell anemia. Both sickle cell carriers and homozygotes are resistant to malaria, so the sickle allele is dominant with respect to this phenotype. On the other hand, sickle cell carriers do not develop sickle cell anemia, so the sickle allele is recessive with respect to anemia. Thus, the short answer to your question is that there are very few traits in humans where dominance applies at all, and even then a single allele can be simultaneously recessive and dominant with respect to multiple phenotypes. So I'm sorry, but there really is no meaningful answer to your question "In an average human, approximately what kind of ratio of dominant to recessive genes(traits) exists, and is this more or less a constant ratio?" It all depends on which traits you look at, and even then most traits don't follow the simple mendelian dominance paradigm. Chris Carlson peterpan47@earthlink.com
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