Mind

Dominance & recessive, is it worthwhile?

Gene ExpressionBy Razib KhanOct 1, 2006 2:16 PM

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This is a shout out to the biologists out there: do you think the concept of dominance and recessive is worthwhile? In other words, does it help in conceptualization more than it hurts? Clearly the idea of recessiveness of deleterious traits helps in comprehending why such alleles exist in the ambient genetic background of a population and can reemerge via inbreeding. On the other hand, my own experience is that if you try to move the conversation to additive polygenic traits, which I think are interesting and need to be understood to really "get" population genetics you have to keep batting down the tendency for people to reconceptualize things in a simple dominance-recessive fashion. I think that there must be a distinction between dominance-recessive dichotomies which are only salient at the level of enzymatic concentration, and have almost no phenotypic impact (e.g., the threshold level of enzyme needed for catalysis is almost never lower in the heteryzogous state than in the homyzogous wild type state), and other traits where the character of classification is crucial. In other words, if for example you define "blonde hair" and "non-blonde hair," clearly the latter is "dominant" as a trait. But, if you look at the melanin concentration in individual hairs I am willing to bet that they will reflective additivity and independence. The issue here is human perception and bias of categories. What I'm getting at is that:a) I am happy to keep the dominance-recessive in cases where the distinction has little fitness implication and the "phenotypic" impact can only be discerned at a biochemical level.b) But I would like to abandon it in the case of gross and somewhat subjective phenotypes. For instance, curly vs. straight hair. My understanding is that the extent of curliness of your hair is proportional to the shape of your follicle, and it seems upon physical inspection that though children who are biracial from black and white parents have "curly" and "frizzy" hair, its extent is far less than in the black parent. I suspect the follicle shape tends to be equidistant in relation to the parental phenotypes, and the topological definition of "curliness" whould reflect this in the offspring.1 - The chance of a recessive allele being expressed is the square of its frequency in a random mating population. So, if the population had a recessive allele in a proportion of 0.01, in the next generation 0.0001 would express the trait, or, 1 out of 10,000 individuals. In other words, the overwhelming majority of individuals who carried the allele would not express a deleterious phenotype, so selection is generally very weak on these traits. The inverse occurs with dominant deleterious traits.

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