Biology Question #489

Bethany Garrison, a 17 year old female from Summerville, SC asks on December 30, 2001,

My mom and I are having an argument. She claims that the gene for blonde hair will be wiped out within the next four generations simply because it is a recessive trait. I told her that just because it is a recessive trait does not mean that it will be weeded out of the gene pool. Having different hair colors is an example of balanced polymorphism. I also mentioned that the gene for achondroplasia is dominant but only 1 in 10,000 babies are born with the syndrom. Clearly if the recessive genes were being eradicated simply because they are recessive genes, the entire population would be dwarfs! Can you please tell me if it is possible for blonde hair to be eradicated, especially in such a short amount of time?

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The answer

Darrell J. Tomkins answered on January 8, 2002

I don't know what your genetics knowledge is but you sound more informed that me about population genetics (I am a cytogeneticist). However, what I do recall about neutral traits, such as blond hair I presume, it is likely that Hardy-Weinberg equilibrium would be maintained in the population. If there is selection against blond hair, then the situation might lead to reduced frequency of the alleles involved. It seems more likely to me that there could be selection for blond hair, although I am sure there is no evidence for that.

In the case of achondroplasia, only a single mutant allele is needed to cause the dominant trait/syndrome. However, in this case, two mutant alleles caused a lethal condition called thanatophoric dwarfism. Therefore, there is selection against the allele, so I don't think we can expect an increase in the frequency of achondroplasia. On the other hand, most cases are caused by a new mutation, so if there are environmental caused for new mutations then we might expect an increase in the frequency of achondroplasia and other dominant syndromes.

David Baillie answered on January 9, 2002

The genetic terms recessive and dominant do not mean the same as the they do in ordinary english. Genes do not get eliminated because they are "recessive", they only get eliminated if they are selected against in a specific circumstance. If they are not selected against the gene frequency does not (indeed can not) change. This of course assumes the population is large (if it is very small, it is possible that either gene can be eliminated by chance). The mistake is the assumption that dominance means the gene dominates the recessive gene at the population level.

This is not what the word means. It simply means that the dominant allele is the one that manifests it's phenotypic consequence in an individual that has both dominant and recessive alleles for the gene. Indeed, the whole point of Mendel's demonstration of "segregation" was that the recessive gene could be recovered from a heterozygote in EXACTLY the frequency that is predicted assuming that it had not been dominated (eliminated) by the other allele.

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