… and what goes out:-Mutation,
Supposing we have a recessive deleterious allele, a:Selection, s
… and the mutation rate is given asGenotypes AA Aa aafitnesses 1 1 1-sfrequency before selection p2 2pq q2genotype freqs after selection p2 2pq q2(1-s), as before
The loss rate of a: when q is small (see selection and the single gene).
The gain rate of a: because .
So, if mutation which produces a and selection which removes a are balanced, we have an equilibrium gene frequency, q* between mutation and selection, so that:
EXAMPLE: Tay Sachs, an autosomal recessive genetic disease in humans has a frequency (q2, assuming random mating) of about 2x10-6; assuming random mating, therefore, the frequency of the Tay-Sachs allele, q is about 1.4x10-3 . Tay-Sachs sufferers usually die within two years (). If this disease is at mutation-selection balance, we can estimate that the mutation rate, .
Some lethal or nearly lethal diseases (for
example sickle-cell anaemia, especially among West Africans, or cystic
fibrosis (q ~ 1/20 among Europeans) are rather common. This
implies that something else is going on, possibly SELECTION for the mutant
alleles in heterozygotes?
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