19.5 Balanced polymorphismThus far we have considered the changes in allelic fre-quency that occur when the homozygous carriers of oneallele, say,A/A, are more Ft than homozygous carriersfor another allele, say,a/a,while the heterozygotes,A/a,are somewhere between A/Aand a/ain their Ftness.But there are other possibilities.Overdominance and underdominance±irst, the heterozygote might be moreFt than either ho-mozygote, a condition termed overdominance in Ftness.When one of the alleles, say,A, is in low frequency, thereare virtually no homozygotes A/Aand the allele occursalmost entirely in heterozygous condition. Since hetero-zygotes are more Ft than homozygotes,Aalleles are al-most all carried by the most Ft genotype, so A will in-crease in frequency, while adecreases in frequency. Onthe other hand when ais in very low frequency, it oc-curs almost entirely in heterozygous condition. In thiscase,aalleles are almost all carried by the most Ft geno-type, and they will increase in frequency at the expenseof Aalleles. The net effect of these two pressures, oneincreasing the frequency of A
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.