Introduction to Genetic Analysis 635

Introduction to Genetic Analysis 635 - 44200_19_p611-642...

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19.5 Balanced polymorphism Thus far we have considered the changes in allelic fre- quency that occur when the homozygous carriers of one allele, say, A/A , are more Ft than homozygous carriers for another allele, say, a/a, while the heterozygotes, A/a , are somewhere between A/A and a/a in their Ftness. But there are other possibilities. Overdominance and underdominance ±irst, the heterozygote might be more Ft than either ho- mozygote, a condition termed overdominance in Ftness. When one of the alleles, say, A , is in low frequency, there are virtually no homozygotes A/A and the allele occurs almost entirely in heterozygous condition. Since hetero- zygotes are more Ft than homozygotes, A alleles are al- most all carried by the most Ft genotype, so A will in- crease in frequency, while a decreases in frequency. On the other hand when a is in very low frequency, it oc- curs almost entirely in heterozygous condition. In this case, a alleles are almost all carried by the most Ft geno- type, and they will increase in frequency at the expense of A alleles. The net effect of these two pressures, one increasing 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.

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