AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

MORE THAN ONE LOCUS

• The attainment of equilibrium in genotype frequencies after one generation of random mating is true of all autosomal loci considered individually.
• But it is not true of the genotypes considered jointly .
• Consider a population made up of equal numbers of AABB and aabb individuals of both sexes. The gene frequency at both loci is 0.5.
• If the individuals are mated at random the possible genotypes are:

AABB	AaBB	aaBB
AABb	AaBb	aaBb
AAbb	Aabb	aabb

• Only three out of nine genotypes would appear in the progeny in the next generation i.e. the two original homozygotes ( AABB and aabb ) and the double heterozygote ( AaBb ).
• The genotype AAbb would be absent though its frequency in equilibrium population would be 1/16.
• The missing genotypes appear in subsequent generations but not immediately at their equilibrium frequency.
• Therefore when two loci are considered together the genotype frequencies will reach equilibrium after several generations of random mating.
• If three loci are considered together, then the number of generations required to reach equilibrium genotype frequencies will be more than that required for two loci considered jointly.

Linked loci

• Under random mating, loci that are linked approach equilibrium more slowly than do loci segregating independently.
• Further more closely the linkage the slower the approach to equilibrium. When equilibrium is reached coupling and repulsion phases are equally frequent:
  • Coupling heterozygotes:
  • AB / ab Repulsion heterozygotes: Ab / aB