IV) Models of Selection – continued

B) Directional Selection with Additive Alleles

Genotype AA Aa aa

Fitness 1 1 – 1/2s 1 – s

OR Fitness 1 1 – hs 1 – s

Dp =

Dp is larger than in the recessive deleterious allele case for the same values of p, q, and s

C) Rate of Change of Allele Frequencies

i. Dominant deleterious – initially very fast but slows because once rare only heterozygotes are available

1. goes to fixation of non-deleterious allele

2. few cases of this (Hodkinsens disease)

ii. Recessive deleterious – rate of change is slower

1. never goes to fixation because hidden in heterozygotes.

V) Maintenance of Polymorphisms Under Selection

A Stabilizing Selection – The most fit type is the heterozygote

i. AKA: Balancing Selection or Overdominance

Genotype AA Aa aa

Fitness 1 + s 1 1 + t

W₁₁ W₁₂ W₂₂

s and t are negative numbers

W₁₂ > W₁₁, W₂₂

AA Homozygote dies with a probability of s

aa Homozygote dies with a probability of t

ii. Probability of “death” of A allele

A allele unites with another A allele with a probability of p (i.e., the frequency) and dies with a probability of s therefore:

loss of A allele = ps

a allele unites with another a allele with a probability of q (i.e., the frequency) and dies with a probability of t therefore:

loss of a allele = qt

When these balance each other there is an EQUILIBRIUM ( – i.e., the loss of the A allele is balanced by the loss of the a allele)

ps = qt

=

C) Disruptive Selection – Heterozygote is least fit (AKA Underdominance)

Genotype AA Aa aa

Fitness 1 + s 1 1 + t

W₁₁ W₁₂ W₂₂

s and t are positive numbers

W₁₂ < W₁₁, W₂₂

=