Download Heterozygote disadvantage

Definitions
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Absolute fitness (W or λ)
Relative fitness ( w or m )
Average or mean fitness (T)
Viability selection
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Selection against a recessive phenotype.
Selection against a dominant phenotype.
The general effects of dominance.
Selection against a X-linked recessive
phenotype.
• Heterozygote disadvantage and advantage.
• The strength of natural selection
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fitness
Selection coefficients
Allele fixation
sheltering effect
Selection against a recessive phenotype
• Selection in Homozygotes
• Allele frequency changes slowing by
generation
• There is no allele fixation
in case of natural selection against recessive alleles
sheltering effect of heterozygotes:
 if q = 0.05  sheltering effect of heterozygotes equals to 19
 if q = 0. 5  sheltering effect of heterozygotes equals to 1
 if q = 0.95  sheltering effect of heterozygotes approximately equals
to 0.52
Selection against a dominant phenotype
• If there is complete dominance:
– shared by the dominant homozygotes and the heterozygotes
– If A is wild-type allele, after starting of selection:
• Natural selection slow to change allele frequency
• In first generations heterozygote frequency increases!!!
• heterozygote frequency in 2pq=0.5
• Then drops again
– There is recessive allele fixation
in case of natural selection against dominant alleles
General Dominance
• Dominance coefficient (h):
– h=1  complete recessive
– h=0  Complete dominant
– h=0.5 codominant (additive gene action)
• Additive gene action has the most rapid overall approach to
equilibrium allele frequency
Allele frequency change in different gene action manners
Heterozygote disadvantage
• natural selection acting against the heterozygote phenotype
• Also known as:
– heterozygote disadvantage
– underdominance for fitness
– disruptive selection
• Each allele with frequency of higher than 0.5 will become
fixed
• In case of p=q=0.5 allele frequency will be unchanged over
generations
• even a slight amount of genetic drift would alter allele
frequencies
• Unstable equilibrium
Heterozygote disadvantage
Unstable
equilibrium
Heterozygote advantage
• natural selection acting to increase the frequency of the heterozygous
genotype
• So called:
– Overdominance for fitness, or
– balancing selection
• Over time allele frequencies shift toward 0.5
• Because of increase in heterozygotes frequency
• Leads to polymorphic equilibrium (balanced polymorphism) in contrast to
other selection modes which lead to monomorphic equilibrium
Heterozygote advantage
Heterozygote advantage
• Allele frequencies in equilibrium:
summary
Natural selection acts to increase mean fitness