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21 terms

Population genetics

STUDY
PLAY
gene pool
sum of all alleles
allele frequency (p)
(number of a particular allele in the population)/(number of all alleles in the population)
p
frequency of allele of the dominant trait (and a short arm... but yeah, confusing much)
q
frequency of the allele of the recessive trait
genotype frequency
(number of individuals with a particular genotype in a population)/(total number of individuals in that population)
frequency of genotype AA
p^2
frequency of genotype Aa
2pq
frequency of genotype aa
q^2
Hardy-Weinberg law (equilibrium)
In the absence of mutation, selection or migration, the allele frequencies of a large, randomly-mating population do not change.
This explains why recessive traits are not bred out.
does equal allele frequency mean equal genotype frequency?
no, in actuality, allele and genotype frequencies may be different. p^2 + 2pq + q^2 = 1 gives an estimated (or "ideal") genotype frequency model
3 useful applications of the Hardy-Weinberg law
a) predicting genotype frequencies from allele frequencies
b) a model for the situation if no evolution occured
c) if a large deviancy from the Hardy-Weinberg law is observed, this can prompt you to look for factors (e.g. heterozygote advantage, assortative mating)
assortative mating
attraction to a mate with the same/similar phenotype to yourself (e.g. in the case of deaf couples)
Using the H-W law for estimating carriers of a recessive autosomal disorder
Double the square root of the disease frequency (q^2 being the disease frequency)
Factors disrupting the H-W equilibrium:
a) mutation
b) selection
c) migration
d) genetic drift/founder effect
e) non-random mating/assortative mating
Mutation
natural mutation rate is low, so this usually does not disrupt the equilibrium.
large mutations may indicate a serious environmental mutagen
migration
as individuals move from one population to another, new alleles can be added to the gene pool, or allele frequencies changed
negative/positive selection
getting rid of deleterious mutations (negative selection), or proliferation of advantageous phenotypes, e.g. carrier advantage (positive selection)
genetic drift/founder effect
population bottlenecks result in disproportionate allele frequencies/reduced genetic variation
the founder effect occurs when a small portion of a much larger population forms a new colony (another form of population bottleneck; e.g. Ashkenazi Jew population)
assortative mating
inbreeding results in an increase in homozygosity
selective forces
change human populations.
consanguinity
incest