Quantitative Genetics Flashcards
H-W assumptions
- random mating
- no selection-all genotypes equally viable and no selection for or against a phenotype
- no new mutations
- population is infinitely large
- no migration
assortative mating
choosing a mate who has the same (positive) or opposite (negative) trait or genotype
-ex: individuals with deafness or achondroplasia
population stratification
presence of a systematic difference in allele frequencies within population subgroups related to the fact that matings across divisions are more common than matings within divisions (ex: African Americans and US Caucasians)
founder mutations
increased allele frequency or disease allele in a population related to the fact that it is more common to inter-marry within the population (ex: BRCA1/2 in AJ, alpha thal in SE Asians and Mediterranean pops)
consanguinity
shared common ancestry predisposes individuals to sharing more alleles that are identical-by-descent, than would be expected in a randomly mating population
-increased risk for AR conditions
coefficient of inbreeding (F)
probability that a person who is homozygous at a particular locus inherited both from a common ancestor; proportion of loci at which an individual is IBD
coefficient of relationship (r)
measure of the degree of consanguinity; amount of shared DNA
consanguinity between third cousins
degree of relationship at this point or further apart is not considered genetically significant
non-random mating
tends to increase the proportion of homozygotes and decrease the proportion of heterozygotes
fitness (f)
probability of an individual to transmit their genes to the next generation compared to the average probability for the population
coefficient of selection (s)
-measurement of the loss of fitness
=1-f
mutation rate of the gene (µ)
equal to the selection against the allele (s) and (multiplied by) its frequency in the population (q)
f=0
complete selection against an allele and all affected individuals result from new mutation (ex: thanatophoric dysplasia)
f=1
no deleterious effect on reproduction for carriers of the allele; means s=0 and there is no selection against it and nearly all affected individuals inherit allele from parent (ex: HD)
0
proportion of affected individuals come from carrier parents, while others result from new mutation
selection in AR conditions
less effective because it takes many generations to decrease the mutant frequency and fitness is normal due to unaffected carriers
-medical treatments do no effect frequencies significantly
heterozygote advantage
example of balancing selection that results in a relatively high allele frequency due to selective forces acting both for and against an allele
-ex: sickle trait in malaria protection
heterozygote disadvantage
allele reduces the reproductive fitness of a carrier, but is still maintained in the population because it does not completely inhibit the production of offspring
-ex: pericentric inversion carriers have more risk for miscarriage, Rh-negative mothers predispose Rh-positive fetus to HDN
genetic drift
pool of gametes formed for next generation represents a random sample of alleles from the population
- frequency in variations are due to random sampling
- effects in smaller populations can be powerful, in large populations usually negligible
founder effects/bottleneck
genetic isolation/genetic drift occurs either by separation of a few individuals or shrinking of population, which can lead to a higher incidence of an allele or condition
-ex: EVC in Penn Dutch, HD in Venezuelan pop, tyrosinemia in FC
gene flow or population admixture
non-random, slow diffusion of an allele across a reproductive (either geographical or ethnic) barrier
-occurs due to migration
