cycle 6 bio families to populations

what is heterozygote advantage (over dominance)?

when heterozygous individuals (A1A2) have higher fitness than either homozyogus genotypes (A1A1 or A2A2)

what phenotypic requirement must be true for heterozygote advantage to occur?

the heterozygote must have a different phenotype than either homozygoye (not complete dominance)

how does heterozygote advantage affect allele frequency?

rare alleles increase in frequency

how does heterozygote advantage affect genetic variation?

it maintains genetic variation; both alleles persist in the population

what is heterozygote disadvantage (underdominance)?

when heterozygotes have lower fitness than either homozygote

how does heterozygote disadvantage affect allele frequencies?

rare alleles decrease in frequency; popoulation tends to become homozygous

how does heterozygote disadvantage affect genetic variation?

it reduces genetic variation; one allele tends to be lost

why are rare alleles mostly found in heterozygotes?

because when an allele is rare, it rarely meet anothers copy of itself to form homozygotes

why can either heterozygote advantage nor disadvantage occur if complete dominance (a dominant allele completely masks the effect of a recessive allele in a heterozygous genotype) exists?

because heterozygotes (A1A2) would have the same phenotype as one homozygote, preventing any fitness difference  

what is absolute fitness (W)?

a measurable quantity such as number of eggs, offspring, lifespan, etc

• ex: W(A1A1) = 20 eggs, W(A1A2) = 15 eggs, W(A2A2) = 12 eggs

what is relative fitness (w)?

absolute fitness divided by the absolute fitness of the most successful genotype, the relative fitness of the most fit genotype is 1

• equation: w = W/Wmax

what is genetic drift?

random sampling error in allele frequency from one generation to the next

does genetic drift only occur during bottlenecks or founder effects?

no, it occurs in every finite population, all the time

in what population size is drift strongest?

small populations

over many generations, what does drift usually do?

removes genetic variation by eliminating rare alleles (usually)

if f(A1) decreases and f(A2) increases due to drift in one generation, what will happen next generation?

frequencies will change again, but unpredictably

what is assortive mating?

• “like with like”

• individuals mate with those having similar phenotypes

what is diassortative mating?

• “opposites attract”

• individuals mate with those having different phenotypes

what is inbreeding?

genome-wide assortative mating, individuals mate with close relatives

what is inbreeding avoidance?

genome-wide dissassortative mating, avoid mating with close relatives

does non-random mating (alone) cause evolution?

only affects genotype frequencies but not allele frequencies, so by itself it dos not cause evolution

why do genetic disorders appear more often in inbred populations?

because harmful alleles are recessive; inbreeding increases frequency of homozygous recessives

what are qualitative traits?

traits with discrete categories (ex: tall/drawf, round/wrinked peas)

what are quantitative traits?

traits with continuous variation (height, weight, blood pressure)

what controls quantitative traits?

many genes (polygenic) + environment effects

give an example of polygenic inheritance with tail length?

3 loci (A, B, C), each with 2 alleles, each "2” allele adds +1cm to base tail length at 10 cm

• 27 possible genotypes → formula n(n+1)/2 n is number of alleles

• 7 possible phenotypes

how does environment affect phenotype?

same genotype can produce different phenotypes depending on environmental conditions (nutrition, stress, etc)

what is the effect of environment on genotype-phenotype correlation?

it reduces the correlation; phenotype becomes less predictable from genotype

what is directional selection?

selection favours one extreme → mean shifts toward that extreme

what is stablizing selection?

selection favours intermediate phenotype → mean stays the same; variation decreases

what is disruptive selection?

selection favours both extremes → mean unchanged; variation increases

what is heritability?

H² = Vg/VP = fraction of pehnotypic variance due to genetic variance

what is phenotypic variance?

Vp =Vg + Ve (genetic + environmental variance)

what does H² = 1 mean?

variation is enitrely due to genetics

what does H² = 1 mean?

variation is entirely due to environment

why is heritability often misunderstood?

it applies to populations, not individuals

example of misunderstood heritability: height

heritability of height approximately 0.8 does no mean genes contribute to exactlu 80% of someone’s height, it means that 80% of the differences in height observed across a population are attributed to genetic differences among those people

why is finger number heritability low?

because deviations from 10 fingers are due to environmental accidents, not genetic variation

how do environment and inequality affect heritability?

• uniform environments → higher heritability

• different environments → lower heritability

if a trait is highly heritable in both populations A and B, does their difference in mean phenotype prove genetic differences?

no, high heritability within each population does not explain differences between populations; environmental differences may still cause population-level differences

what is allele frequency?

proportion of each allele in the gene pool

what is genotype frequency?

proportion of each genotype in the population

what is a gene pool?

all alleles at all loci in all individuals in a population

formula for expected Hardy-Weinberg Equilibrium (HWE) genotype frequencies?

• p² + 2pq + q² = 1

• p = frequency of dominant allele

• q = frequency of recessive allele

• p² = frequency of the homozygous dominant genotype

• 2pq = frequency of the heterozygous genotype

• q² = frequency of the homozygous recessive genotype

• these genotype frequencies must add up to 1 for a population in equilibrium

what does it mean if observed frequencies match HWE expectations?

no evolutionary forces acting on that locus; random mating for that locus

what does it mean if population is not in HWE?

at least one HWE assumption violated (selection, drift, gene flow, mutation, non-random mating)