bio cycle 6 families to populations

what is heterozygote advantage (overdominance)?

when individuals with 2 different alleles (heterozygotes) have higher fitness than either homozygote

what is heterozygote disadvantage (underdominance)?

when heterozygotes have lower fitness than either homozygote

what is required for either heterozygote advantage or disadvantage to occur?

heterozygotes must have a different phenotype than either homozygote, it can’t occur if one allele is completely dominant and the other completely recessive

how do heterozygote advantage and disadvantage affect genetic variation?

• advantage: maintains/increases variation (rare alleles increase)

• disadvantage: reduces variation (rare alleles decrease)

whey are rare alleles mostly found in heterozygotes?

because when an allele is rare, it is unlikely to meet another copy of itself to form homozygotes

how does the starting allele frequency influence heterozygote advantage or disadvantage?

• for advantage, allele frequencies will move toward equilbrium regardless of starting point

• for disadvantage, outcome depeds heavily on starting frequencies (can lead to fixation or loss of an allele)

what is absolute fitness (W)?

a measurable value of reproductive success (ex: number of eggs, lifespan, grand offspring)

what is relative fitness (w)?

the ratio of a genotype’s absolute fitness to the highest absolute fitness in the population

• w = W/Wmax

what is the relative fitness of the most successful genotype?

1

relative fitness example: if WA1A1 = 20, WA1A2 = 15, WA2A2 = 12, what are the relative fitnesses?

• WA1A1 = 1

• WA1A2 = 0.75

• WA2A2 = 0.6

what is genetic drift?

the change in allele frequency in a population due to random chance (sampling errors), not natural selection 

in what population is genetic drift most powerful?

small populations, though it occurs in all finite populations

is drift restricted to bottlenecks (sharp reduction in size of population due to environmental/human activity) and founder events (loss of genetic variation occuring when new population is established by a very small number of individuals from a larger population)?

no, drift always occurs when population is finite

what happens to allele frequencies under drift?

they change unpredictably each generation, overtime, variation decreases and one allele may become fixed with the rare allele often disappearing from the gene pool 

what is assortative mating?

“Like with like” individuals mate with those who have similar phenotypes for a trait

what is disassortative mating?

“Opposites attract” individuals mate with those who have different phenotypes

what is inbreeding?

genome-wide assortative mating, mating between relatives

what is inbreeding avoidance?

genome-wide disassortative mating, avoiding mating with close relatives

does non-random mating (assortative, disassortative, inbreeding) cause evolution?

• if fitness is equal: no, allele frequencies don’t change, but genotype frequencies do 

• if fitness differs: yes, can lead to evolution

does inbreeding cause evolution?

no, it changes genotype frequencies, not allele frequencies

why are genetic disorders more common in inbred populations?

inbreeding increases the frequency of recessive homozygotes that express harmful alleles

what is the difference between qualitative and quantitative traits?

• qualitative: few distinct phenotypes; often controlled by one/few genes

• quantitative: continous variation; controlled by manyg genes (polygenic) + environment

give examples of qualitative traits

pea shape (round/wrinkled), flower colour, pig spotting pattern

give examples of quantitative traits

height, weight, milk yield, blood pressure, intelligence 

what are polygenic traits?

traits controlled by many genes, often with environmental influence

how does environment affect quantitative traits?

it can cause the same genotype to produce different phenotypes (phenotypic plasticity)

example: how can 2 birds with different genotypes have the same tail length?

environmental effect (like poor nutrition) can offset genetic potential for longer tails

what is directional selection?

favours one extreme phenotype → shifts the mean toward that extreme

what is stablizing selection?

favours the average phenotype → reduces variation

what is disruptive selection?

favours both extremes over the mean → increases variation

what does heritability (H²) measure?

the fraction of phenotypic variation (Vp) due to genetic variation (Vg)

• H² = Vg / Vp = Vg / Vg + Ve (variation due to environment)

what does a heritability of 1 mean?

all variation is genetic (no environmental influence)

what does a heritability of 0 mean?

all variation is environmental

does high heritability mean a trait is inherited?

not necessarily, it refers to variation within a population, not whether the trait has a genetic basis

why is heritability population specific?

it depends on both genetic variation and environmental uniformity

how can equality in society affect heritability?

more equal environments → lower Ve → higher heritability

if a trait has high heritabilty, what does that suggest about selection potential?

the trait can respond quickly to selection, since genetic vairation drives differences

what does low heritaility suggest?

the trait is mostly influenced by the environment, not genes

if two populations have high within-population heritability but different mean phenotypes, what can we infer?

we cannot assume the difference is genetic, environment may differ between populations

how do you calculate allele frequencies from genotype frequencies?

• p = f(A1A1) + 0.5f(A1A2)

• q = f(A2A2) + 0.5(A1A2)

what are the expected genotype frequencies under HWE?

p², 2pq, q²

what does it mean if a population is in HWE?

no evolution is occuring at that locus (allele frequencies stable)

what does it mean if a population is not in HWE?

at least one evolutionary agent (selection, drift, mutation, migration, or non-random mating) is acting 

which agents of evolution increase, maintain, or decrease variation?

• increase: mutation, gene flow

• maintain: balancing selection, heterozygote advantage

• decrease: drift, directional selection, heterozygote disadvantage