Lecture 5: Hardy-Weinberg Equilibrium

polymorhpism

the presence of multiple forms of variants among individuals

what is mendel’s law of segregation

the two alleles at a locus segregate into different gametes during meiosis

what is mendel’s law of independent assortnment

alleles at different loci are incorporated independently into gametes during meiosis

what is the difference between mendelian genetics and population genetics

mendelian genetics tracks genotypes within families, while population genetics relates allele frequencies to genotype frequencies in populationsw

hat is allele frequency

the proportion of individuls with a particular genotype in a population

what is genotype frequency

the proportion of individuals with a particular genotype in a population

for a two-allele locus, what do p and q represent

p is the frequency of allele A and q is the frequency of allele a

what is the relationship between p and q

p+q = 1

how are allele frequencies calculated from genotype frequincies

p = AA + ½ Aa and q = ½ Aa

what is the hardy-weinberg equilibrium (HWE)

a population-genetic model in which genotype frequencies are determined by allele frequencies under specific assumptions

what is the hardy-weinberg eq

p² + 2pq + q² = 1

what does p² represent

expected frequency of genotype AA

what does 2pq represent

expected frequency of genotype Aa

what does q² represent

expected frequency of genotype aa

what happens to genotype frequencies under random mating

genotypes frequencies depend only on allele frequencies

what are the assumptions of the HWE

random mating, no natural selection, no migration, no mutation, and non-overlapping generations

what additional assumption about mating is often emphasized for HWE

no inbreeding; the population is panmictic/random mating

what is the wahlund effect

a deficiency of heterozygotes that occurs when genetically distinct populations are sampled together and treated as one population

why does the whlund effect occur

Combining genetically differentiated populations produces fewer heterozygotes than expected from treating the pooled sample as one randomly mating population

what is the practical application of HWE to mixed-stock fisheries

HWE can help determine whether a fishery sample contains more than one genetic population

how can you test whether a population conforms to HWE expectations

use a chi-square (x²) testw

what is the null hypothesis when testing HWE

the population is not in HWEw

what degrees of freedom are typically used in an HWE chi-squared calculation

number of genotypes − number of alleles; usually df = 1

what is the alternative hypothesis when testing HWE

The population is not in Hardy-Weinberg equilibrium

what type of data should be used in the

observed numbers, not ratios or proportions

what does a significant x² result indicate in an HWE test

the observed genotype frequencies differ significantly from HWE expectations

how can inbreeding affect HWE

it produces an excess of homozygotes and a deficiency of heterozygotes

what is assortative mating

non-random mating in which individuals preferentially mate based on particular characteristics

how can directional selection affect allele frequencies

it can increase the frequency of a particular allele

what is balancing selection

selection that maintains multiple alleles in a population and can produce an excess of heterozygotes

how does mutation affect HWE

mutation introduces variation but usually has negligible effects on genotype proportions unless selection acts on the new mutation

how does migration affect HWE

migration introduces alleles from another population and may cause an excess of heterozygotes

what is admixture

the movement of alleles into a population from an adjacent or previously separate populationw

hat does it mean for HWE to be robust

mild violations of its assumptions amy still produce approx. HW genotype proportions

what is the relatinship between observed and expected heterozygosity under HWE

under HWE, observed and expected heterozygosity should be approximately equal

what does Ho ≠ He indicate

at least one assumption of HWE may have been violated