Bio 1B: Lec 4: Hardy-Weinberg as a null model

What does it mean that Hardy-Weinberg is a null model?

It means that if smth were in HW equilibrium, nothing is happening evolutionarily

Why do we use a null model?

It tells us if this situation is interesting or not

If it fits the null model, nothing interesting is going on, so we don’t care

Basics of HW equilibrium

If observed genotypes differ from expected, it is not in HW equilibrium

HW equilibrium equation

p² + 2pq + q² = 1

p + q = 1

How to calculate p and q from given genotype frequencies?

PP, PQ, QQ

PP = p² and PQ = 2pq

to get p, do [PP + 1/2PQ]/#population

to get q, do [QQ + 1/2PQ]/ #population

Explain the malaria/sickle-cell thing

AA = normal hemoglobin, but susceptible to malaria

AS = carrier of sickle-cell disease so some mild anemia, protected from malaria

SS = severe sickle-cell disease, dies young

HW assumptions & what it means if it is violated

Random mating: non-random mating/inbreeding/assortative mating

No natural selection: differential survival or reproduction

No mutation: new alleles entering the gene pool

Extremely large population: genetic drift/sampling error

No gene flow: migration/gamete movement

What is HW good and no good for?

Good: predict genotypic frequencies from allele frequencies, detect a mismatch between expected and observed genotypes, frame testable hypotheses about mechanisms

Not good: identify the EXACT mechanism causing every deviation, PROVE natural selection without biological evidence, REPLACE sampling/statistical analysis in real studies