Lecture 25

Variation and Selection in Population

Population Genetics

extension on Mendelian principles; tool to learn about biological function, evolutionary mechanisms, and human history

Population

individuals in a species that occupy the same region and can interbreed; same time and place

Gene Pool

total of alleles carried in all members of a population

Sample

number of individuals used to make inferences about a population

Phenotype Frequency

proportion of individuals in a population that a specific phenotype

Genotype Frequency

proportion of individuals in a population that carry a specific genotype

Allele Frequency

proportion of gene copies in a population that a specific allele

Total # of Gene Copies

2x # of individuals

Hardy Weinberg Assumptions

infinite # of individuals, random mating, no new mutations, no migration in/out, genotypes have no effect on ability to survive and transmit alleles to next generation

Equal segregation and random mating have two important consequences

allele frequencies should be same in adults and gametes + allele frequencies in gametes can be used to calculate expected genotype frequencies in zygotes of next generation

Hardy Weinberg Formula

p2 + 2pq + q2 = 1; p = dominant allele frequency and q = recessive allele frequency

Fitness

ability to survive to reproductive age and transmit genes to next generation

Relationship between a and A frequency

when a is low → AA frequency dominates; when a is high → aa frequency dominates; when a and A are equal → Aa frequency dominates

In populations not at HWE

one generation of random mating can reshuffle alleles INTO equilibrium → allele frequencies remain constant and genotype frequency changes

Sex Linked Genes

take several generations to reach HWE if allele frequencies are not originally in equilibrium

Males → 1 X while females → 2 X

allele frequencies in males in each successive generation = allele frequency in females of the generation before because males get X from mother

The allele frequencies of females of each successive generation

equal to the average frequency in males and females of the prior generation because females receive one X from each parent

Many human loci

are near HWE → humans don’t select mates based on specific genotype; many loci do not affect phenotype and are usually used in solving crimes and identifying remains

CODIS

Combined DNA Indexing System; loci are simple sequence repeat loci, 20 loci are unlinked and highly variable

Match Probability

find the p^2, 2pq, or q^2 based on whether the individual is homo or hetero, then multiply these numbers together

DNA Fingerprinting

analyzes individuals based on occurrence of repetitive sequences in their genome → creates bands that have a unique size and order for each individual

DNA Fingerprinting Properties

done using PCR to amplify microsatellites; used to identify suspects, paternity