Overview Forces responsible for genetic variation mutation de novo mutation rates constant among populations intrinsic error rate in DNA polymerase founder effect if one member of a small community carries a triat, as the population expands there will be a higher frequency of that trait in the new community than there is in the general population Ex.) Pennsylvania Amish and Ellis-van Creveld syndrome genetic drift a dramatic change in allele frequency based on chance small populations are more vulnerable to genetic drift natural selection ↑ in allelic frequency that ↑ species fitness ↓ in allelic frequency that ↓ species fitness some genes ↑ species fitness as heterozygote but ↓ species fitness as a homozygote ex.) sickle cell trait lowers malarial infections, while sickle cell anemia is detrimental bottleneck Even when fitness is equal for all phenotypes, a population bottleneck can result in disrupted allelic frequencies or loss of a genotype all together by chance gene flow transfer of alleles from one population to another Hardy-Weinberg equilibrium states that genotype and allele frequencies remain constant through generations disease prevalence equation p2+ 2pq + q2 = 1 where p = frequency of allele A where q = frequency of allele B p2 = frequency of homozygous individuals for allele A q2 = frequency of homozygous individuals for allele B 2pq = frequency of heterozygotes requirements for validity large population random mating the genotypic frequencies of the population will remain stable from generation to generation assumptions no mutation no selection for any of the genotypes at the locus no migration other notes prevalence of an X-linked recessive disease in males = q prevalence of an X-linked recessive disease in females = q2 possible to assume in most cases that p = 1 as the wild-type allele is approximately 1