Flashcards in Lecture 30 Deck (19)
What is Hardy-Weinberg equilibrium?
1) This equilibrium describes how Mendelian inheritance leads to the stable maintenance of alleles in a particular population from generation to generation
2) It also predicts, for a given allele frequency, the equilibrium proportions of individuals with particular genotypes
What is the Hardy-Weinberg equation for total number of alleles?
p + q = 1, where p is the normal allele frequency and q is the mutant allele frequency
What are the conditions required for Hardy-Weinberg equilibrium to hold true?
1) Very large population size (about 10,000+ people) - small population allows genetic drift
2) Random mating - geographical isolation, stratification, assortative mating can distort random mating
3) No selection either for or against particular genotypes - heterozygous advantage selects against homozygous recessive
4) No net migration into or out of the population - founder effect can have a grand influence on a population
5) No introduction of new alleles by mutation - founder effect can have a grand influence on a population
What is the Hardy-Weinberg equilibrium equation?
p2 + 2pq + q2 = 1
For what do we use the Hardy-Weinberg relationship?
Disease allele frequencies in a population are used to assess risk for individual carrier status when the presence of a disease allele is unknown (for example, when genetic testing for disease alleles is not available)
For an X-linked disease, what Hardy-Weinberg assumptions can be made?
1) Disease incidence in males directly estimates q (only one X chromosome)
2) Disease incidence in females directly estimates q2 (two X chromosomes)
What Hardy-Weinberg estimate can be made in autosomal dominant diseases?
2pq is approximately equal to 2q
What is genetic drift?
1) In small populations, chance sampling of gametes make it likely that allele frequencies will change every generation
2) New DNA alleles may be "fixed" or "lost" in the DNA by chance (drift) in these small isolated mating populations
What is stratification?
1) Religious or social barriers within or between populations
2) Geographical barriers in the world wide population
What is assortative mating?
Choosing a mate because of shared specific traits
What is consanguineous mating?
Mating with a person being descended from the same ancestor as you
Why does Hardy Weinberg equilibrium depend on the absence of selection?
Selection distorts allelic frequencies
What is the heterozygote advantage?
Mutations that confer a selective advantage to heterozygotes may spread despite major disadvantages to homozygotes
What is the Founder Effect?
In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population
What is an example of the Founder Effect in Venezuela?
1) Huntington's Disease is an autosomal dominant disease with a worldwide disease incidence of 1/25,000
2) In tiny, fishing villages around Lake Maracaibo, Venezuela, up to 40% of the population is at risk for Huntington's disease
3) All affected people are descendants of a Portuguese sailor who married a local woman in the 19th Century. They had 20 children.
Most mutant alleles are found in ______ (homozygotes; heterozygotes)
In autosomal recessive disease, what happens when affected homozygotes do not survive?
In recessive disease, the frequency of homozygotes is usually very small. Thus even when they do not survive, the effect on the allele frequency in the population is very small
In autosomal dominant disease, what happens when there is selection against disease?
In dominant disease, frequency is due primarily to heterozygotes. Homozygote frequencies are still small. Selection against disease keep disease allele frequencies low