Lecture 30

Question 1

What is Hardy-Weinberg equilibrium?

Answer 1

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

Question 2

What is the Hardy-Weinberg equation for total number of alleles?

Answer 2

p + q = 1, where p is the normal allele frequency and q is the mutant allele frequency

Question 3

What are the conditions required for Hardy-Weinberg equilibrium to hold true?

Answer 3

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

Question 4

What is the Hardy-Weinberg equilibrium equation?

Answer 4

p2 + 2pq + q2 = 1

Question 5

For what do we use the Hardy-Weinberg relationship?

Answer 5

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)

Question 6

For an X-linked disease, what Hardy-Weinberg assumptions can be made?

Answer 6

1) Disease incidence in males directly estimates q (only one X chromosome)
2) Disease incidence in females directly estimates q2 (two X chromosomes)

Question 7

What Hardy-Weinberg estimate can be made in autosomal dominant diseases?

Answer 7

2pq is approximately equal to 2q

Question 8

What is genetic drift?

Answer 8

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

Question 9

What is stratification?

Answer 9

1) Religious or social barriers within or between populations
2) Geographical barriers in the world wide population

Question 10

What is assortative mating?

Answer 10

Choosing a mate because of shared specific traits

Question 11

What is consanguineous mating?

Answer 11

Mating with a person being descended from the same ancestor as you

Question 12

Why does Hardy Weinberg equilibrium depend on the absence of selection?

Answer 12

Selection distorts allelic frequencies

Question 13

What is the heterozygote advantage?

Answer 13

Mutations that confer a selective advantage to heterozygotes may spread despite major disadvantages to homozygotes

Question 14

What is the Founder Effect?

Answer 14

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

Question 15

What is an example of the Founder Effect in Venezuela?

Answer 15

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.

Question 16

Most mutant alleles are found in ______ (homozygotes; heterozygotes)

Answer 16

Heterozygotes

Question 17

In autosomal recessive disease, what happens when affected homozygotes do not survive?

Answer 17

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

Question 18

In autosomal dominant disease, what happens when there is selection against disease?

Answer 18

In dominant disease, frequency is due primarily to heterozygotes. Homozygote frequencies are still small. Selection against disease keep disease allele frequencies low

Question 19

What Hardy-Weinberg estimate can be made in X-linked recessive diseases in males?

Answer 19

Since males only have one X chromosome, their q is a direct measure of homozygous recessive