21 Intro to Population Genetics

Question 1

What is a locus?

Answer 1

The physical position of a gene or marker along a chromosome

Question 2

What is a polymorphism?

Answer 2

A gene or phenotype with more than one form

Question 3

What does population genetics study?

Answer 3

The frequencies of alleles in a population

Question 4

What forces determine the genetic composition of a population?

Answer 4

Natural selection
Mating system
Migration
Mutation
Random genetic drift

Question 5

What do p and q represent?

Answer 5

The frequency of the different alleles in the population
p+q=1
Always true when only two alleles

Question 6

How are allele frequencies estimated?

Answer 6

1) Calculate total number of alleles in the population (no in pop x2)
2) Calculate how many of each allele there is
3) To find frequency of p, divide the number of this allele by total number of alleles
4) q = 1 - p

Question 7

What is the Hardy-Weinberg equilibrium?

Answer 7

Allele frequencies in one generation can be used to predict genotype frequencies in the next generation

Frequency of A1A1 = p^2
Frequency of A1A2 = 2pq
Frequency of A2A2 = q^2
p^2 + 2pq + q^2 = 1

In a random mating population, genotype frequencies
p^2 : 2pq : q^2

Question 8

What are the Hardy-Weinberg assumptions?

Answer 8

• Random mating
• No natural selection
• Large population size
• No migration
• No mutation

Question 9

Why is HWE useful?

Answer 9

Provided description of how genetic variation is maintained
- Frequencies constant over time
- Shows no blending inheritance

Departures from HWE mean some assumptions not met
- e.g. selection must be going on

Is useful in medical genetics
- Carrier frequencies

Question 10

How are departures from HWE tested?

Answer 10

1) Estimate allele frequencies
2) Calculate expected genotype frequencies under HWE (p^2, 2pq, q^2)
3) Calculate expected genotype counts in sample (observed x expected proportion)
4) Calculate chi-squared statistic

Question 11

What are the degrees of freedom (df)?

Answer 11

no. genotypes - no. parameters tested -1
e.g. 3 genotypes
1 parameter (just p, because q=1-p)
so df = 3-1-1 = 1

Question 12

When df=1 when is the null hypothesis rejected?

Answer 12

At significance level of 0.05 when x^2 > 3.84

(Null hypothesis = pop in HWE)

Question 13

Why can migration be important for population genetics?

Answer 13

Can restore genetic variation