Population Genetics Studying the Distribution of Alleles and Genotypes in a Population

Human polymorphism for ABO blood type

Evolution Change in characteristics of organismsChange in characteristics of organisms Change in genetic composition (frequency of alleles) in a populationChange in genetic composition (frequency of alleles) in a population Population = group of organisms of same species occupying a particular geographic regionPopulation = group of organisms of same species occupying a particular geographic region To study evolution: measure changes in allele frequencies from equilibrium valuesTo study evolution: measure changes in allele frequencies from equilibrium values

Hardy-Weinberg Equilibrium Allelic Frequencies p + q = 1 p = frequency of A q = frequency of a Genotypic Frequencies p 2 + 2pq + q 2 = 1 p 2 = f(AA), q 2 = f(aa) 2pq = f(Aa) Assumptions Large population size Random mating No selection No migration No mutation

Example Using Hardy-Weinberg Equilibrium Problem 1, page 4-1 Albinism, a lack of skin pigmentation, is caused by the autosomal recessive allele a. The dominant allele A causes skin pigmentation. For a particular population, the frequency of albinos is Albinism, a lack of skin pigmentation, is caused by the autosomal recessive allele a. The dominant allele A causes skin pigmentation. For a particular population, the frequency of albinos is a.What is the frequency of the allele for albinism? b.What is the frequency of the allele for skin pigmentation? c.What is the frequency of individuals with skin pigmentation? d.What is the frequency of individuals who are homozygous for the skin pigmentation allele? e.What is the frequency of individuals who are heterozygous for the albino allele?

Genotypic Frequencies as Related to Allele Frequencies for Populations in Hardy-Weinberg Equilibrium

Hardy-Weinberg Equilibrium Applied to Sex-linked Genes Problem 2a, page 4-1Problem 2a, page 4-1 Colorblindness is an X-linked recessive allele. In a particular population in Hardy- Weinberg Equilibrium, 1 in 20 males are colorblind. What is the frequency of colorblind females in this population?Colorblindness is an X-linked recessive allele. In a particular population in Hardy- Weinberg Equilibrium, 1 in 20 males are colorblind. What is the frequency of colorblind females in this population? Same frequencies of alleles in both gendersSame frequencies of alleles in both genders

Solution to Problem 2a, page 4-1

Hardy-Weinberg Equilibrium Applied to Sex-linked Genes Problem 2b, page 4-1Problem 2b, page 4-1 In a population that is not in Hardy-Weinberg Equilibrium, the frequency of the colorblind allele is 0.1 in females and 0.7 in males. Considering random mating to produce the next generation, what proportion of male offspring will be colorblind? What proportion of the female offspring will be colorblind?In a population that is not in Hardy-Weinberg Equilibrium, the frequency of the colorblind allele is 0.1 in females and 0.7 in males. Considering random mating to produce the next generation, what proportion of male offspring will be colorblind? What proportion of the female offspring will be colorblind? Genders do not have the same allele frequenciesGenders do not have the same allele frequencies FemalesMales q p0.90.3

Solution to Problem 2b, page 4-1 Considering only female offspring: P(colorblind female) = Considering only male offspring: P(colorblind male) =

Perturbations of Hardy-Weinberg Equilibrium Genetic Drift Changes in allele frequency due to small population sizes

Causes of Genetic Drift 1.Population is reduced in size due to limitations in resources 2.Founder Effect: small group of individuals starts a new population 3.Population Bottleneck: population size decreased by chance

Perturbations of Hardy-Weinberg Equilibrium Non-Random Mating Alters the genotypic frequencies of a population

Inbreeding Mating between close relatives is called inbreeding.Mating between close relatives is called inbreeding. Inbreeding increases the probability of homozygosity by descent: becoming homozygous by inheriting two copies of the same allele from one ancestor.Inbreeding increases the probability of homozygosity by descent: becoming homozygous by inheriting two copies of the same allele from one ancestor. Homozygosity by descent is calculated as (1/2) n where n=number of ancestors in the common pathway.Homozygosity by descent is calculated as (1/2) n where n=number of ancestors in the common pathway.

From male parent ½ x ½ x ½ = 1/8 ½ x ½ x ½ = 1/8 From female parent ½ x ½ x ½ = 1/8 p=1/2 P(homozygosity by descent) = 1/8 + 1/8 = 1/4 or (1/2) 3 + (1/2) 3 = 1/4 or A 2 /A 3 or A 2 /A 2 A1/A2A1/A2 Matching allele A 1 or A 2 p=1/2 A 4 /A 4 Matching allele A 3 or A 4 p=1/2 or A 3 /A 3 A 2 /A 3 or A 2 /A 4 A1/A2A1/A2

Additional Problems Involving Inbreeding Problem 3, page 4-1Problem 3, page 4-1 What is the probability of homozygosity by descent of the offspring of aWhat is the probability of homozygosity by descent of the offspring of a a.parent child - mating? b.aunt-nephew or uncle-niece mating? c.first cousin mating?

Solutions to Inbreeding Problems Problem 3a, page 4-1:Problem 3a, page 4-1: Problem 3b, page 4-1:Problem 3b, page 4-1: Problem 3c, page 4-1:Problem 3c, page 4-1:

Perturbations of Hardy-Weinberg Equilibrium Natural Selection Increases the frequency of genotypes with higher fitness

Perturbations of Hardy-Weinberg Equilibrium Migration Can add new alleles, remove alleles or change allele frequency

Hardy-Weinberg Equilibrium Applied to Migration Problem 2c, page 4-1Problem 2c, page 4-1 Twenty percent of the males in a certain population are colorblind. A representative group of 1000 migrates to a small, isolated island where there are already 1000 individuals where 30% of the males are colorblind. What is the frequency of colorblind males and females in the new mixed population immediately after immigration? Assume both populations are in Hardy- Weinberg Equilibrium at all times.Twenty percent of the males in a certain population are colorblind. A representative group of 1000 migrates to a small, isolated island where there are already 1000 individuals where 30% of the males are colorblind. What is the frequency of colorblind males and females in the new mixed population immediately after immigration? Assume both populations are in Hardy- Weinberg Equilibrium at all times.

Solution to Problem 2c, page 4-1 q = F(X b Y) = F(X b X b ) =

Perturbations of Hardy-Weinberg Equilibrium Mutation Alters allele frequency, causes formation of new genotypes