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Population Genetics. Studying the Distribution of Alleles and Genotypes in a Population. Human polymorphism for ABO blood type. Evolution. Change in characteristics of organisms Change in genetic composition (frequency of alleles) in a population

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Population genetics

Population Genetics

Studying the Distribution of Alleles and Genotypes in a Population



Evolution
Evolution

  • Change in characteristics of organisms

  • Change in genetic composition (frequency of alleles) in a population

  • Population = group of organisms of same species occupying a particular geographic region

  • To study evolution: measure changes in allele frequencies from equilibrium values



Example using hardy weinberg equilibrium
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 0.09.

  • What is the frequency of the allele for albinism?

  • What is the frequency of the allele for skin pigmentation?

  • What is the frequency of individuals with skin pigmentation?

  • What is the frequency of individuals who are homozygous for the skin pigmentation allele?

  • What is the frequency of individuals who are heterozygous for the albino allele?


.7A .3a

.49AA

.21Aa

.7A

.3a

.09aa

.21Aa


Genotypic Frequencies as Related to Allele Frequencies

for Populations in Hardy-Weinberg Equilibrium


Hardy weinberg equilibrium applied to sex linked genes
Hardy-Weinberg Equilibrium Applied to Sex-linked Genes

  • Problem 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?

  • Same frequencies of alleles in both genders


Solution to Problem 2a, page 4-1

.95XB .05Xb

.9025 XBXB

.0475 XBXb

.95XB

.05Xb

.0025XbXb

.0475 XBXb


Hardy weinberg equilibrium applied to sex linked genes1
Hardy-Weinberg Equilibrium Applied to Sex-linked Genes

  • Problem 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?

  • Genders do not have the same allele frequencies


Solution to Problem 2b, page 4-1

Considering only female offspring:

P(colorblind female) = P(Xb) from female x P(Xb) from male

P(colorblind female) = 0.1 x 0.7 = 0.07

.9XB .1Xb

.27XBXB

.3XB

.7Xb

.03 XBXb

.63 XBXb

.07 XbXb

Considering only male offspring: P(colorblind male) = P(Xb from female) = 0.1

Y

.9 XBY

.9XB

.1Xb

.1 XbY


Perturbations of hardy weinberg equilibrium
Perturbations of Hardy-Weinberg Equilibrium


Causes of genetic drift
Causes of Genetic Drift

  • Population is reduced in size due to limitations in resources

  • Founder Effect: small group of individuals starts a new population

  • Population Bottleneck: population size decreased by chance


Perturbations of hardy weinberg equilibrium1
Perturbations of Hardy-Weinberg Equilibrium


Inbreeding
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.

  • Homozygosity by descent is calculated as (1/2)n where n=number of ancestors in the common pathway.


A1/A2

A1/A2

p=1/2

p=1/2

or A2/A3

Matching alleleA1 or A2

Matching allele A3 or A4

A2/A3 or A2/A4

p=1/2

p=1/2

p=1/2

or A2/A2

orA3/A3

A4/A4

From male parent

½ x ½ x ½ = 1/8

From female parent

½ x ½ x ½ = 1/8

P(homozygosity by descent) = 1/8 + 1/8 = 1/4

or (1/2)3 + (1/2)3 = 1/4


Additional problems involving inbreeding
Additional Problems Involving Inbreeding

  • Problem 3, page 4-1

  • What is the probability of homozygosity by descent of the offspring of a

  • parent child - mating?

  • aunt-nephew or uncle-niece mating?

  • first cousin mating?


Solutions to Inbreeding Problems

  • Problem 3a, page 4-1: 1/4

  • Problem 3b, page 4-1: 1/8

  • Problem 3c, page 4-1: 1/16


Perturbations of hardy weinberg equilibrium2
Perturbations of Hardy-Weinberg Equilibrium


Perturbations of hardy weinberg equilibrium3
Perturbations of Hardy-Weinberg Equilibrium


Hardy weinberg equilibrium applied to migration
Hardy-Weinberg Equilibrium Applied to Migration

  • Problem 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.


Solution to Problem 2c, page 4-1

Assume males represent half of each population.

0.2 x 500 = 100 colorblind males migrate

0.3 x 500 = 150 colorblind males in receiving population

q = 100 + 150/1000 = 0.25

F(XbY) = q = 0.25

F(XbXb) = q2 = 0.0625


Perturbations of hardy weinberg equilibrium4
Perturbations of Hardy-Weinberg Equilibrium


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