Active Lecture PowerPoint® Presentation for

Essentials of GeneticsSeventh Edition

Klug, Cummings, Spencer, Palladino

Copyright © 2010 Pearson Education, Inc.

Chapter 23

Population and Evolutionary Genetics

Genetic compositions of groups/populations - how they change and why…

The nature of genesHow traits

are passed from one generation to the next

Population Genetics

• Study of genetic variation in groups or populations and how it changes over time.

• A population is a group of individuals with a common set of genes that lives in same geographic area and interbreeds

23.1 Genetic variation is present in most populations & species

• This variation reflects the variation in the alleles distributed among populations of a species

• A population's gene pool is all of the alleles present in that population

• Due to population dynamics, the gene pool can change over time

Copyright © 2010 Pearson Education, Inc.

Genetic variation in Lady-bird beetles

Detecting genetic variation by artificial selection

• Dog example

• All dogs are descendents from wolves

• Dramatic change in phenotype occurred in about 15,000

years.

• Broad array of sizes, shapes, colors and behaviors seen in

dogs all arose from variation present in wild wolves.

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Figure 23-1 Alcohol dehydrogenase (Adh) gene in Drosophylla

Copyright © 2010 Pearson Education, Inc.

Figure 23-2 DNA sequence variation in Adh gene in Drosophylla

23.2 The Hardy–Weinberg Law

Describes what happens to alleles and genotypes in an “ideal” population assuming

1. No selection

2. No mutation

3. No migration

4. Large population

5. Random mating

The Hardy–Weinberg Laws

1. Frequency of alleles in the gene pool does not change over time

2. After one generation of random mating, genotype frequencies for two alleles can be calculated as p2 + 2pq + q2 = 1

where p = frequency of allele A and q = frequency of allele a

Allele vs. Genotype Frequency

• AllelesFreq of allele A =0.7 freq of allele a =0.3

• GenotypesAA, Aa, aa

• HW law explains the relationship between allele and genotype frequency in a population

Allele frequencies after one generation under HW Law

• Parental generationFreq of allele A =0.7 freq of allele a =0.3

• How do you calculate allele frequencies after one generation?

Allele frequencies in the next generation

AA - 49%Aa - 42%aa - 9%

Frequency of allele A = 0.49 + 0.21 = 0.70

Frequency of allele a = 0.09 + 0.21 = 0.30

Allele frequency has not changed from the previous generation

The Hardy–Weinberg Law

• The Hardy-Weinberg law allows the frequency of heterozygotes in a population to be estimated

• In general, the frequencies of all three genotypes can be estimated once frequency of either allele is known and Hardy-Weinberg assumptions are invoked

The Hardy–Weinberg Law

• Problem 1:

Human blood types, MM, MN and NN are controlled by two alleles (M & N) of a single gene. In a population of 100 individuals, 49% are of the NN blood type.

What percentage is expected to be MN assuming Hardy-Weinberg equilibrium conditions?

The Hardy–Weinberg Law

• Problem 2:

In a population of 100 that meets the Hardy-Weinberg equilibrium assumptions, 81 individuals are homozygous for a recessive allele.

What percentage of the individuals would be expected to be homozygous for the dominant allele in the next generation?

The Hardy–Weinberg Law Assumes

1. There is no selection

2. That no new alleles arise from mutation

3. That there is no migration into or out of

the population

4. That the population is infinitely large

5. That random mating occurs

Change in allele frequency is evolution!!!!

23.3 The Hardy–Weinberg Law can be applied to human populations

• CCR5 gene – Chemokine co-receptor 5

• Normal allele –CCR5-1

• Δ32 mutation – 32 bp deletion in the gene

• In a population of 100 people

– Homozygous for CCR5 - 1/1 79

– Heterozygous -1/ Δ32 29

– Homozygous for Δ32 - Δ32/ Δ32 1

Forces Driving Allele Frequency Change (p 491)

• Natural selection

• Mutation

• Migration

• Genetic drift in small populations

• Non-random mating

Natural Selection

• Natural selection takes place when individuals with adaptive trait produce a greater number of offspring than that produced by others in a population

• In the next generation, the offspring with the adaptive trait appear in greater frequency

• What are some examples?

Natural Selection

• If individuals are subject to natural selection and do not have equal rates of survival and reproductive success, allele frequencies may change from one generation to the next

• Natural selection is the principal force that shifts allele frequencies within large populations

Population with varied inherited traits1

Elimination of individuals with certain traits2

Reproduction of survivors3

Natural selectionacts on polygenic

traits too

Mutation

• Mutation is the only process that creates new alleles in a gene pool

• Mutation creates new alleles on which natural selection acts upon

• Change in allele frequency due to mutation depends on the fitness they confer and the action of natural selection

Migration or Gene Flow

• Migration occurs when individuals move between the populations

• Migration may have a large effect on allele frequency if:– the rate of migration is large

or– if the allele frequency of the migrant population

differs greatly from that of the population to which it is moving

Migration – B allele of ABO locus

Genetic Drift

• Genetic drift occurs when # of reproducing individuals in a population is too small to ensure all alleles in the gene pool will be passed on to next generation in their existing frequencies

• Genetic drift may result in one allele becoming fixed and one allele disappearing in a population

Nonrandom Mating

• Nonrandom mating can change the genotype frequency in a population

• But not the allele frequency

Forms of Nonrandom Mating

• Positive assortive mating in which similar genotypes are more likely to mate than dissimilar ones

• Negative assortive mating in which dissimilar genotypes are more likely to mate than similar ones

• Inbreeding in which mating individuals are related

Inbreeding

• One consequence of inbreeding is an increased chance that an individual will be homozygous for a recessive deleterious allele

• The significance of this fact is that inbred populations often have a lowered mean fitness, called inbreeding depression

Inbreeding in plants

• Self-fertilization is a form of inbreeding• Continuous self-fertilization can lead to fully

homozygous plants called inbred lines

• If members of two inbred lines are mated, the offspring often display hybrid vigor.

• Hybrid vigor is highest in the F1 generation and

generally declines thereafter.

Inbreeding: First-cousin marriage