Part 5: Genetics of Organisms and Populations
25.1 Allele Frequencies in Population Gene Pools Vary in Space and Time
A population is a group of individuals with a common set of genes that lives in the same geographic area and can or does interbreed. 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.
The Hardy-Weinberg Law describes the relationship between
frequencies in an ideal
Extremely large population size.
No gene flow due to migration.
No mutations .
No natural selection .
Mendelian inheritance preserves allele frequencies in a population, resulting in constant genetic variation.
At Hardy-Weinberg equilibrium, the distribution of 2 alleles
in a population can be modeled by these equations:
The allele frequencies are described by
= 1, where
is the dominant allele frequency and
is the recessive allele frequency.
The genotype frequencies are described by
= 1, where
are frequencies of the homozygous genotype and 2
is the frequency of the heterozygous genotype.
The Hardy-Weinberg law makes two predictions:
(1) the frequency of the alleles in the gene pool does not change over time; and
(2) after one generation of random mating, the genotype frequencies for two alleles can be calculated as p2�+�2pq�+�q2 = 1 where p equals the frequency of allele A and q is the frequency of allele a
are polygenic and often multifactorial but have a small number of discrete phenotypic
25.3 The Hardy-Weinberg Law Can Be Applied to Human Populations
An example of how the Hardy-Weinberg law can be applied to humans is analysis of susceptibility to HIV-1 infection based on the genotype for the CCR5 HIV-1 receptor gene
25.4 The Hardy-Weinberg Law Can Be Used for Multiple Alleles, X-Linked Traits, and Estimating Heterozygote Frequencies
Frequencies for multiple alleles can be calculated using the Hardy-Weinberg equation by adding more variables.
For instance, in a situation involving three alleles (p�+�q�+�r = 1), the frequencies of the genotypes are given by (p�+�q�+�r)2 = p2�+�q2�+�r2 +�2pq�+�2pr�+�2qr = 1.
An example of genotype frequency calculations for ABO blood type is given in
In using the Hardy-Weinberg equation to calculate allele and genotype frequencies for X-linked traits in mammals, the frequency of the X-linked allele in the gene pool will equal the frequency of males expressing the X-linked trait.
For females, the frequency of having the allele in question will be q2 if the allele frequency is q.
The Hardy-Weinberg law also allows the frequency of heterozygotes in a population to be estimated. In general, the frequencies of all three genotypes can be estimated once the frequency of either allele is known and Hardy-Weinberg assumptions are invoked
25.5 Natural Selection Is a Major Force Driving Allele Frequency Change
If individuals of all genotypes 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.
Hardy-Weinberg analysis allows fitness w to be examined for each genotype. For a homozygous recessive individual that dies before producing offspring, w = 0, and the frequency of the recessive allele will decrease in each generation
The rate at which the frequency of a deleterious allele declines depends on the strength of selection applied
Selection in natural populations works as predicted to increase the frequency of the allele to which selective pressure is applied. No such increase is observed in populations not subjected to the selection
Selection acting on quantitative traits can be directional, stabilizing, or disruptive
In directional selection, the genotype conferring one phenotypic extreme is selected, resulting in a change in the population mean over time.
In stabilizing selection, intermediate types are favored, and both extreme phenotypes are selected against. This will reduce the population variance over time but not the mean.
In disruptive selection, both phenotypic extremes are selected for, and the intermediates are selected against. This will result in a population with an increasingly bimodal distribution for the trait
25.6 Mutation Creates New Alleles in a Gene Pool Mutation is the only process that creates new alleles in a gene pool. Because most mutations are recessive, indirect methods using probability and statistics are often employed to determine the mutation rate.
If the mutation rate is known, the extent to which mutation can cause allele frequencies to change from one generation to the next can be estimated.
In general, although mutation provides the raw material for evolution, mutation by itself plays a relatively insignificant role in changing allele frequencies
25.7 Migration and Gene Flow Can Alter Allele Frequencies When a species divides into populations that are separated geographically, the allele frequencies in these new populations may differ over time due to migration.
Migration occurs when individuals move between the populations and 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.
25.8 Genetic Drift Causes Random Changes in Allele Frequency in Small Populations
Genetic drift occurs when the number of reproducing individuals in a population is too small to ensure that all the alleles in the gene pool will be passed on to the next generation in their existing frequencies. Genetic drift may result in one allele becoming fixed and one allele disappearing in a population.
25.9 Nonrandom Mating Changes Genotype Frequency but Not Allele Frequency
Nonrandom mating can take the form of 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, and inbreeding in which mating individuals are related.
For a given allele, inbreeding increases the proportion of homozygotes in the population, and a completely inbred population theoretically will consist only of homozygotes.
Self-fertilization is a form of inbreeding common in plants. The rate of homozygotes in a self-fertilizing population rapidly increases over a few generations, but the overall allele frequency does not change
A coefficient of inbreeding can be calculated to give the probability that two alleles of a given gene in an individual are identical because they are descended from the same single copy of the allele in an ancestor
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.
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 may increase the efficiency with which selection removes a deleterious allele from a population.
Dec 3, 2008