Download POPULATION GENETICS Learning Objectives • Define Population

POPULATION GENETICS
Learning Objectives
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Define Population genetics
Define genotype and allele frequency
Define Hardy- Weinberg equilibrium
Enlist the factors responsible for genetic variation in / among the
population
Define genetic drift, genetic flow
Population Genetics
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1. The branch of genetics concerned with the hereditary makeup of
populations.
2. the study of changes in gene frequencies in population of organisms
and the effects of such changes on evolution and adaptation.
Also called quantitative genetics
Population genetics is the study of allele frequency distribution and
change under the influence of the four main evolutionary processes:
natural selection,
genetic drift,
mutation and
gene flow.
Genotype and Allele Frequency
Genetic variation in populations can be analyzed and quantified by the
frequency of alleles. Two fundamental calculations are central to
population genetics
allele frequencies and genotype frequencies.
Genotype frequency in a population is the number of individuals with
a given genotype divided by the total number of individuals in population
Hardy- Weinberg equilibrium
Defines as allele and genotype frequencies in a population will remain
constant from generation to generation in the absence of other
evolutionary influences.
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These influences include
non-random mating,
mutation,
selection,
genetic drift,
gene flow
meiotic drive.
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Non-random mating : 
 is inbreeding,
which causes an increase in homozygosity for all genes.
Selection  in general, causes allele frequencies to change, quite
rapidly.
While directional selection  leads to the loss of all alleles except the
favored one, some forms of selection, such as balancing selection, lead
to equilibrium without loss of alleles.
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Mutation: Recurrent mutation will maintain alleles in the population,
even if there is strong selection against them.
Migration genetically links two or more populations together.
Distributions
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Distribution of a trait in a population = proportion
of individuals that have each of the possible phenotypes
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Mean = peak of distribution
i /N
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Variance = spread of distribution estimated by squared deviation from
2
the mean s2=∑(xi /N-1
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Standard deviation = square root of the variance
s =√ s2
Normal Distribution
Normal distribution = symmetrical curve produced by data in which half
points are above and half points are below mean
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~68% of a population have a phenotype within one standard deviation
(s) of the mean
~95% - within 2 s
~99.7% - within 3 s
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The distribution of a trait in a population implies nothing about its
inheritance
Fig. 15.5
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Phenotypic Variation
Variation of a trait can be separated into genetic and environmental
components
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Genotypic variance g2 = variation in phenotype caused by differences
in genotype
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Environmental variance e2 = variation in phenotype caused by
environment
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Total variance p2 = combined effects of genotypic and
environmental variance
p2 = g2 + e2
Phenotypic Variation
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Genotype and environment can interact or they can be associated
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Genotype-environment (G-E) interaction = environmental effects on
phenotype differ according to genotype
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Genotype-by-sex interaction: same genotype produces different
phenotype in males and females (distribution of height among women
and men).
Genetic Variation
Genotype-environment (G-E) association = certain genotypes are
preferentially associated with certain environments
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There is no genotypic variance in a genetically homogeneous
population g2 = 0
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When the number of genes affecting a quantitative trait is not too large,
the number, n, of genes contributing to the trait is
n = D2/8g2
D = difference between parental strains
Factors Responsible for Genetic Variation in / Among the Population
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Mutation  Branched chain ketoacid dehydrogenase deficiency
Natural selection.
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GENETIC DRIFT
Genetic drift is the phenomenon of change in the frequency of alleles
(variants of a gene) in a population of organisms due to chance or
random events.
Genetic drift or allelic drift is the change in the frequency of a gene
variant (allele) in a population due to random sampling.
Genetic Flow
Gene flow (also known as gene migration) is the transfer of alleles or
genes from one population to another.