Download Population Genetics

POPULATION GENETICS
• Population genetics is the genetics of a
population dealing principally with gene and
genotype frequencies through time and
space.
• Population is a collection of individuals or data.
• It may be static or dynamic.
• A static population is one that existed at a
particular time only.
• A dynamic population is one that existed over
time and is maintained from generation to
generation through procreation of new
individuals, i.e. progeny.
GENETIC PROPERTIES OF POPULATION
i) Gene frequency
• A character in an individual that passes on from
generation to generation due to breeding of the
parents is controlled by genes.
• The distribution of genes in different individual of
the population is one of the properties of a
population and is expressed as ‘Gene frequency’.
ESTIMATION OF ALLELE FREQUENCIES
• The frequency of an allele is given by:
2Ho + He
p = ---------------------2N
Where
Ho = number of homozygotes for that allele,
He = number of heterozygotes for the allele and
N = number individuals scored at the locus.
HARDY - WEINBERG LAW
• The gene and genotype frequencies remain
constant from generation to generation in the
absence of mutation, migration and selection.
• The population then is said to be in HardyWeinberg equilibrium.
DERIVATIVES OF LAW
A large population that is not in equilibrium attains
equilibrium
after
one
generation of random
mating, provided mutation, migration and selection
are absent.
The genotype frequency of the progeny is fully
determined by the gene frequency of the parents in
an equilibrium population and the gene frequency
is
obtained by taking
square-root of
the
homozygote frequency.
The frequency of the heterozygotes in an equilibrium
population is never more than 50 per cent.
FORCES CHANGING GENE FREQUENCY IN POPULATION
The magnitudes of change in the gene and genotype
frequency are as follows.
• Non-random mating:
1)Inbreeding
2)Mutation
3)Migration
4)Selection
5)Small populations
6)Genetic drift
INBREEDING
• Mating among close relatives will lead to a
surplus of homozygotes.
• Assortative mating – If individuals prefer to
mate with other individuals with similar
genotypes this may lead to a surplus of
homozygotes.
• Disassortative mating - If individuals prefer to
mate with other individuals with different
genotypes, then this may lead to a surplus of
heterozygotes.
• Wahlund effect – This denotes a surplus of
homozygotes due to the presence of individuals
that represent different populations and do not inter
breed.
MUTATION
• It is a rare event in nature.
• It can occur in both the directions, i.e. forward
and backward.
• If it is recurrent, it can change the gene and
genotype frequency of the population.
• If mutation occurs continuously in a gene it is
called recurrent mutation.
• Some mutations increase fitness or lead to the
evolution of new species by creating new or
improved phenotypes.
• Some mutations lower fitness.
• Many mutant alleles produce phenotypes that are
so abnormal that they either cause death or
reduce viability severely.
• Other cause only small reductions in viability.
MIGRATION
• The change in gene frequency due to migration depends
on the preparation of individuals that enter into the
population and their gene frequency.
• The change in gene frequency due to migration
q = m (qm-q)
q- initial gene frequency of the population
qm-gene frequency in the immigrants and
‘m’ is the proportion of immigrants entering into the
population
SELECTION
• It is defined as the non-random differential propagation
of the genotypes. In other words.
• Selection acts on the phenotype of the next generation
and is considered to select in favor by nature.
• One who contributes the highest number of progeny is
called as the fittest individual.
• Has highest fitness (F=1).
• The change in gene frequency due to selection
depends on the coefficient of selection or the degree of
disadvantage of the phenotype and its frequency in the
population.
SMALL POPULATIONS
• In a large population the gene frequencies are
inherently stable in the absence of mutation, migration
and selection.
• The gametes that transmit genes to the next generation
carry a sample of genes in the parent generation and, if
the sample is not large, the frequencies are liable to
change can be predicted, the direction of change
cannot be predicted.
• This results in dispersion of gene frequencies and is
called the dispersive process (random genetic drift).
GENETIC DRIFT
• It is a phenomenon that leads to a random changes in the
gene frequency in a founder population, which may not
carry some alleles due to sampling error.
• Genetic drift leads to loss or fixation of alleles within
populations.
• Genetic drift can irreversibly alter gene frequencies and
eliminates alleles, which can decrease a populations ability
to survive or to adapt to an altered environment, and it can
preclude future selection.
CONSEQUENCES OF RANDOM GENETIC DRIFT
• The random change in gene frequency found in small
populations from generation to generation is called as
‘random genetic drift’.
• Random drift occurring independently in different subpopulations possess significantly different gene and
genotype frequencies. Uniformity within sub-populations.
• Increased homozygosity
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