Transcript Hardy Weinberg Equilibrium, Gene and Genotypic frequencies

Hardy Weinberg Equilibrium,
Gene and Genotypic
frequencies
The Hardy Weinberg law
states that allele and
genotype frequencies in a
population will remain
constant from generation to
generation in the absence of
other evolutionary influences.
<> In case of single locus with two alleles denoted
“A” and “a” with frequencies f(A) = p and f(a) = q
respectively.
<> The expected frequencies are f(AA) = p2 for AA
homozygotes, f(aa) = q2 for the aa homozygotes
and,
<> f(Aa) = 2pq for the heterozygotes.
<> p2, q2, 2pq are called Hardy Weinberg
proportions.
<> Sum of all genotype frequencies is the square of
the sum of p and q.
<> (p+q)2 = p2 + q2 + 2pq
<> Solution of this equation is q = 1-p
Derivation
• A locus in the population has two alleles “A” and ‘a”
occurring with initial frequency f’(A)=p
and f’(a)=q respectively.
• The allele frequencies at each generation are
obtained by pooling together the alleles from each
genotype of the same generation according to the
homozygote and heterozygote genotypes, which are
1 and ½ respectively.
• f”(A) = f”(AA) + 1/2f”(Aa)
• f”(a) = f”(aa) = 1/2f”(Aa)
Equation (1) and (2) gives out
<> f1(A) = f1(AA) = 1/2f1(Aa) =
p2 + pq = p(p+q) = p = f’(A)
<> f1(a) = f1(aa) + 1/2f1(Aa) =
q2 + pq = q(p+q) = q = f’(a)
Limitation of Hardy Weinberg
Law
• Mutations : These are permanent heritable changes in the
genetic material of individuals. Gene mutations produce
new alleles.
• Recombinations : During meiosis, crossing over causes
reshuffling of gene combinations which provides new
combinations of existing genes and alleles.
• Gene Migration : If the migrating individuals breeds
within the new population the immigrants will add new
alleles to the local gene pool of the host population.
• Genetic Drift : The random change in the allele frequency
occurring by chance alone are called genetic drift.
Gene Frequency
It is relative frequency of an allele at
a particular locus in a population,
expressed as a fraction or
percentage.
Genotypic Frequency
It is the frequency of a genotype –
Homozygous recessive, homozygous
dominant, or heterozygous in a
population.
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