Hardy weinberg supplement

Hardy-WeinbergLaw of Population Genetics

Prepared by Pratheep SandrasaigaranLecturer at Manipal International University

Constant Allele Frequencies

• Population genetics looks at phenotypes and genotypes among large numbers of individuals.

• Tracking allele frequencies from one generation to the next can reveal evolution in action.

Prepared by Pratheep Sandrasaigaran

Hardy-Weinberg Equilibrium

• In 1908 Godfrey Harold Hardy (mathematician) and Wilhelm Weinberg (German physician).

• Independently (never met) used algebra to explain how genotypic frequencies are related to allele frequencies.


Relating alleles to genotypes

• What is an equilibrium?

• What is Hardy-Weinberg equilibrium?

• Allele and genotype frequencies will remain unchanged, generation after generation, as long as certain conditions are met; four assumptions.

• When one of the assumptions isn’t met, the relationship between allele frequency and genotype frequency usually starts to fall apart


Four conditions for Hardy-Weinberg Equilibrium

• The organism must reproduce sexually and be diploid.

• The allele frequencies must be the same in both sexes.

• The loci must segregate independently.

• Mating must be random with respect to genotype


Graphically illustrated Hardy-Weinberg Equilibrium

• The Y axis is genotypic frequency in percentage.

• The X axis is the frequency of the recessive allele in percentage.

• What proportion of the population is homozygous aa when the allele frequency of a is 40 percent?

• 20 percent of the population is expected to be aa when 40 percent of the population carries the a allele


Relating alleles to genotypes• If frequency of allele a is 40%, then the

frequency of allele A is 60%, because p + q = 1.

• What is this line for?

• The frequency of heterozygotes, Aa.

• The highest proportion of the population that can be heterozygous is 50 percent


Relating alleles to genotypes• When 50 percent of the population is

heterozygous, the Hardy-Weinberg equilibrium predicts that 25 percent of the population will be homozygous for the A allele, and 25 percent will be homozygous for the a allele.

• This situation occurs only when p is equal to q.

• p = q = 50%


Relating alleles to genotypes• The relationship between allele

frequencies and genotype frequencies is described by the equation p2 + 2pq + q2.

• Thus, the line marked aa is described by the equation p2.

• The line marked AA is described by the equation q2.

• 2pq describes the frequency of heterozygotes (Aa)


Violating the law• There are several ways that populations can wind

up out of Hardy-Weinberg equilibrium.

• How to ensure no violation?– Large population– No mutation– No natural selection– No migration– Randomly mating populations


Solving a Problem• Consider an autosomal recessive trait: a middle finger shorter than

the second and fourth fingers.

• If we know the frequencies of the dominant and recessive alleles, then we can calculate the frequencies of the genotypes and phenotypes and trace the trait through the next generation.


Solving a Problem• The dominant allele D confers normal-length

fingers; the recessive allele d confers a short middle finger.

• If 9 out of 100 individuals in a population have short fingers (dd) —the frequency is 9/100 or 0.09.

• Since dd equals q2 , then q equals 0.3.

• Since p + q = 1.0, knowing that q is 0.3 tells us that p is 0.7


Solving a Problem• Calculate the proportions of the three genotypes

that arise when gametes combine at random.

• Homozygous dominant = DD (p2) • 0.7 × 0.7 = 0.49

• Homozygous recessive = dd (q2) • 0.3 × 0.3 = 0.09

• Heterozygous = Dd + dD (2pq)• (0.7)(0.3) + (0.3)(0.7) = 0.42


Solving a Problem• Within a population of mouse, the color black (B)

is dominant over the white color. 40% of all mice are white. Calculate the following

– The percentage of mice in the population those are heterozygous.

– The frequency of homozygous dominant individuals


Science

Hardy weinberg supplement