The document discusses the Hardy-Weinberg law of population genetics, which explains how allele and genotype frequencies remain constant over generations under specific conditions. It outlines the four assumptions necessary for Hardy-Weinberg equilibrium and illustrates how to calculate allele frequencies and genotypic ratios using real-world examples. Additionally, it addresses factors that can lead to deviations from this equilibrium and provides guidance on maintaining ideal population conditions.

1. Hardy-Weinberg
Law of Population Genetics
Prepared by Pratheep Sandrasaigaran
Lecturer at Manipal International University

2. 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

3. 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.
Prepared by Pratheep Sandrasaigaran

4. 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
Prepared by Pratheep Sandrasaigaran

5. 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
Prepared by Pratheep Sandrasaigaran

6. 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
Prepared by Pratheep Sandrasaigaran

7. 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
Prepared by Pratheep Sandrasaigaran

8. 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%
Prepared by Pratheep Sandrasaigaran

9. 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)
Prepared by Pratheep Sandrasaigaran

10. 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
Prepared by Pratheep Sandrasaigaran

11. 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.
Prepared by Pratheep Sandrasaigaran

12. 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
Prepared by Pratheep Sandrasaigaran

13. 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
Prepared by Pratheep Sandrasaigaran

14. 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
Prepared by Pratheep Sandrasaigaran