AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

HARDY-WEINBERG LAW: SEX-LINKED GENES

• In mammals, the female is the homozygous sex, with two X chromosomes (XX), while the male is heterozygous, with one X and one Y chromosome (XY).
• Genes on the X or Y chromosome are called sex linked genes.
• The relationship between gene frequency and genotype frequency in the homogametic sex is the same as with an autosomal gene; but the heterogametic sex has only two genotypes and carries only one gene instead of two.
• So , two-thirds of the sex-linked genes in the population are carried by the homogametic sex and one-third by the heterogametic sex.
• Consider two alleles A and a with frequency p and q

	Female			Male
Genotype	AA	Aa	aa	A	a
Genotype Frequency	P	H	Q	R	S

• The frequency of A among the females is

• The frequency of A among the male is

• The frequency of A in the whole populations is

• Hence, if gene frequencies among males and among females are different, the population will not be in equilibrium.
• The gene frequency in the population as a whole does not change; but its distribution between the two sexes oscillates as the population approaches equilibrium.
• This oscillation is because of getting sex linked genes in males only from their mother.

• Females get their sex linked genes equally from both parents

• The difference between the frequencies in the two sexes is

• It is half the difference in the previous generation. Therefore, if the gene frequencies are different in males and females then one generation of random mating is not sufficient to achieve Hardy-Weinberg equilibrium.
• It may take several generations and the number of generations will depend on the magnitude of difference between the sexes in gene frequency.
• The difference in gene frequency between the sexes will be halved as compared to the previous generation and the sign will be opposite.

• The most important implication is that sex-linked characters are expected to occur with different frequencies in males and females.
• This is relevant to the sex-linked recessive traits for which the frequency of the condition in males (q) is expected to be much higher than the frequency of the conditions in females (q²).