Hardy-Weinberg Law: Sex-linked genes
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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).
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Genes on the X or Y chromosome are called sex linked genes.
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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.
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So , two-thirds of the sex-linked genes in the population are carried by the homogametic sex and one-third by the heterogametic sex.
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Consider two alleles A and a with frequency p and q
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Female
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Male
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Genotype
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AA
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Aa
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aa
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A
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a
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P
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H
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Q
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R
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S
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Hence, if gene frequencies among males and among females are different, the population will not be in equilibrium.
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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.
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This oscillation is because of getting sex linked genes in males only from their mother.
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The most important implication is that sex-linked characters are expected to occur with different frequencies in males and females.
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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).
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Last modified: Wednesday, 11 January 2012, 7:09 AM