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Calculating Gene Frequencies Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 23, 2011

Sex-Linked Loci

  1. Dominant and Recessive Alleles. Since each male possesses only one sex-linked allele, the frequency of a sex-linked trait among males is a direct measure of the allelic frequency in the population, assuming that the allelic frequencies thus determined are representative of the allelic frequencies among females as well.
  2. Codominant Alleles. Data from both males and females can be used in the direct computation of sex-linked codominant allelic frequencies. Bear in mind that in organisms with an X-Y mechanism of sex determination, the heterozygous condition can only appear in females. Males are hemizygous for sex-linked genes.

EXAMPLE 9.4   In domestic cats, black melanin pigment is deposited in the hair by a sex-linked gene; its alternative allele produces yellow hair. Random inactivation of one of the X chromosomes occurs in each cell of female embryos. Heterozygous females are thus genetic mosaics, having patches of all-black and all-yellow hairs called tortoise-shell pattern. Since only one sex-linked allele is active in any cell, the inheritance is not really codominant, but the genetic symbolism used is the same as that for codominant alleles.

Sex-Linked Loci

Testing a Locus for Equilibrium

In cases where dominance is involved, the heterozygous class is indistinguishable phenotypically from the homozygous dominant class. Hence, there is no way of checking the Hardy-Weinberg expectations against observed sample data unless the dominant phenotypes have been genetically analyzed by observation of their progeny from test crosses. Only when codominant alleles are involved can we easily check our observations against the expected equilibrium values through the chi-square test.

The number of variables in chi-square tests of Hardy-Weinberg equilibrium is not simply the number of phenotypes minus 1 (as in chi-square tests of classical Mendelian ratios). The number of observed variables (number of phenotypes = k) is further restricted by testing their conformity to an expected Hardy-Weinberg frequency ratio generated by a number of additional variables (number of alleles, or allelic frequencies = r).We have (k – 1) degrees of freedom in the number of phenotypes, (r – 1) degrees of freedom in establishing the frequencies for the r alleles. The combined number of degrees of freedom is (k – 1) – (r – 1) = kr. Even in most chi-square tests for equilibrium involving multiple alleles, the number of degrees of freedom is the number of phenotypes minus the number of alleles.

Practice problems for these concepts can be found at:

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