In a certain population of 540 cats, a single gene with two alleles controls thickness of the cats' fur. Allele C provides for thick fur and is dominant, and c provides thin fur. In this population, 49 cats have thin fur. What are the frequencies of the two alleles?

Since thin fur is recessive, the odds of having thin fur are the odds that an allele codes for thin fur, squared. The odds of thin fur are 49/540. The square root of 49/540 is roughly 7/23, making the frequency of c roughly 7/23 and C roughly 16/23. The allele frequency of an x-linked recessive disorder is the same as the frequency with which it occurs in men. In real life, the 1 in 10,000 figure is an approximation so we can't meaningfully use it to compute an exact allele frequency, and because the frequency in women is almost negligible, we can say the frequency in men is roughly 1 in 5000 and that that's the approximate allele frequency.