Question

Whilst doing community service at the local animal shelter, a genetics student noticed the coat colours in cats: 54 yellow males, 76 black males, 0 calico males, and 0 tortoiseshell males; 22 yellow females, 68 tortoiseshell females, 43 calico females and 15 black females. Is the stray cat population for the town (which the shelter services) likely to be in HWE for the coat colour gene?

Answer 1

Case 1: Male cat coat colour expression is quite simple. As only one allele exist for the gene in each cell, it will be expressed. And all males will either be yellow (XBY) or black (XbY) (Let B and b be the alleles for yellow and black coat colour). There will be no heterozygotes, because the Y does not carry the allele. Let frequency of yellow and black coat colour be p and q respectively. Total number of male cats in the population=54+76+0+0=130

Frequency of XBY genotype,

Frequency of XbY genotype,

So according to Hardy Weinberg equilibrium equation, ; therefore the male population cat will be in Hardy Weinberg equilibrium.

Case 2: The genotype (XBXB) produces yellow coat colour in female cats; genotype (XbXb) produces black coat colour and genotype (XBXb) produce both calico and tortoiseshell coat colour. Total number of female cats = 22+68+43+15=148.

Frequency of XBXB genotype,    and frequency of XbXb genotype,   and frequency of XBXb genotype,

So according to Hardy Weinberg equilibrium equation, , so the female cat population is also in Hardy Weinberg equilibrium.

Case 3: Total number of yellow cats (both males and females)=54+22=76; total number of black cats (both males and females)=76+15=91; total number of heterozygous cats (tortoiseshell and calico)=68+43=111; total number of cats in the population=130+148=278

Frequency of yellow coat colour genotype (XBY & XBXB), and frequency of black coat colour genotype (XbY & XbXb), and frequency of heterozygous tortoiseshell and calico genotype, (XBXb),

So according to Hardy Weinberg equilibrium equation, , so the stray cat population for the town is in Hardy Weinberg equilibrium for the coat colour gene.