Black coat color in Cocker Spaniels is governed by a dominant allele and red coa

Question

Black coat color in Cocker Spaniels is governed by a dominant allele and red coat by a recessive allele; solid pattern is governed by the dominant allele of another gene and spotted pattern is recessive. A solid black male is mated with a solid red female which produces a litter of 6 pups; two solid black , two solid red, one black and white and one red and white. Determine the genotypes of the parents. I need to be very specific with my answer by showing a punnet square with the answer to this question.

Explanation / Answer

So we start out knowing that we are dealing with 2 genes, so we need to follow both to determine ratios. Let's assign the letter "B" for colour because black is dominant, and the letter "S" for the pattern type. B = Black Coat b = Red Coat S = solid s = spotted We know the male is black with a solid colour. This means at least one allele for each gene is dominant, but we DO NOT know what the second gene is because even if it is recessive, the dominant gene is... well... dominant. So we have no way of knowing until we look at the offspring. So the male genotype will be B__S__. The female is a red and also has a solid coat. Since red is the recessive phenotype, we know she must have both alleles recessive for that gene, but again, we aren’t sure about the pattern genotype. The female genotype is bbS__. Now we have to look at the pups: 2 solid black, 2 solid red, one black and white, one red and white. I’m going to go ahead and assume “and white” means spotted and that you aren’t trying to calculate polygenic traits with codominant inheritance patterns haha. It is easiest to break down the phenotypes of the pups into categories of the genes you are looking for. If we break down the ratio in coat colour, we see that 3 pups have black coats and 3 have red coats. That is a perfect 50/50 split, which proves the male is a heterozygote in 2 ways. Firstly, if he was homozygous dominant, there would be no red pups at all – he would only be able to donate dominant alleles to his offspring, which would mask the recessive red trait. Also, if you were to draw a simple 2x2 Punnett square, you would see that the expected ratio of a heterozygote and a homozygous recessive cross is 50/50. When looking at the ratio of colour pattern, we see 4 are solid and 2 are spotted. This shows that both parents are heterozygous for 2 reasons yet again. This ratio is much closer to a 3:1 cross than a 1:1 cross, which is the result of a dihybrid cross (Punnett square it up if you need to, it’s another simple 2x2 cross). Also, again, if either parent were to be dominant, they would only be able to pass the dominant allele to the pups, and all pups would have solid coats. Therefore the male genotype is BbSs, and the female is bbSs.

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