I'm dealing with a pedigree problem, and I'm having some trouble dealing with problems of unknown parental genotypes (where there are multiple possibilities). This is not a homework question; it's an exam practice problem.

enter image description here

The question asks: Suppose brother and sister 6 and 7 are mated. What is the probability that their first pup will be albino?

The answer choices are 1/4, 1/8, 1/16, 1/32, or Not Enough Info.

Here's my attempt at the problem:

Since #2 is black, and some of its offsprings ended up sepia, #2 must carry the sepia gene (since the male mate is cream, and sepia is dominant over cream). So, we know that #2 is $Cc^k$.

Now, the male mate can be either $c^d c^d$, or $c^d c^a$.

This means that in order for the offspring of #6 and #7 to be albino, we need:

1) Male parent needs to carry the albino gene (i.e. male needs to be $c^d c^a$).

2) Male parent needs to pass on the albino gene to both #6 and #7.

3) #6 and #7 both need to pass on the albino gene.

I think the probability that the male is $c^d c^a$ is $\frac{1}{2}$, since the male parent can be either $c^d c^a$ or $c^d c^d$.

If we draw out a punnett square, the probability that the albino gene gets passed onto 1 individual is $\frac{1}{2}$ (since we already know the offsprings are sepia, we only need to consider the genotypes that contain $c^d$).

The probability that #6 and #7 pass on the albino gene is $\frac{1}{4}$.

This led me to think that the final probability is then P(male parent has albino gene) * P(#6 inherits albino gene) * P(#7 inherits albino gene) * P(#6 and #7 pass on the albino gene), which is $\frac{1}{2}$ * $\frac{1}{2}$ * $\frac{1}{2}$ * $\frac{1}{4}$, which gives us $\frac{1}{32}$, but I do not feel confident in my answer. Specifically, I do not know how to account for the fact that the male parent can be either $c^d c^d$ or $c^d c^a$.

Is my thought process correct? If not, where in my logic did I go wrong? Thank you for your help.


Based on the information given in the pedigree, you can actually be certain that #2's mate has a genotype of CdCa because of the following: Two of 5's siblings are albinos. Since albino is a recessive trait (lowest in hierarchy of dominance), both parents (4 and 4's mate) must have the Ca allele. Let's call 4's mate "8". Since 8 has the Ca allele, at least one of 8's parents must also have the Ca allele. Since you have already (correctly) determined that #2 is CCk, #2's mate must have the Ca allele, and must therefore have a genotype of CdCa.

Thus you can eliminate the extra (1/2) from your calculation, making the answer (1/2) * (1/2) * (1/4), or (1/16).

| improve this answer | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.