Question

In guinea pigs, black fur (b) is dominant over white fur (w). You have a black guinea pig, and want to try to determine its genotype. One way to do so would be to mate your guinea pig with another guinea pig and see what the resulting offspring looked like. What genotype of breeder guinea pig would be most helpful to determine your guinea pig's genotype? (hint: there are two answers that would both work in this case, but one is more likely to be helpful than the other.)

Answer 1

The genotype of the guinea pig that you would want to mate it with the black one is ww, which is also a white guinea pig, as all white guinea pigs must have the genotype ww as w is recessive to b. Also, make sure you know that the black guinea pig should have the genotype of bx, where x is either b or w.

So, if you mate the black guinea pig with the ww guinea pig (white), the offsprings will either be all black, or half black half white.

If the offsprings are all black, you can be sure that the black guinea pig (parent) will have the bb genotype. As each offspring must receive one gene from each parent, so if all the offsprings have black fur, they must have one b gene from the black guinea pig.

On the other hand, if the offsprings are half black half white, the genotype of the black guinea pig (parent) should be bw. This is because in order to have white offsprings, each parent must pass on a w gene so that the offspring would have genotype ww, white.