Tutorial to help answer the question

Predicting the genotype of offspring

There are four possible combinations of gametes for the AaBb parent. Half of the gametes get a dominant A and a dominant B allele; the other half of the gametes get a recessive a and a recessive b allele. Both parents produce 25% each of AB, Ab, aB, and ab. (Review problem #3's tutorial if necessary).

Since each parent has four different combinations of alleles in the gametes, there are sixteen possible combinations for this cross. Therefore we use a 4 square by 4 square Punnett Square.

Now we can predict the outcome of the genetic cross of AaBb x AaBb.

To determine the fraction of the offspring homozygous for both traits, first determine the genotype of an offspring homozygous for both traits. It would be aabb.

There is only one of 16 possible combinations with this genotype.

The predicted fraction is therefore 1/16.