## Dihybrid Cross

### Problem 7: Homozygous offspring of a dihybrid cross.

Tutorial to help answer the question

In a dihybrid cross, AaBb x AaBb, what fraction of the offspring will be homozygous for both recessive traits?

Tutorial

#### 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).

#### Possible gametes for each AaBb parent

 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.

#### Alleles from both parents

 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.

#### Offspring homozygous for both recessive traits

 There is only one of 16 possible combinations with this genotype. The predicted fraction is therefore 1/16.

The Biology Project
University of Arizona
Tuesday, August 13, 1996
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