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?


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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