# Dihybrid test cross

Consider the following two genotypes: AABB , aabb where A and B are dominant and a and b are recessive:

1) What is the genotype of the F1 progeny from an AABB x aabb cross?

2) Construct a Punnet square of the F1 self cross (i.e. F1 x F1). What are the genotypic ratios from this testcross? What are the phenotypic ratios?

3) Construct a Punnet square of the F1 genotype with the AABB parental. What are the genotypic ratios from thsi cross? What are the phenotypic ratios?

4) Construct a Punnet square of the F1 genotype with the aabb parental. What are the genotypic ratios? What are the phenotypic ratios?

5) If you had an organism that was A- B- and wanted to determine its exact genotype, which cross would you use? Explain your answer.

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#### Solution Preview

1) What is the genotype of the F1 progeny from an AABB x aabb cross?

First, determine the gametes possible for each parent. AABB can only produce AB gametes. aabb can only produce gametes aa. The only possible offspring are AaBb

2) Construct a Punnet square of the F1 self cross (i.e. F1 x F1). What are the genotypic ratios from this testcross? What are the phenotypic ratios?

Since all the offspring on the cross are AaBb, the F1 cross would be AaBb x AaBb

Punnett square

AB Ab aB ab

AB AABB AABb AaBB ...

#### Solution Summary

Detailed explanation of dihybrid test crosses with Punnett squares.