You perform a test cross of the dihybrid AaBb and score the phenotypes of 1000 progeny. Assuming independent assortment, how many of the progeny do you expect to display the dominant phenotype for both the A and B genes?
A) 500
B) 750
C) 250
D) 100
E) 200

Question

Quizplus · Accepted Answer

In a dihybrid cross AaBb x aabb, assuming independent assortment, each gene pair sorts independently, resulting in a 9:3:3:1 ratio for the phenotypes in the offspring. The dominant phenotype for both genes (A-B-) will be represented in 9 out of 16 parts of this ratio. Therefore, for 1000 progeny, the expected number is 1000 * (9/16) = 562.5, which is not an option provided. However, the question seems to be misinterpreted in the explanation. The correct calculation for the expected number of progeny displaying the dominant phenotype for both genes, assuming independent assortment and a dihybrid cross of AaBb (with an unknown partner, which should ideally be aabb for a test cross, but isn't specified), would be 3/4 (for A-) times 3/4 (for B-), which equals 9/16. However, without the correct partner specified, the calculation based on the provided options and typical understanding of a test cross (AaBb x aabb) leads to confusion. Given the options and typical expectations for a dihybrid cross with independent assortment, the closest correct approach without the confusion would be to calculate the proportion of offspring with both dominant phenotypes, which is incorrectly reasoned in the initial explanation. The correct expectation for a dihybrid cross AaBb with independent assortment, without specifying the cross partner, generally leads to expecting 9/16 of the offspring to show both dominant phenotypes if both parents contribute dominant alleles, but this does not align with the options provided, indicating a misunderstanding in the explanation process.

You Perform a Test Cross of the Dihybrid AaBb and Score