AP BIOLOGY

Detailed explanation-1: -In a dihybrid cross, AaBb x AaBb, what fraction of the offspring will be homozygous for both recessive traits? 1/4 of the gametes of each parent will be ab. The fraction of the offspring homozygous for both recessive traits will be 1/4 times 1/4, or 1/16.

Detailed explanation-2: -Therefore, the “cross” is Bb x bb; half the children from such a cross are expected to show the dominant (brown-eyed) phenotype.

Detailed explanation-3: -When crossing organisms that are heterozygous for two traits, the result is a dihybrid cross. The phenotypic ratio of a heterozygous dihybrid cross for autosomal (none-sex-linked) traits is always 9:3:3:1. Nine offspring will show both dominant traits (AABB, AaBB, AABb, AaBb).

Detailed explanation-4: -However, if the male is Bb as above and you perform a testcross, 50% of all offspring should have the bb genotype and a white phenotype. A testcross to a heterozygous individual should always yield about a 1:1 ratio of the dominant to recessive phenotype. So, both the genotypic and phenotypic ratios here are 50:50.