Advanced Placement (AP) Biology Practice Exam

What is the result of a cross between BB and bb true-breeding parents?

1. All hybrids

2. All BB progeny

3. All bb progeny

4. Heterozygous offspring only

The correct answer is: All hybrids

The result of crossing true-breeding parents BB (homozygous dominant) and bb (homozygous recessive) will produce hybrid offspring that are all heterozygous (Bb). This is because each offspring will inherit one dominant allele (B) from the BB parent and one recessive allele (b) from the bb parent. Since the dominant allele masks the effect of the recessive allele, all progeny will express the dominant phenotype. The term "hybrid" here refers to the genetic variation created from the combination of different alleles (B and b), resulting in all offspring being Bb. This cross exemplifies the principles of Mendelian genetics, demonstrating how alleles segregate during gamete formation and combine during fertilization.