Advanced Placement (AP) Biology Practice Exam

Which of the following describes a dihybrid cross?

1. A cross involving more than two pairs of contrasting traits

2. A cross between individuals with only recessive traits

3. A cross involving two pairs of contrasting traits

4. A genetic analysis of linked traits only

The correct answer is: A cross involving two pairs of contrasting traits

A dihybrid cross specifically refers to the genetic crossing of individuals that examines the inheritance of two distinct traits, each represented by different alleles. In a dihybrid cross, researchers look at how these two traits are inherited simultaneously, making it a powerful tool to analyze the principle of independent assortment as outlined by Mendel’s laws of inheritance. This type of cross is typically represented using a Punnett square that encompasses all combinations of allele pairs from each parent for the two traits being studied. For example, if one trait is seed shape (round vs. wrinkled) and another is seed color (yellow vs. green), the dihybrid cross would involve parents that are heterozygous for both traits, yielding offspring that can display various combinations of these traits. The other options do not accurately characterize a dihybrid cross. For instance, a cross involving more than two pairs of contrasting traits would be referred to as a multi-hybrid cross, while a cross between individuals with only recessive traits would not illustrate the interaction between dominant and recessive traits typical of a dihybrid cross. Additionally, a genetic analysis of linked traits pertains to traits located close together on the same chromosome and is not confined to the concept of examining two contrasting