Kaplan Nursing Entrance Practice Exam

In the genetic cross of mice, which combination will result in all short, long-tailed offspring?

1. TtLl x TtLl

2. TtLl x TTll

3. TtLl x TTLL

4. TTLL x TtLl

The correct answer is: TtLl x TTLL

In the genetic cross of TtLl x TTLL, we are analyzing two traits: tail length (T for long, t for short) and another trait represented by L. The uppercase letters indicate dominant alleles, while lowercase letters signify recessive alleles. In this combination, one parent is homozygous dominant for the tail length trait (TT), meaning it has two alleles for long tails, while the other parent is heterozygous (Tt), which has one long tail and one short tail allele. Because all offspring will receive at least one dominant allele for tail length (either T or T), it guarantees that all offspring will exhibit the dominant trait of long tails. For the other trait, both parents contribute one of their L (dominant) or l (recessive) alleles. As the second parent is also homozygous dominant for this trait (LL), regardless of whether the first parent contributes an L or an l allele, each offspring will also have at least one dominant allele for this trait, leading to all offspring displaying the dominant phenotype. Consequently, this genetic cross results in all short, long-tailed offspring because for tail length, they cannot inherit a recessive phenotype (short tail