A male mouse from a true-breeding strain of hyperactive animals is crossed with a female mouse from a true-breeding strain of lethargic animals. (These are both hypothetical strains.) All the progeny are lethargic. In the F₂ generation, all offspring are lethargic. What is the best genetic explanation for these observations? Propose a cross to test your explanation.

Verified step by step guidance

1. Step 1: Analyze the parental generation (P generation). The male mouse is from a true-breeding hyperactive strain, and the female mouse is from a true-breeding lethargic strain. True-breeding means both parents are homozygous for their respective traits. Assign alleles: let 'H' represent the hyperactive allele and 'h' represent the lethargic allele.
2. Step 2: Examine the F₁ generation. All progeny are lethargic, which suggests that the lethargic trait is dominant over the hyperactive trait. This means the F₁ generation must have the genotype 'Hh', where 'h' (lethargic) is dominant and 'H' (hyperactive) is recessive.
3. Step 3: Analyze the F₂ generation. If all offspring in the F₂ generation are lethargic, this suggests that the hyperactive allele ('H') is either lethal in the homozygous state (HH) or that the lethargic allele ('h') is the only functional allele. This would explain why no hyperactive offspring are observed.
4. Step 4: Propose a genetic explanation. The lethargic allele ('h') is dominant, and the hyperactive allele ('H') is recessive but may also be lethal in the homozygous state (HH). This would result in only lethargic offspring being viable in the F₂ generation.
5. Step 5: Propose a test cross. To confirm this explanation, cross an F₁ heterozygous mouse (Hh) with another F₁ heterozygous mouse (Hh). If the hyperactive allele is lethal in the homozygous state, the expected offspring ratio would be 2 lethargic (Hh): 1 lethargic (hh), with no viable hyperactive (HH) offspring.

Key Concepts

True-breeding Strains

True-breeding strains are genetically uniform populations that consistently produce offspring with the same phenotype when crossed. In this scenario, the hyperactive male and lethargic female represent true-breeding strains, meaning their traits are stable and predictable. This concept is crucial for understanding how traits are inherited and expressed in the progeny.

Dominance and Recessiveness

In genetics, dominance refers to the phenomenon where one allele masks the expression of another allele at the same locus. In this case, the lethargic trait appears to be dominant since all progeny from the cross exhibit lethargy, regardless of the hyperactive male's genotype. Understanding dominance is essential for explaining the observed phenotypes in the offspring.

F₂ Generation and Mendelian Ratios

The F₂ generation refers to the second filial generation, produced by crossing two F₁ individuals. According to Mendelian genetics, the expected phenotypic ratio in a typical monohybrid cross is 3:1 for dominant to recessive traits. However, in this case, the observation that all F₂ offspring are lethargic suggests that the lethargic trait is likely homozygous dominant, indicating a need for further testing to confirm the genetic basis.