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. <span>Step 1: Identify the phenotypes and genotypes of the parental generation (P generation). The male mouse is hyperactive, and the female mouse is lethargic. Since they are true-breeding, the male is homozygous for the hyperactive allele (let's denote it as HH) and the female is homozygous for the lethargic allele (let's denote it as LL).</span>
2. <span>Step 2: Determine the genotype of the F₁ generation. Since all progeny are lethargic, the lethargic trait is dominant. Therefore, the F₁ generation must be heterozygous (HL) and express the lethargic phenotype.</span>
3. <span>Step 3: Analyze the F₂ generation. Since all offspring in the F₂ generation are lethargic, it suggests that the lethargic allele is dominant and the hyperactive allele is recessive. This implies that the F₁ generation (HL) crossed with itself (HL x HL) results in all lethargic offspring, indicating a possible lethal effect of the homozygous recessive genotype (HH).</span>
4. <span>Step 4: Propose a test cross to confirm the hypothesis. Cross an F₁ lethargic mouse (HL) with a homozygous hyperactive mouse (HH). If the hypothesis is correct, the offspring should all be lethargic (HL), confirming the lethargic allele's dominance and the recessive lethality of the hyperactive allele.</span>
5. <span>Step 5: Consider alternative explanations. If the test cross does not yield the expected results, consider other genetic mechanisms such as epistasis or incomplete dominance that might explain the observed phenotypes.</span>

Key Concepts

True-breeding Strains

True-breeding strains are genetically uniform populations that consistently produce offspring with the same phenotype when self-crossed. In this scenario, the hyperactive male and lethargic female represent two distinct true-breeding strains, which helps establish the genetic basis for the traits being studied. Understanding true-breeding is crucial for predicting the inheritance patterns in subsequent generations.

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, if lethargy is a dominant trait, it would explain why all progeny from the cross exhibit lethargy, regardless of the hyperactive male's genotype. Recognizing the dominance relationship between traits is essential for interpreting the results of genetic crosses.

F₂ Generation and Mendelian Ratios

The F₂ generation refers to the second filial generation, which results from 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 a different inheritance pattern, possibly indicating that lethargy is a trait governed by multiple alleles or epistatic interactions.