Question

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 ${\mathrm{F}}_2$ generation, all offspring are lethargic. What is the best genetic explanation for these observations? Propose a cross to test your explanation.

Short answer

Answer: The inheritance pattern suggests that the lethargic allele (L) is autosomal dominant over the hyperactive allele (H). From a cross between two F1 generation mice, the expected phenotypes of the offspring would be 75% lethargic and 25% hyperactive.

Step-by-step solution

In the initial cross, we have a true-breeding hyperactive male mouse and a true-breeding lethargic female mouse. Since both parents are true-breeding, their genotypes can be represented as HH (for hyperactive) and LL (for lethargic), respectively. #Step 2: Determine the F1 generation genotypes and phenotypes#

When crossing the parents (HH x LL), all the offspring in the F1 generation will have the genotype HL or LH (both representing the same heterozygous state). As all F1 offspring are lethargic, we can infer that the lethargic trait (L) is dominant over the hyperactive trait (H). #Step 3: Analyze the F2 generation and confirm the inheritance pattern#

Since all F2 generation offspring are lethargic, this implies a 100% chance of inheriting the dominant lethargic trait (L). This indicates that the lethargic allele is likely autosomal dominant, meaning that the presence of one or two lethargic alleles (L) will result in a lethargic phenotype. #Step 4: Propose a cross to test the genetic explanation#

To test whether the lethargic allele (L) is indeed autosomal dominant over the hyperactive allele (H), we can design a cross between two F1 generation mice (both with genotype HL or LH). The resulting offspring should have the following genotypes and phenotypes: Expected genotypes: - 25% LL (homozygous lethargic, predicted to exhibit the lethargic phenotype) - 50% LH or HL (heterozygous, also predicted to exhibit the lethargic phenotype) - 25% HH (homozygous hyperactive, predicted to exhibit the hyperactive phenotype) Expected phenotypes: - 75% lethargic (LL and LH/HL offspring) - 25% hyperactive (HH offspring) If the observed offspring phenotypes match these expected ratios, it would support our hypothesis that the lethargic allele (L) is autosomal dominant over the hyperactive allele (H).