In Drosophila, the X-linked recessive mutation vermilion (v) causes bright red eyes, in contrast to the brick-red eyes of wild type. A separate autosomal recessive mutation, suppressor of vermilion (su-v), causes flies homozygous or hemizygous for v to have wild-type eyes. In the absence of vermilion alleles, su-v has no effect on eye color. Determine the F₁ and F₂ phenotypic ratios from a cross between a female with wild-type alleles at the vermilion locus, but who is homozygous for su-v, with a vermilion male who has wild-type alleles at the su-v locus.

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Step 1: Define the genotypes of the parents. The female is wild-type for vermilion (X^+X^+) but homozygous recessive for the suppressor (su-v/su-v). The male is vermilion mutant (X^vY) but wild-type for the suppressor (Su-v+/Su-v+).

Step 2: Determine the possible gametes from each parent. The female can produce X^+ chromosomes with su-v alleles, while the male can produce X^v or Y chromosomes with Su-v+ alleles.

Step 3: Construct the F₁ genotypes by combining the gametes. For females, combine X^+ (su-v) from the mother with X^v (Su-v+) from the father. For males, combine X^+ (su-v) from the mother with Y (Su-v+) from the father.

Step 4: Analyze the F₁ phenotypes based on the genotypes. Remember that vermilion is X-linked recessive, and su-v suppresses vermilion only when homozygous or hemizygous with vermilion. Determine which flies show wild-type or vermilion eye color.

Step 5: For the F₂ generation, consider a cross between F₁ individuals. Determine all possible allele combinations for both loci, and then predict the phenotypic ratios by applying the inheritance patterns of X-linked vermilion and autosomal suppressor su-v, including the suppression effect.

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주요 개념

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X-linked Recessive Inheritance

X-linked recessive traits are controlled by genes on the X chromosome. Males, having only one X chromosome, express the trait if they inherit the mutant allele, while females require two copies. This pattern affects phenotypic ratios differently in males and females, important for predicting offspring outcomes in crosses involving X-linked genes.

Autosomal Recessive Suppressor Mutation

A suppressor mutation is a secondary mutation that counteracts the effect of a primary mutation. When autosomal and recessive, it requires two copies to show its effect. In this case, the suppressor (su-v) masks the vermilion phenotype only when vermilion alleles are present, illustrating gene interaction and epistasis.

Genetic Crosses and Phenotypic Ratios

Analyzing genetic crosses involves determining genotypes of parents, predicting gametes, and combining them to find offspring genotypes and phenotypes. Understanding how linked and unlinked genes segregate and interact allows calculation of expected phenotypic ratios in F₁ and F₂ generations, essential for solving this problem.