While vermilion is X-linked in Drosophila and causes the eye color to be bright red, brown is an autosomal recessive mutation that causes the eye to be brown. Flies carrying both mutations lose all pigmentation and are white-eyed. Predict the F₁ and F₂ results of the following crosses:
white females x wild-type males

Verified step by step guidance

1. Step 1: Define the alleles and their inheritance patterns. Vermilion (v) is X-linked, with the wild-type allele (V) dominant over vermilion (v). Brown (b) is autosomal recessive, with the wild-type allele (B) dominant over brown (b). The white-eyed phenotype occurs when both mutations (v and b) are present together, resulting in loss of pigmentation.
2. Step 2: Determine the genotypes of the parental flies. The white-eyed females must be homozygous for brown (bb) and carry the vermilion mutation on both X chromosomes (X\^v X\^v). The wild-type males have normal alleles: X\^V Y for the X-linked gene and BB or Bb for the autosomal gene (assuming homozygous dominant BB for simplicity).
3. Step 3: Predict the F₁ genotypes by performing the cross. For the X-linked gene, females contribute X\^v, males contribute X\^V or Y. For the autosomal gene, females contribute b, males contribute B. Write out the possible combinations of alleles for the F₁ offspring, considering sex and autosomal genotype.
4. Step 4: Determine the F₁ phenotypes based on the genotypes. Remember that vermilion is X-linked recessive, so only males with X\^v Y will show vermilion eyes, and brown is recessive, so only bb individuals show brown eyes. Also, individuals with both mutations (v and b) will be white-eyed.
5. Step 5: Predict the F₂ generation by crossing F₁ individuals. Set up a Punnett square for both the X-linked and autosomal genes, considering the genotypes of F₁ males and females. Calculate the expected phenotypic ratios for the F₂ offspring, including wild-type, vermilion, brown, and white-eyed flies.

Key Concepts

X-linked Inheritance

X-linked inheritance refers to genes located on the X chromosome. In Drosophila, males have one X and one Y chromosome, so a single recessive mutation on the X chromosome will express the trait in males. Females have two X chromosomes, so they can be carriers if heterozygous. This pattern affects the distribution of traits like the vermilion eye color.

Autosomal Recessive Inheritance

Autosomal recessive inheritance involves genes on non-sex chromosomes where two copies of the recessive allele are needed to express the trait. Brown eye color in Drosophila is caused by such a mutation. Heterozygous individuals are carriers without showing the phenotype, influencing the expected ratios in offspring.

Epistasis and Interaction of Multiple Genes

Epistasis occurs when one gene masks or modifies the expression of another. In this case, the combination of vermilion (X-linked) and brown (autosomal recessive) mutations results in white eyes due to loss of pigmentation. Understanding gene interactions is essential to predict phenotypic ratios in F₁ and F₂ generations.