A recessive eye color mutation (se) in fruit flies leading to a color called "sepia" is determined by an autosomal gene. Another recessive mutation called "brown" (bw) causes brown eye color, and is determined by an autosomal gene on a different chromosome. Gene bw, when recessive homozygous, is epistatic to se or se+. Cross a male (whose true-breeding father had sepia eyes and true breeding mother, brown eyes) to a female of the same genotype. Give scheme of cross ratios of resulting phenotypes.

Question

A recessive eye color mutation (se) in fruit flies leading to a color called "sepia" is determined by an autosomal gene. Another recessive mutation called "brown" (bw) causes brown eye color, and is determined by an autosomal gene on a different chromosome. Gene bw, when recessive homozygous, is epistatic to se or se+. Cross a male (whose true-breeding father had sepia eyes and true breeding mother, brown eyes) to a female of the same genotype. Give scheme of cross ratios of resulting phenotypes.

Answer 1

To determine the cross ratios of resulting phenotypes, we need to analyze the genotypes of the male and female flies and understand how the alleles for eye color are inherited.

The male fly's genotype is se+/se bw+/bw, which means he carries one copy of the sepia allele (se) and one copy of the brown allele (bw). The "+" symbol represents the wild-type allele, which is dominant to both se and bw.

The female fly also has the genotype se+/se bw+/bw, meaning she has one copy of each allele.

To determine the cross ratios, we need to consider the possible combinations of alleles that can be inherited by their offspring. Let's represent the sepia allele with "s" and the brown allele with "b" for simplicity.

Since both the male and female flies are heterozygous for both alleles, there are four possible combinations of alleles that their offspring can inherit:

1. se bw (s b)
2. se bw+ (s b+)
3. se+ bw (s+ b)
4. se+ bw+ (s+ b+)

Now, let's assign phenotypes to each genotype:

1. se bw (s b) - This genotype has the sepia phenotype.
2. se bw+ (s b+) - This genotype also has the sepia phenotype.
3. se+ bw (s+ b) - This genotype has the brown phenotype.
4. se+ bw+ (s+ b+) - This genotype has the wild-type (non-sepia, non-brown) phenotype.

Based on these findings, let's calculate the cross ratios for the resulting phenotypes:

- Ratio of sepia eyes: 2 (sepia) : 1 (brown).
- Ratio of wild-type eyes: 1 (wild-type) : 2 (brown).

Therefore, the scheme of cross ratios of resulting phenotypes is 2:1:2, with 2 sepia-eyed flies, 1 brown-eyed fly, and 2 wild-type-eyed flies.