Eye color in a particular strain of fly is influenced by one gene with two alleles: a dominant allele that results in red eyes and a recessive allele that results in sepia eyes. A red-eyed female from a true-breeding population is mated with a sepia-eyed male. The Fl offspring are all red-eyed. The Fl flies are allowed to interbreed, producing the following in the F2 generation.
Females: 40 red eyes; 13 sepia eyes
Males: 39 red eyes; 11 sepia eyes
Which of the following best describes the likely mode of inheritance for the eye-color gene?
a. The eye-color gene is likely autosomal because males and females have similar phenotype ratios.
b. The eye-color gene is likely autosomal because more females have sepia eyes than males do.
c. The eye-color gene is likely sex-linked because the males and females have similar phenotype ratios.
d. The eye-color gene is likely sex-linked because the males and females display both phenotypes.

If the eye-color gene was sex-linked, the phenotypic ratios would be different. There would be a higher proportion of red-eyed females than red-eyed males. Females would only express red eyes, while males would express red and sepia eyes. This is

Cross: X⁺X⁻ x X⁺Y (Note: The symbol + represents the allele R

Phenotype: red-eyed both and the symbol - the allele r)

Gametes) X⁺ X⁻ X⁺ Y

Punnett square) X⁺ X⁻

X⁺ X⁺X⁺ X⁺X⁻

Y X⁺Y X⁻Y

F2) 3/4 of the whole progeny would be red-eyed

1/4 of the whole progeny would be sepia-eyed

BUT

Females: 100% red-eyed

Males: 50% red-eyed and 50% sepia-eyed

These proportions are not coincident with the exposed real proportions

So,

c. The eye-color gene is likely sex-linked because the males and females have similar phenotype ratios.

If this gene was sex-linked, Males and females would not have similar phenotype ratios. This is FALSE.

d. The eye-color gene is likely sex-linked because the males and females display both phenotypes.

If this gene was sex-linked, Males would display both phenotypes, but females would only be red-eyed. This is also FALSE.

According to this, we can assume that this is an autosomal gene. If this is true, then

Cross: Rr XX x Rr XY

Gametes) RX, RX, rX, rX

RX, RY, rX, rY

Punnett square:

RX RX rX rX

RX RRXX RRXX RrXX RrXX

rX RrXX RrXX rrXX rrXX

RY RRXY RRXY RrXY RrXY

rY RrXY RrXY rrXY rrXY

F2) From the whole progeny

12/16 = 3/4 = 75% would be red-eyed
4/16 = 1/4 = 25% would be sepia-eyed

Phenotypic ratio 3:1

Among females

6/8 = 3/4 = 75% would be red-eyed, R-XX
2/8 = 1/4 = 25% would be sepia-eyed, rrXX

Phenotypic ratio 3:1

Among males

6/8 = 3/4 = 75% would be red-eyed, R-XY
2/8 = 1/4 = 25% would be sepia-eyed, rrXY

Phenotypic ratio 3:1

The phenotypic ratio in all cases are 3:1.

b. The eye-color gene is likely autosomal because more females have sepia eyes than males do.

Both males and females exhibit sepia eyes in equal proportions. This is FALSE

a. The eye-color gene is likely autosomal because males and females have similar phenotype ratios.

Males and females do have similar phenotype proportions of red eyes and sepia eyes. This is TRUE