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.

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

Explanation:

Available data:

              Males: 39 red eyes; 11 sepia eyes

77% of the total progeny has red eyes and 23% has sepia eyes.

Among females, 75% is red-eyed, and 25% is sepia-eyed

Among males, 78% is red-eyed, and 22% is sepia-eyed.

The general phenotypic ratio is 3:1

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

       Phenotypic ratio 3:1

       Among females

        Phenotypic ratio 3:1

      Among males

       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

dz nuts.   eeneneneeenenne

b is the best answer for this question.