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.

Answer:

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

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