Chapter 13, Problem 5TYK

Summary Introduction

To review:

The phenotypic ratio for eye color in the two sexes of the offspring. White-eyed Drosophila, which is female, is crossed with a male that is normal red-eyed. Thereafter, the F₁ female progeny is crossed with her father, and the ${\text{F}}_1$ male progeny is crossed with his mother.

Introduction:

When the genes are present on X-chromosomes, it is known as X-linked trait. The example can be seen in Drosophila, where the genes for colored eyes are present on the X-chromosome. The red-colored eye is dominant in Drosophila over the white-colored eye.

Explanation of Solution

Genes for colored eyes in Drosophila are present on the X-chromosome. The white-colored eyes is an X-linked recessive trait found in Drosophila. The red-colored eyes are dominant in them.

Let ${\text{X}}^{w^{-}}{\text{X}}^{w^{-}}$ represent white-eyed female and ${\text{X}}^{w^{+}}\text{Y}$ represent red-eyed male.

The cross can be made as:

Parents ${\text{X}}^{w^{-}}{\text{X}}^{w^{-}}\times {\text{\hspace{0.17em}\hspace{0.17em}X}}^{w^{+}}\text{Y}$

Gametes ${\text{X}}^{w^{-}},{\text{X}}^{w^{-}}$ ${\text{X}}^{w^{+}}\text{,Y}$

${\text{F}}_1$ progeny

Gametes	${\text{X}}^{w^{+}}$		$\text{Y}$
${\text{X}}^{w^{-}}$	${\text{X}}^{w^{-}}{\text{X}}^{w^{+}}$ (red eyes)		${\text{X}}^{w^{-}}\text{Y}$ (white eyes)
${\text{X}}^{w^{-}}$	${\text{X}}^{w^{-}}{\text{X}}^{w^{+}}$ (red eyes)		${\text{X}}^{w^{-}}\text{Y}$ (white eyes)

F₁ female progeny is crossed with her father:

The cross can be made as:

Parents ${\text{X}}^{w^{-}}{\text{X}}^{w^{+}}\times {\text{\hspace{0.17em}\hspace{0.17em}X}}^{w^{+}}\text{Y}$

Gametes ${\text{X}}^{w^{-}},{\text{X}}^{w^{+}}$ ${\text{X}}^{w^{+}}\text{,Y}$

F₁ progeny

Gametes	${\text{X}}^{w^{+}}$	$\text{Y}$
${\text{X}}^{w^{-}}$	${\text{X}}^{w^{-}}{\text{X}}^{w^{+}}$ (red)	${\text{X}}^{w^{-}}\text{Y}$ (white)
${\text{X}}^{w^{+}}$	${\text{X}}^{w^{+}}{\text{X}}^{w^{+}}$ (red)	${\text{X}}^{w^{+}}\text{Y}$ (red)

So, the ratio of female progenies, red to white is 2:0 and for male progenies, red to white is 1:1.

Now, ${\text{F}}_1$ male progeny is crossed with his mother.

The cross can be made as follows:

Parents ${\text{X}}^{w^{-}}{\text{X}}^{w^{-}}\times {\text{\hspace{0.17em}\hspace{0.17em}X}}^{w^{-}}\text{Y}$

Gametes ${\text{X}}^{w^{-}},{\text{X}}^{w^{-}}$ ${\text{X}}^{w^{-}}\text{,Y}$

${\text{F}}_1$ progeny

Gametes	${\text{X}}^{w^{-}}$	$\text{Y}$
${\text{X}}^{w^{-}}$	${\text{X}}^{w^{-}}{\text{X}}^{w^{-}}$ (white)	${\text{X}}^{w^{-}}\text{Y}$ (white)
${\text{X}}^{w^{-}}$	${\text{X}}^{w^{-}}{\text{X}}^{w^{-}}$ (white)	${\text{X}}^{w^{-}}\text{Y}$ (white)

Therefore, the ratio of red to white female progenies is $0:2$, and that of red to white male progenies is $0:2$.

Conclusion

Thus, it can be concluded that when the F₁ female progeny is crossed with her father, the ratio of female progenies, red to white, is 2:0, whereas, for male progenies, the ratio of red to white is 1:1. When the F₁ male progeny is crossed with his mother, the ratio of female progenies, red to white, is 0:2, whereas, for male progenies, the ratio of red to white is 0:2.