Question

Colorblindness is a recessive, X chromosome sex-link disorder. Cross the parents to figure out which percentage of their offspring will be colorblind.
A) 0%
B) 25%
C) 50%
D) 75%

Answer 1

Answer:

B) 25%

Explanation:

X-linked disease is a disease caused by mutation of gene on X chromosome.

Color blindness is X-linked recessive inherited disorder. It occurs mostly in male population. Colorblindness represents the decreased ability to see color or differences in color. The most common cause of color blindness is due to an error in the development of one or more of the three sets of color sensing cones in the eye. There are three types of cones that are responsible for trichromacy in color perception. If there is a defect in these retinal cells and cones, it results in congenital color vision defect (CVD).

It is  transmitted through females. In case of a mutant homozygous chromosome, it leads to the onset of disease in male in greater proportion than female, it needs 2 mutant X chromosome to cause disease. It is known that heterozygous female are carriers of these disease. All sons of an affected mother are affected, thus there is a 100% chance that the son will be color blind. The daughter will have normal vision but carry the recessive trait.

The male’s sperm carries one of the three gene combinations for color blindness such as: 1. An X chromosome with the gene for normal vision (X⁺). 2. An X chromosome with the gene for colorblindness (X⁰) and/or 3. Y chromosome. In the female egg, it is carried one of the two combinations: 1. An X chromosome with the gene for normal vision X⁺, 2. An X chromosome with the gene for colorblindness (X⁰), without the Y chromosome.

There are four different possible outcomes in case of the offsprings: X⁰Y color blind boy (1/4 or 25%), X⁺Y normal vision boy (1/4 or 25%), X⁰Y⁰ color-blind girl (1/4 or 25%) and X⁺Y⁰ heterozygous normal vision girl (1/4 or 25%). In this case, the heterozygous X⁺Y⁰ offspring is the carrier for the recessive gene for colorblindness. Thus, this gene will be passed onto 50% of her male offspring and 50% of her female offspring.

In certain disease, heterozygous have a  certain degree of disease due to differences in X chromosome inactivation.