Question

A normal woman is married to a colour blind man. The children are expected to be
A. All normal
B. $50\% $ sons are colour blind
C. All daughters are normal but carrier whereas all sons are normal phenotypically as well as genotypically
D. $50\% $ daughters are colour blind

Answer 1

Hint:- The Human eye consists of specialized photoreceptor cells called rods and cones. Rods are responsible for night vision whereas cones are responsible for colour vision. Different cones are sensitive to different colours of light. There are three types of colour pigments present in each of the different cones, thus making the cones selectively sensitive to different colours such as blue, green or red. White colour is seen when all the three different types of cones- red, green and blue are stimulated equally.

Complete Answer:-
When even a single group of colour receptive cones is missing from the eye, the person is unable to distinguish the different types of colours. This is called Colour blindness.
It is a genetic disorder (X- linked recessive disorder) that occurs exclusively in males. That is genes in the female X chromosome code for the respective cones. In females, the chances of colour blindness are very less because they have two X chromosomes out of which at least one X chromosome almost always has a normal gene for each type of cone. As the male has only one X chromosome, a missing gene can lead to colour blindness.
If a normal woman having genotype XX gets married to a colourblind man having genotype \[{X^c}Y\], then the progenies produced may have the following genotypes - \[\;{X^c}X\] or XY. Thus, the progeny having genotype \[\;{X^c}X\] will be normal (carrier for colour blindness) and the progeny with genotype XY will be completely normal. Thus, daughters produced will be normal but carriers and the sons produced will also be normal.

So, the correct answer is option C.

Note:- A colourblind person is unable to differentiate basic colours like red, green and blue. A person without red cone cells will have red colour blindness (protanopia). Similarly, in green colour blindness (deuteranopia) and blue colour blindness (tritanopia), green cones and blue cone cells are lost respectively. Colour blindness is not a lethal disorder, so it can be found in both males and females but the chances of colour blindness in males are higher than in females. It is detected with the help of color test charts.