Question

A colour blind daughter born in case of
a. Colour blind mother, Normal father
b. Carrier mother, colour blind father
c. Normal mother, colour blind father
d. Carrier mother, normal father

Answer 1

Hint: Colour blindness is a recessive sex-linked disease in human beings. The gene for colour blindness is present on X chromosome only. In females, colour blindness visible only when both the sex chromosomes carries recessive gene.

Complete answer:
Color blindness is a human disease that causes the loss of eyes ability to differentiate between red and green colour. It is considered as an X- linked recessive disease. But the genes which are responsible for normal vision are always dominant. The normal gene and its recessive alleles are carried by X chromosome.

In case of women, colour blindness is occur only when both the sex chromosomes (XX) carries recessive gene. In many cases it is possible that females may have normal vision (XXc) but it may function as a carrier only if a single recessive gene for colour blindness is present. On the other hand, in case of human males the colour blindness appears by the presence of a recessive gene (XcY). Colour blindness shows criss-cross inheritance like other sex-linked traits in which cross between a carrier mother and a colour blind father, there will be a 50% chance that their sons will be color blind and however, daughters will have a 50% chance of being color blind and 100% chance of gene carrier.

This can be understood by the help of a diagram in which a father is colour blind and mother is a carrier of colour blind gene (that means only one X carries the colour blind gene). After fertilization the offering either carrier or blind depends on the transfer of genes.

As we know it is a recessive disease, male have only one X chromosome and it has only 50% chances of being blind but in the case of daughters, there is a probability of 50% being colour blind but it would definitely be a carrier.

Hence, the correct answer is option (B).

Note: Carrier mother and colour blind father. It is a sex-linked recessive disease that carried by X- chromosomes only and hence a daughter can be colour blind if father is colour blind and mother is either colour blind or caries the colour blindness gene.