Question

Both husband and wife have normal vision though their fathers were colour blind.
The probability of their daughter becoming colour blind is?
A. $0\%$
B. $25\%$
C. $50\%$
D. $75\%$

Answer 1

Colour blindness: Colour blindness is a recessive sex-linked trait in which the eye fails to distinguish red and green colours. The gene for normal vision is dominant The normal gene and its recessive allele are named by X-chromosome.

Complete answer:
In female, colour blindness appears only when both the sex chromosomes carry the recessive gene $\left( {XcXc.{\text{ }}} \right)$The females have normal vision but function as carrier if a single recessive gene for colour blindness is present $\left( {XXc} \right).$However, in human males the defect appears in the presence of a single recessive gene $\left( {Xc} \right)$because Y-chromosomes of males do not carry any gene in colour Vision. Male transmits his trait to his grandson through daughter, while a female transmits the traits to her grand daughter through her son or it is transfer of trait from one sex to the offspring of the opposite sex.
Now as it is cross linked inheritance the traits of father of female will be transferred to his grandson and not granddaughter And father transmits traits through daughter so male will play no role here.
Now let us match this with options:-.
$0\%$:- this is exactly the correct option as traits of father of a female will be transferred to his grandson and not granddaughter And father transmits traits through daughter so male will play no role here.
$25\%$:- it is a cross linked inheritance so traits will be passed to opposite generations thus this option is not correct.
$50\%$:- only the son of male and female will be colour blind due to cross linked inheritance . Thus this option is not correct.
$75\%$:traits of father of a female will be transferred to his grandson and not granddaughter And father transmits traits through daughter so male will play no role here. So this option is not correct.

Our required answer is A that is $0\%$

Note: It should be very much clear, colour blindness does not mean not seeing any colour at all, it means that those who are colour-blind have trouble in seeing the differences between certain colours. They can not tell the difference between red and green . it is a cross linked inheritance so traits will be passed to opposite generations.