Question

A woman with normal vision marries a man with normal vision and gives birth to a colour blind son. Her husband dies and she marries a colour blind man. What is the probability of her children having the abnormality?
A. $50%$ colour blind sons $+50%$ colour blind daughters
B. All sons colour blind and daughter carrier
C. All daughters colour blind and sons normal
D. $50%$ sons colour blind and all daughters normal

Answer 1

Hint: The most common cause of colour blindness is that it is an inherited disease. Among humans, males are more likely to be colour blind than females because the gene responsible for the most common form of colour blindness is on the X-chromosome.

Complete answer: Colour blindness is an inheritable X- chromosome-linked recessive disorder. Son has an X and a Y chromosome and gets its X- chromosome from the mother, while gets the Y-chromosome from the father, hence, the normal mother of a colour blind son will be a carrier $\left( {{\text{X}}^{\text{c}}}\text{X} \right)$. Females have two X-chromosomes, so a defect in one type is typically compensated for by the other. Non-colour blind females can carry genes for colour blindness and pass them on to their children. Males have only one X-chromosome and therefore always express the genetic disorder if they have a recessive gene. Among the children of a carrier female $\left( {{\text{X}}^{\text{c}}}\text{X} \right)$ and colour-blind man $\left( {{\text{X}}^{\text{c}}}\text{X} \right)$, $50%$ sons and $50%$ daughters will be affected.
So, the correct answer is option A.

Additional information: Most people with colour vision deficiency have difficulty distinguishing between shades of red, yellow and green. This is known as “red-green” colour vision deficiency. It’s a common problem that affects around $1$ in $12$men and $1$ in $200$women. Common symptoms of colorblindness include:
a) Difficulty distinguishing between colours.
b) Inability to see shades or tones of the same colour.

Note: Colour blindness can also result from the physical or chemical damages to the eye, optic nerve or parts of the brain. Diagnosis is typical with the ‘Ishihara’ colour test. Other methods include genetic testing.