question_answer

A man has normal red-green colour vision. His blood group is rhesus negative (homozygous recessive). His wife also has normal colour vision but is rhesus positive. She is heterozygous at both the red-green colour vision locus and the blood group locus. What is the probability that their first child will be rhesus negative, red green colourblind boy?

A) 0                                 

B) 0.0626  

C) 0.125

D) 0.26

Correct Answer: C

Solution :

[c]

A normal red-green colour vision man with \[R{{h}^{-}}\] blood group is \[XY\,R{{h}^{-}}R{{h}^{-}}.\] A normal woman with \[R{{h}^{+}}\] blood group with heterozygocity at both red-green colour vision and blood group is\[X{{X}^{C}}R{{h}^{+}}R{{h}^{-}}.\]

The inheritance of colour vision in the progeny is

i.e., 50% boys are colourblind also 50% of offspring are boys. Thus, 0.25 is the probability the first boy is colourblind.

The inheritance of Rhesus blood group in the progeny is

i.e., 50% are Rhesus positive and 50% Rhesus negative or 0.5 is the probability of the progeny being Rhesus negative.

Probability of their first child being a rhesus negative = 0.5.

Probability of their first child being a red green colourblind boy =0.25.

Therefore, probability of their first child being both \[=0.5\times 0.25=0.125.\]