Question

8. The gene for colourblindness is 15 map units from the gene for muscular dystrophy, and both genes are on the X-chromosome. Recessive mutations result in colourblindness or muscular dystrophy. A woman whose father was colourblind and whose mother's father had muscular dystrophy marries a colourblind man. Their first son is colourblind. The probability that he also has muscular dystrophy is: a) 50% b) 7.5% c) 3.75% d) 15% e) none of the above

Answer 1

Ans C 3.75%

Women will be always carrier for both disease as it is running in her family and male is color blindness

XX^{​​​​​cm.}  X X^cY

I

XX^{​​​​​C.} X^{​​​​cm}​​​​X^c XY. X^{​​​​​​Cm}​Y

So half of son is color blindness which probability is 1/2 and the probabilty of same son to be muscular dystrophy is 1/2

So probabilty of son to have both color blindness and muscular dystrophy is 1/2x1/2=1/4

And distance between two gene is 13cM means 13 % so final probabilty will be 1/4x 13%=3.75%

Hope it's clear..thanks