Select the correct answers to the questions from the given options. (Do not copy the questions, write the correct answer only.)
A family has a history of colour blindness. During genetic testing it was found that the mother is a carrier of colour blindness (XᶜX) and the father has normal vision (XY). What is the probability of their sons being colour blind?
Topic: Sex-linked inheritance — colour blindness
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Answer
(b) 50%.
Work the cross out rather than guessing.
Mother: XᶜX (carrier — she has one defective X but normal vision, because the normal allele is dominant) Father: XY (normal vision)
The four possible offspring are: XᶜX — carrier daughter, normal vision XX — normal daughter XᶜY — COLOUR BLIND SON XY — normal son
THE TRAP IS THE WORD "SONS". Of all four children, one in four is a colour-blind son, so it is tempting to answer 25%. But the question asks for the probability among the SONS — and of the two possible sons, one is colour blind. That is 50%.
Read the denominator in every probability question: "of their children" gives 25%, "of their sons" gives 50%.
WHY ONLY SONS ARE AFFECTED: a son receives his single X from his mother and a Y from his father. The Y carries no allele for colour vision, so there is nothing to mask a defective X. A daughter would need a defective X from BOTH parents to be colour blind.
50%.