Listed below are three different families with genotypes related to colour blindness which is an X-linked recessive disorder.
Family 1: mother XᴿXʳ, father XᴿY
Family 2: mother XʳXʳ, father XᴿY
Family 3: mother XᴿXʳ, father XʳY
Among the families, which ones can have a colour-blind daughter?
A) 2 and 3
B) 1 and 2
C) 3
D) 1
E) 2
IMAT 2023 · Question 17
Q17 - Inheritance patterns
Pinned
The correct answer is C) 3.
A daughter always gets her father's X. For her to be colour-blind (XʳXʳ), the father must be XʳY and the mother must be able to pass on Xʳ.
Families 1 and 2 have an XᴿY father, so their daughters always get a normal X: at worst they're carriers. Only family 3 has an XʳY father, and the carrier mother gives a 50% chance of an XʳXʳ daughter.
Only family 3.
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