3. The ABO blood type tn human is under the control of autosomal multiple alleles. Colour blindness is recessive X-linked trait. A male with a blood type A and normal vision marries a female who also has blood type A and normal vision. The couple’s first child a mate who is colour blind and has a blood group O. What is the probability that their next female child has normal vision and a blood group O?
(1) 1/4 (2) 3/4
(3) 1/8 (4) 1
Step-by-step solution
Step 1: Deductions from first child (male, colour-blind, O)
ABO locus (autosomal)
Blood group O = ii. Both type A parents must be IAi (AO) [web:129].
P(child O from IAi × IAi) = 1/4.
Colour blindness (X-linked recessive)
Let X⁺ = normal, Xᶜ = colour-blind.
Colour-blind son = XᶜY → mother carrier X⁺Xᶜ, father X⁺Y.
All daughters receive father’s X⁺ → normal vision (P=1) [web:123].
Step 2: Probability for next female child
Question interpreted as conditioned on child being female:
P(normal vision | female) = 1
P(blood group O | female) = 1/4
de>P(normal, O | female) = 1 × 1/4 = 1/4
Option-wise explanation
- 1/4: correct – among daughters, 1/4 are O [web:130]
- 3/4: fraction of type A from AO × AO (not O)
- 1/8: joint probability including sex (female × O)
- 1: ignores blood group O requirement
Therefore, probability next female child has normal vision and blood group O is 1/4.
