Red–green colour blindness is inherited as a recessive X-linked trait.
Q.48 What will be the probability of having the colour-blind son to a woman with phenotypically normal parents and a colour-blind brother, and married to a normal man?
(Assume that she has no previous children)
Red-Green Color Blindness Probability Carrier Mother Normal Father
Correct Answer: (B) 50%
The woman is an obligate carrier (XᶜX) because her color-blind brother inherited Xᶜ from their mother. Married to a normal man (XY), each son has a 50% chance of inheriting her Xᶜ chromosome and being color-blind (XᶜY).
🔬 Detailed Pedigree Analysis & Probability Calculation
Family Genetics:
- Mother’s genotype: Must be XᶜX (carrier) since son is color-blind (XᶜY)
- Woman: Inherits one X from mother → 100% chance XᶜX (obligate carrier)
- Husband: XY (normal vision)
- Son possibilities: XᶜY (50%, color-blind) or XY (50%, normal)
Punnett Square:
| X | Y | |
|---|---|---|
| Xᶜ | XᶜX | XᶜY (colorblind) |
| X | XX | XY (normal) |
Option Analysis
| Option | Probability | Explanation |
|---|---|---|
| (A) | 100% | Incorrect. Sons have 50% chance of normal XY genotype. |
| (B) | 50% | Correct. Standard X-linked recessive carrier × normal male produces 50% affected sons. |
| (C) | 25% | Incorrect. Applies to autosomal recessive traits. |
| (D) | 12.5% | Incorrect. Would require both parents as carriers (1/2 × 1/2 × 1/2). |
Key Insights:
Red-green colour blindness X-linked recessive inheritance probability calculation for carrier mother with colour-blind brother defines Q.48 genetics problem essential for biotechnology pedigree analysis.
Bayesian Carrier Risk Confirmation
Prior: Normal parents → mother carrier probability = 1/2 (general population)
Brother color-blind: Updates to posterior = 1 (obligate carrier)
P(carrier|brother) = P(brother|carrier) × P(carrier) / P(brother)
= (1/2 × 1/2) / (1/2) = 1
X-Linked Pedigree Pattern
Key Indicator: Affected male → all sisters are obligate carriers (100%).
🎯 Probability Cascade
- Maternal grandmother carrier: 1/2
- Mother inherits Xᶜ: 1/2 → but brother confirms (1)
- Woman inherits Xᶜ: 1 (from carrier mother)
- Son inherits Xᶜ: 1/2
Progeny Outcomes:
- Sons: 50% color-blind (XᶜY)
- Daughters: 50% carriers (XᶜX), all phenotypically normal
