Red–green colour blindness is inherited as a recessive X-linked trait.
Q.49 What will be the probability of having the colour-blind daughter to a phenotypically normal woman, who already had one colour-blind son, and is married to a colour-blind man?
Probability of Color-Blind Daughter in X-Linked Red-Green Color Blindness Inheritance
Red-green color blindness follows X-linked recessive inheritance, where the woman must be a carrier due to her color-blind son, and the color-blind husband contributes the recessive allele. The probability of a color-blind daughter is 50%. This detailed genetic analysis explains the Punnett square outcomes and evaluates all options.
Genetic Background
Red-green color blindness is an X-linked recessive trait, denoted as Xc for the recessive allele and X for the dominant normal allele. Males (XY) express the trait with XcY, while females (XX) need XcXc to be affected; XXc females are carriers with normal vision.[web:1][web:3]
Mother’s Genotype
The phenotypically normal woman had a color-blind son (XcY), so she must carry Xc (genotype XXc). She passed Xc to that son, confirming she is not XX. Her normal phenotype requires one normal X allele.[web:1]
Father’s Genotype
The husband is color-blind, so his genotype is XcY. He passes Xc to daughters and Y to sons.[web:4]
Punnett Square Analysis
Cross XXc × XcY:
X |
Xc |
|
|---|---|---|
Xc |
XXc(carrier female) |
XcXc(color-blind female) |
| Y | XY(normal male) |
XcY(color-blind male) |
- Daughters: 50%
XXc(normal), 50%XcXc(color-blind). - Probability of color-blind daughter:
1/2 = 50%.[web:4][web:1]
Option Evaluation
Real-World Implications
In families with X-linked recessive traits like red-green color blindness, genetic counseling aids probability assessment. Males show the trait 8% vs. 0.5% in females globally.[web:7][web:9]
