Q.100 Red-Green color blindness is an X-linked recessive disorder. In a population which is in the Hardy-
Weinberg equilibrium, the incidence of occurrence of this in males is 1:1000. What will be the
expected incidence of affected homozygous females?
(A) 1 in 1002000 (B) 1 in 2000000 (C) 1 in 1001000 (D) 1 in 1000000
Red-green color blindness females incidence in Hardy-Weinberg equilibrium is crucial for GATE Life Sciences. With male incidence 1:1000 (X-linked recessive), calculate affected homozygous females using allele frequency q.
Hardy-Weinberg for X-Linked Traits
For X-linked recessive traits, males (XY) are affected if they inherit the recessive allele (frequency q), so q = incidence in males = 1/1000.
Females (XX) are affected only if homozygous recessive (q²).
Thus, female incidence = (1/1000)² = 1/1,000,000.
Option Analysis
| Option | Incidence | Explanation |
|---|---|---|
| (A) 1 in 1002000 | 1/1,002,000 | Incorrect; no basis in calculation. |
| (B) 1 in 2000000 | 1/2,000,000 | Incorrect; confuses with carrier frequency 2pq ≈ 2/1000 = 1/500. |
| (C) 1 in 1001000 | 1/1,001,000 | Incorrect; arithmetic error. |
| (D) 1 in 1000000 | 1/1,000,000 | Correct; q² = (1/1000)². |
X-Linked Recessive Genetics
Red-green color blindness follows X-linked recessive inheritance: males express with one mutant allele, females need two.
Allele frequency q = male incidence = 1/1000; p = 1 – q ≈ 0.999.
Calculation Steps
- q = 0.001 (from males).
- Female affected = q² = 0.001² = 0.000001 = 1/1,000,000.
| Genotype | Males Frequency | Females Frequency |
|---|---|---|
| Normal | p = 0.999 | p² ≈ 0.998 |
| Carrier | – | 2pq ≈ 0.002 |
| Affected | q = 0.001 | q² = 0.000001 |
Option Breakdown
- (A)/(C): Invalid approximations.
- (B): Matches ~q/2 or carrier error.
- (D): Exact q².
