Q.98 A newly discovered, recessively-inherited disease-susceptibility trait (DS) is observed only in
cotton plants with white flowers, although the flower colour (R) and DS are independently
inherited. In a breeding programme, one variety that is homozygous for the absence of DS, but
heterozygous for R was mated to another having white flowers but heterozygous for DS. What is
the probability that a given plant among the cross progeny will be susceptible to the disease?
(A) 25 % (B) 12.5 % (C) 75 % (D) 0 %
The recessive disease susceptibility cotton plants white flowers scenario tests epistasis in plant breeding crosses. DS trait appears only in homozygous recessive white flower genotype (rr), despite independent inheritance of flower color (R) and DS loci.
Cross Setup and Genotypes
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Parent 1: Homozygous absence DS (DD), heterozygous flower color (Rr) → Rr DD
-
Parent 2: White flowers (rr), heterozygous DS (Dd) → rr Dd
-
Gametes: Parent 1 (RD, rD); Parent 2 (rD, rd)
Progeny Punnett square (independent assortment):
| rD (Parent 2) | rd (Parent 2) | |
|---|---|---|
| RD (P1) | Rr DD | Rr Dd |
| rD (P1) | rr DD | rr Dd |
Genotypic ratios: 25% Rr DD, 25% Rr Dd, 25% rr DD, 25% rr Dd.
Disease Expression Rule
DS observed only in white flowers (rr). Thus, susceptible plants must be rr dd (recessive homozygous for both). From progeny:
-
Only rr dd expresses disease (none produced here).
-
rr progeny: 50% (rr DD + rr Dd), but all carry ≥1 D allele → disease-free.
Probability Calculation
No progeny are rr dd → probability = 0%. DS requires rr (25% chance) × dd (25% chance from Dd × DD), but Parent 1 contributes only D gametes, yielding 0% dd overall.
Option Analysis
(A) 25% Incorrect. Matches single-locus recessive (dd) but ignores rr requirement and cross specifics.
(B) 12.5% Incorrect. Would be (1/4 rr) × (1/2 dd from Dd self), but Parent 1 blocks dd.
(C) 75% Incorrect. Might reflect dominant expression, opposite of recessive DS.
(D) 0% Correct. No rr dd progeny possible; all rr plants inherit D from Parent 1.
| Condition for DS | Progeny Frequency | Reason Excluded |
|---|---|---|
| rr (white) | 50% | Must also be dd |
| dd (susceptible) | 0% | Parent 1 DD only |
| rr dd (both) | 0% | Correct answer |
GATE Genetics Insights
This dihybrid epistasis problem (conditional recessive) emphasizes reading “observed only in” as phenotype modifier. Practice similar plant breeding PYQs: cotton fiber color follows similar dominance patterns.
