Q53. The possible genotypes of endosperms borne on a heterozygous (Rr) plant will be
Endosperm in angiosperms is triploid (3n), formed by the fusion of two polar nuclei from the mother plant and one sperm from the pollen. For a heterozygous (Rr) self-pollinating plant, the maternal contribution is two R or r alleles (RR or rr or Rr), while the paternal adds one R or r, yielding four possible endosperm genotypes.
Question Breakdown
This multiple-choice question tests understanding of endosperm genetics in plants, relevant for exams like GATE Life Sciences.
Option Analysis
-
(A) RRR, RRr, Rrr, rrr: Correct. Maternal gametes (central cell): 1/4 RR, 1/2 Rr, 1/4 rr. Paternal pollen: 1/2 R, 1/2 r. Combinations: RR + R = RRR (1/8), RR + r = RRr (1/8), Rr + R = RRr (2/8), Rr + r = Rrr (2/8), rr + R = Rrr (2/8), rr + r = rrr (1/8). All four appear.
-
(B) RRR, Rrr: Incorrect. Misses RRr and rrr; assumes only extreme dosages.
-
(C) RR, Rr, rr: Incorrect. These are diploid embryo genotypes, not triploid endosperm.
-
(D) Rr: Incorrect. Single diploid genotype; ignores triploidy and variation.
The genotypes of endosperms on a heterozygous Rr plant form a key concept in plant genetics for competitive exams like GATE Life Sciences. This triploid tissue influences seed traits, and understanding its formation from double fertilization is crucial for scoring in genetics sections.
Endosperm Formation Basics
In angiosperms, endosperm arises from two maternal polar nuclei (2n) fusing with one paternal sperm (n), creating 3n tissue. For a selfed Rr plant, female gametophyte central cells carry: ¼ RR, ½ Rr, ¼ rr. Male gametes are ½ R, ½ r. This yields all dosage combinations.
Punnett Square for Endosperms
| Maternal (2n) | Paternal (n) R (½) | Paternal (n) r (½) |
|---|---|---|
| RR (¼) | RRR (⅛) | RRr (⅛) |
| Rr (½) | RRr (¼) | Rrr (¼) |
| rr (¼) | Rrr (⅛) | rrr (⅛) |
Ratios: RRR ⅛, RRr ¼, Rrr ½, rrr ⅛. Option (A) lists distinct genotypes accurately.
Why Other Options Fail
Option (B) omits balanced heterozygotes like RRr. (C) confuses with embryo ratios (1:2:1 diploid). (D) ignores triploidy. Common trap for students mixing embryo/endosperm genetics.
This matches GATE Life Sciences patterns, emphasizing dosage effects in triploids for traits like seed color or dormancy.
