15. Mendel during his experiment on garden pea observed F2 ratio of 3:1 on selfing round seed plants where 3 were round and 1 was wrinkle. If all round seed plants are selfed to raise the generation the observed ratio will be
(1) Half of plants with round seed and rest with wrinkled seed
(2) 3/4 of plants with round seed and 1/4 with wrinkled Seeds
(3) All plants with round seeds
(4) 5/6 of plants with round seeds and 1/6 of plants with wrinkled seeds
Detailed explanation
Trait: seed shape in pea
- Round = dominant (R)
- Wrinkled = recessive (r)
F2 from standard monohybrid cross (Rr × Rr)
Genotypic ratio: 1 RR : 2 Rr : 1 rr
Phenotypic ratio: 3 round (RR + Rr) : 1 wrinkled (rr)
Among the 3 round F2 plants:
- 1 is RR (true-breeding round)
- 2 are Rr (heterozygous round)
All 3 round classes are selfed
RR selfed: RR × RR → all progeny RR (all round)
Each Rr selfed: Rr × Rr → genotypes: 1 RR : 2 Rr : 1 rr
Phenotypes: 3 round : 1 wrinkled
Weighted proportion from all F2 round plants
Among F2 rounds: 1/3 RR, 2/3 Rr
RR selfing → 0 wrinkled
Rr selfing → 1/4 wrinkled
Weighted wrinkled fraction: (2/3) × (1/4) = 2/12 = 1/6
Round fraction: 1 − 1/6 = 5/6
Question options
- Half of plants with round seed and rest with wrinkled seed
- 3/4 round and 1/4 wrinkled
- All round
- 5/6 round and 1/6 wrinkled
Only option (4) matches the proper weighted calculation [web:17][web:18].
Why other options are wrong
- (1) Half round, half wrinkled: would require all parents to be heterozygous
- (2) 3/4 round, 1/4 wrinkled: from selfing a single heterozygous plant only
- (3) All round: would occur only if every F2 round plant were RR
Therefore, the correct option is (4) 5/6 round seeds and 1/6 wrinkled seeds.
