Q.89 Mendel deduced the genetic principle of inheritance by experimenting on sweet pea
plants. One of the experiments involved crossing plants with two contrasting
characters, tall (dominant) and dwarf (recessive), which yielded all tall plants in the
first generation. When the same genetic cross was independently repeated by a
researcher, only short plants were obtained. Which one of the following can
possibly explain the altered outcome?
(A) Tall plants were heterozygous
(B) An enhancer for the tall allele is present in the dwarf plant
(C) A suppressor for the tall allele is present in the dwarf plant
(D) Dwarf plants are homozygous
Correct Answer: (C) A suppressor for the tall allele is present in the dwarf plant
Mendel’s monohybrid cross between pure tall (TT) and dwarf (tt) pea plants normally produces all tall F1 hybrids (Tt) due to dominance of the tall allele. The altered outcome of all short plants suggests a genetic modifier from the dwarf parent inhibiting tall allele expression in F1 heterozygotes.
Option Analysis
(A) Tall plants were heterozygous
Heterozygous tall plants (Tt) crossed with dwarf (tt) yield 50% tall and 50% dwarf F1 plants, not all short. This violates Mendel’s dominance principle where T masks t.
(B) An enhancer for the tall allele is present in the dwarf plant
An enhancer would amplify tall allele effects, producing taller F1 plants, not short ones. Enhancers promote rather than block dominant traits.
(C) A suppressor for the tall allele is present in the dwarf plant (Correct)
Suppressor mutations in the dwarf parent inhibit tall allele function in F1 (T/suppressed t), yielding all short plants. Genetic suppressors block dominant phenotypes via epistasis or pathway interference.
(D) Dwarf plants are homozygous
Homozygous dwarf (tt) is standard in Mendel’s setup and produces tall F1 with dominant tall parent. This doesn’t alter the expected outcome.
Mendel’s foundational tall dwarf cross in sweet pea plants established dominance and segregation laws, yet replicated experiments sometimes show unexpected F1 results like all short plants. This deviation highlights suppressor genetics crucial for CSIR NET Life Sciences preparation.
Understanding Mendel’s Standard Experiment
Pure tall (TT) × dwarf (tt) yields all tall F1 (Tt), with 3:1 tall:dwarf F2 ratio. Dominance ensures tall phenotype despite recessive dwarf allele presence.
Genetic Basis of Altered F1 Outcome
A suppressor gene in the dwarf parent blocks tall allele expression in heterozygotes, producing short F1 plants. Suppressors act via epistasis, inhibiting dominant pathways like GA signaling in height regulation.
Suppressor vs Enhancer Mechanisms
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Suppressors (correct explanation): Inhibit tall allele, common in modifier QTLs affecting DELLA proteins.
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Enhancers: Promote tallness, opposite of observed short phenotype.
| Mechanism | Effect on Tall Allele | F1 Phenotype |
|---|---|---|
| Suppressor | Inhibits expression | All short |
| Enhancer | Amplifies expression | Taller plants |
| Standard dominance | Masks recessive | All tall |
This table clarifies why only suppressors explain the anomaly for competitive exams like CSIR NET.
