Q1. [5] deep thorough-understanding
A plant hormone is produced by an enzyme encoded by gene T. Plants with genotype TT produce a large amount of this hormone, Tt plants produce a moderate amount, yet both TT and Tt plants are equally tall. Plants with genotype tt produce very little hormone and are short.
(i) Using this information, explain at the molecular level why T is dominant over t.
(ii) In a cross between two Tt plants, what fraction of the offspring would be short? Show the cross.
(iii) If variations in height arose in an asexually reproducing plant population, what would be the likely source of those variations?
Generated by claude-sonnet-4-6 · 2026-06-26 01:05 · grounding rag
Model Answer
(i) Gene T encodes an enzyme that produces a plant hormone. In genotype TT, two functional copies produce a large amount of hormone. In Tt, one functional copy (T) produces a moderate amount — which is still enough to make the plant tall. Since even one copy of T produces sufficient hormone for full height, T is dominant. In tt, no functional enzyme is produced, so hormone level is too low and the plant is short. Thus, dominance of T is due to the sufficiency of a single functional allele to produce the required hormone.
(ii) Cross: Tt × Tt
| | T | t |
|---|----|----|
| T | TT | Tt |
| t | Tt | tt |
Offspring ratio: 1 TT : 2 Tt : 1 tt
Short plants = tt → 1/4 of the offspring would be short.
(iii) In asexually reproducing plants, variations cannot arise from genetic recombination (no sexual reproduction). The likely source of variation would be mutations — random changes in DNA during cell division (DNA copying errors).
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Explanation
- Part (i) tests molecular understanding of dominance: one functional allele of T produces enough enzyme/hormone for full height — this is the key molecular reason. Don't just say "T is dominant because it is expressed."
- Part (ii) always show the Punnett square; the answer 1/4 (25%) must follow logically from it.
- Part (iii) is directly from the chapter concept: asexual reproduction produces very little variation; what variation does arise comes from DNA copying errors/mutations, not recombination.