A tall pea plant with genotype TT is crossed with a short plant (tt) to produce F1 plants (Tt), which are all tall. The F1 plants are then self-pollinated to produce F2 offspring. Answer the following:
(a) How is the gene T transmitted from the parent plant to a germ cell? What happens to the chromosome pair carrying this gene during germ cell formation?
(b) In the F1 plant (Tt), both T and t alleles are present. Why is only the tall trait expressed? What determines whether a trait is expressed or hidden?
(c) In the F2 generation, a plant inherits alleles for both height (T/t) and seed colour (Y/y), which are located on different chromosomes. Explain how the chromosome mechanism during germ cell formation allows these two traits to be inherited independently.
Generated by claude-sonnet-4-6 · 2026-06-26 01:05 · grounding rag
Model Answer
(a) Gene T is located on a chromosome. During germ cell (gamete) formation, the process of meiosis occurs, in which the chromosome pair carrying gene T separates. Each germ cell receives only one chromosome from the pair — so it carries either T or t, not both. This ensures each gamete has a single copy of the gene.
(b) In the F1 plant (Tt), only the tall trait is expressed because T is dominant over t. A single copy of T is sufficient to produce the tall phenotype. The trait 't' (shortness) is recessive — it is hidden but not lost. Whether a trait is expressed or hidden depends on whether it is dominant or recessive; recessive traits are expressed only when both copies are identical (tt).
(c) When genes for height (T/t) and seed colour (Y/y) are on different (non-homologous) chromosomes, they assort independently during meiosis. The chromosome pairs separate independently of each other, so all combinations — TY, Ty, tY, ty — are equally possible in gametes. This independent assortment produces new trait combinations in F2 offspring, as Mendel demonstrated.
Source: Chapter 8, Section 8.2.2
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Explanation
- (a) Examiners expect the term "meiosis" and the idea of separation of chromosome pairs so each gamete gets one allele.
- (b) Key terms: dominant, recessive. Quote the textbook logic: "a single copy of T is enough to make the plant tall; both copies must be t for the plant to be short."
- (c) The phrase "independent assortment" (or independent inheritance) and the reason — genes on different chromosomes separate independently — are the scoring points. Mention new combinations in F2 for full marks.