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2020Q7(a)(ii)Genetics – Some Basic Fundamentals
Short answerHard1 mark
7(a)

A homozygous dominant tall pea plant bearing red flowers (TTRR) is crossed with a homozygous recessive dwarf pea plant bearing white flowers (ttrr).

Write the possible combination of gametes that are obtained when two F1 hybrid plants are crossed.

Topic: Gametes of a dihybrid

Previous-year question practice database

Answer

Exam answer

Each TtRr plant produces FOUR kinds of gamete: TR, Tr, tR and tr

Explanation

THE RULE: each gamete receives ONE allele from each pair. With two pairs (T/t and R/r), the number of possible combinations is 2 × 2 = FOUR.

Write them systematically so none is missed: take T with R, then T with r, then t with R, then t with r.

THE REASON there are four rather than two is MENDEL’S LAW OF INDEPENDENT ASSORTMENT — the T/t pair separates independently of the R/r pair, so T can go with either R or r.

Since each parent makes 4 kinds of gamete, the Punnett square has 4 × 4 = 16 boxes, which is where the 9:3:3:1 ratio comes from.

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