Q64. In a diploid plant species, the T allele produces tall individuals and is
completely dominant over the t allele that produces short individuals.
Similarly, the W allele produces round seeds and is completely dominant
over the w allele that produces wrinkled seeds (assume T and W loci not
linked). If a parent with TTWW genotype is crossed to another parent with
ttww genotype, the fraction of the F2 population produced by the fusion of
both recombinant gametes would be ___________. (Round-off to two
decimal places.)

The fraction of the F2 population produced by the fusion of both recombinant gametes is 0.25.

Cross Overview

A dihybrid cross between TTWW (tall, round seeds) and ttww (short, wrinkled seeds) produces F1 heterozygotes TtWw. These F1 plants self-cross to form F2, yielding 16 genotypic combinations due to independent assortment.

Gamete Types

F1 TtWw plants produce four gametes equally: TW (parental), Tw (recombinant), tW (recombinant), tw (parental), each at 1/4 probability.

Recombinant Fusion

Fusion of both recombinant gametes (Tw × tW or tW × Tw) occurs with probability (2 × (1/4) × (1/4)) = 1/8 for ordered pairs, but since Punnett square counts unique zygotes, it produces four genotypes: TTww, Ttww, TtwW, ttWW, each 1/16, totaling 4/16 = 0.25 of F2.

Phenotypic Note

These zygotes yield tall wrinkled (TTww, Ttww), short round (ttWW), and tall round (TtwW), confirming recombinant offspring from non-parental gamete unions.

Dihybrid crosses like the TTWW (tall, round) x ttww (short, wrinkled) diploid plant example test independent assortment in genetics exams such as CSIR NET Life Sciences. Understanding the fraction of F2 from recombinant gametes fusion is key for scoring in molecular biology and inheritance questions.

Step-by-Step Dihybrid Cross Solution

• P Generation: TTWW × ttww yields all TtWw F1 (tall, round).

• F1 Gametes: TtWw produces TW, Tw, tW, tw (each 25%). Tw and tW are recombinant (non-parental).

• F2 Punnett Square: 16 boxes. Both recombinant gametes fuse in 4 boxes: Tw + tW → Ttww, TTww, ttWW, TtwW (4/16 = 0.25).

• Probability Calculation: P(recombinant gamete) = 0.5, so both = 0.5 × 0.5 = 0.25.

No options provided, but common distractors:

• 0.50: Confuses with single recombinant gamete proportion.

• 0.125: (1/4 × 1/4) without doubling Tw × tW and tW × Tw.

• 0.0625: Double recessive only (tw × tw).

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