CLASS 10 SCIENCE · HEREDITY

Heredity: Mendel's ratios and probability

Monohybrid cross (Tt × Tt): phenotype 3 : 1, genotype 1 TT : 2 Tt : 1 tt. Dihybrid cross (RrYy × RrYy): 9 : 3 : 3 : 1. Test cross (Tt × tt): 1 : 1.

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Worked examples

Example 1 · level 1

Two heterozygous tall pea plants (Tt × Tt) are crossed. What is the probability that an offspring is a plant with genotype Tt? (Give a fraction.)

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Tt × Tt → 1 TT : 2 Tt : 1 tt.
Phenotypes: 3 tall : 1 dwarf.
P(a plant with genotype Tt) = 1/2.

Answer: 1/2

Example 2 · level 2

In a school garden in Ranchi, pea plants with genotypes RR × rr are crossed (R = round-seeded, dominant). Out of 1000 offspring, how many are expected to be round-seeded?

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Offspring: all Rr.
Fraction round-seeded = 1/1.
1000 × 1/1 = 1000.

Answer: 1,000

Example 3 · level 3

Pea plants heterozygous for seed shape and colour (RrYy, round R and yellow Y dominant) are self-pollinated, giving 1600 seeds. How many are expected to be round and green?

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Fraction round and green = 3/16.
1600 × 3/16 = 300.

Answer: 300