A pea plant has the genotype Tt, where T (tall) is dominant over t (dwarf). Describe what happens when this plant is crossed with a homozygous recessive plant, including the genotypes and phenotypes of all possible offspring.

Pearson Edexcel International GCSE Biology (4BI1) — 3.2 Inheritance · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

In pea plants, stem height is controlled by a single gene. The allele for tall (T) is dominant over the allele for dwarf (t).

Model answer (4 marks)

Crossing a heterozygous tall plant (Tt) with a homozygous recessive dwarf plant (tt) gives two possible genotypes:
1. Tt – 50 % of the offspring. These plants are tall because the dominant T allele is expressed.
2. tt – 50 % of the offspring. These plants are dwarf as they carry only the recessive t allele.

Examiner tips

  • Use a Punnett square to show the 1:1 ratio clearly.
  • State both the genotype and the corresponding phenotype for each type of offspring.

Mark scheme (4 marks)

  1. The homozygous recessive parent has the genotype tt
  2. Offspring genotypes are Tt and tt (in equal proportions / 50% each)
  3. Tt offspring show the tall phenotype (dominant allele expressed)
  4. tt offspring show the dwarf phenotype (only recessive alleles present)

Key terms in this question

genotype · phenotype · dominant · homozygous

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