In pea plants, the allele for tall stems (T) is dominant over the allele for short stems (t). A tall pea plant is crossed with a short pea plant and approximately half the offspring are tall and half are short. Explain what the result of this cross tells us about the genotype of the tall parent plant.

Cambridge International IGCSE Biology (0610) — 17.4 Monohybrid inheritance · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The tall parent must be heterozygous (Tt). The short parent is homozygous recessive (tt). A 1:1 ratio of tall to short offspring is produced, which can only occur if the tall parent is heterozygous. If the tall parent were homozygous dominant (TT), all offspring would be tall.

Examiner tips

  • State the genotype of the tall parent first (Tt)
  • Explain that the short parent is tt and why
  • Show that a 1:1 ratio arises only from a heterozygous tall parent
  • Mention the TT scenario to demonstrate understanding

Common mistakes

  • Assuming the tall parent is TT and explaining all tall offspring
  • Not mentioning the short parent’s genotype
  • Failing to link the 1:1 ratio to heterozygosity

Mark scheme (4 marks)

  1. The tall parent must be heterozygous (Tt)
  2. The short parent must be homozygous recessive (tt), as it only has the recessive allele
  3. A 1:1 ratio of tall to short offspring is produced, which is only possible if the tall parent is heterozygous
  4. If the tall parent were homozygous dominant (TT), all offspring would be tall / show the dominant phenotype

Key terms in this question

dominant · genotype

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