In pea plants, the allele for tall stems (T) is dominant over the allele for dwarf stems (t). A tall pea plant is crossed with a dwarf pea plant. All of the offspring produced are tall. Explain what the genotypes of the two parent plants must be, and why all of the offspring are tall.

Eduqas GCSE Biology — 2.3.2 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 homozygous dominant (TT). The dwarf parent must be homozygous recessive (tt). All offspring receive one T allele from the tall parent and one t allele from the dwarf parent, giving genotype Tt. T is dominant over t, so all offspring with genotype Tt show the tall phenotype.

Examiner tips

  • State the parental genotypes first, then explain the inheritance of alleles, and finish with the dominance rule.
  • Use the exact terms ‘homozygous dominant’, ‘homozygous recessive’, ‘Tt’, and ‘dominant’ as the mark scheme expects.

Common mistakes

  • Assuming the tall parent is heterozygous (Tt) – this would produce some dwarf offspring.
  • Using the wrong allele notation (e.g., tT instead of Tt).
  • Failing to mention that T is dominant over t.

Mark scheme (4 marks)

  1. The tall parent must be homozygous dominant (TT)
  2. The dwarf parent must be homozygous recessive (tt)
  3. All offspring receive one T allele (from the tall parent) and one t allele (from the dwarf parent), giving genotype Tt
  4. T is dominant over t, so all offspring with genotype Tt show the tall phenotype

Key terms in this question

dominant · genotype · allele

Related

More Inheritance questions

▶ Try answering this question with AI marking (free) →