# 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

## 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

- [dominant](https://www.gradenine.co.uk/glossary/dominant)
- [genotype](https://www.gradenine.co.uk/glossary/genotype)

## Related

- [Revision notes for Cambridge International IGCSE Biology (0610)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/in-pea-plants-the-allele-for-769bdcb0) · Published by Druglandscape Ltd.