# In guinea pigs, the allele for short hair (S) is dominant over the allele for long hair (s). A breeder crosses two guinea pigs, both with short hair, and obtains offspring in an approximate ratio of 3 short-haired : 1 long-haired. Explain how this ratio arises, including the genotypes of the parents and the possible genotypes of the offspring.

> Cambridge International IGCSE Biology (0610) — 17.4 Monohybrid inheritance · Explain · 4 marks

## Mark scheme (4 marks)

1. Both parents are heterozygous (Ss)
2. Each parent produces two types of gamete: S and s
3. The possible offspring genotypes are SS, Ss, Ss and ss (three short-haired : one long-haired)
4. S is dominant, so SS and Ss both show the short-hair phenotype; only ss shows the long-hair phenotype, producing the 3:1 ratio

## 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-guinea-pigs-the-allele-for-4977e984) · Published by Druglandscape Ltd.