# In guinea pigs, coat length is controlled by a single gene. The allele for short coat (S) is dominant over the allele for long coat (s). Two guinea pigs, both with short coats, are crossed. Among their offspring, some have long coats. Explain how two short-coated parents can produce offspring with a long coat.

> Pearson Edexcel International GCSE Biology (4BI1) — 3.2 Inheritance · Explain · 4 marks

## Mark scheme (4 marks)

1. Both parents are heterozygous (Ss)
2. Each parent produces two types of gamete: gametes carrying S and gametes carrying s
3. There is a chance that two s gametes combine at fertilisation to give the genotype ss
4. The ss offspring is homozygous recessive so the long coat phenotype is expressed

## Key terms

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

## Related

- [Revision notes for Pearson Edexcel International GCSE Biology (4BI1)](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-coat-length-is-e2e45012) · Published by Druglandscape Ltd.