# In dogs, the allele for smooth coat (S) is dominant over the allele for rough coat (s). A breeder has a dog with a smooth coat and does not know whether it is homozygous dominant or heterozygous. Describe how the breeder could carry out a test cross to find out the genotype of the smooth-coated dog, and explain how the results would indicate whether the dog is homozygous dominant or heterozygous.

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

## Mark scheme (4 marks)

1. The smooth-coated dog must be crossed with a homozygous recessive (rough-coated) individual
2. Observe / record the phenotypes of the offspring produced
3. If all offspring have smooth coats, the dog is homozygous dominant (SS)
4. If approximately half the offspring have rough coats, the dog is heterozygous (Ss)

## Key terms

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

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Source: [GradeNine](https://www.gradenine.co.uk/q/in-dogs-the-allele-for-smooth-c99658cb) · Published by Druglandscape Ltd.