# In rabbits, the allele for dark fur (D) is dominant over the allele for white fur (d). A rabbit breeder suspects that a dark-furred rabbit may be heterozygous. Describe how a test cross could be carried out to determine the genotype of this dark-furred rabbit, and explain how the results would allow the breeder to identify whether the rabbit is homozygous dominant or heterozygous.

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

## Mark scheme (4 marks)

1. The dark-furred rabbit must be crossed with a homozygous recessive (white-furred) rabbit.
2. If all offspring show the dominant phenotype (dark fur), the original rabbit is homozygous dominant (DD).
3. If approximately half the offspring show the recessive phenotype (white fur), the original rabbit is heterozygous (Dd).
4. A large number of offspring should be produced / the more offspring, the more reliable the result.

## Key terms

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

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Source: [GradeNine](https://www.gradenine.co.uk/q/in-rabbits-the-allele-for-dark-b5fdfb03) · Published by Druglandscape Ltd.