# In cats, the allele for black fur (B) is dominant over the allele for ginger fur (b). A breeder wishes to find out whether a black cat is homozygous dominant or heterozygous. Describe how a test cross could be carried out and explain how the offspring would indicate the genotype of the black cat.

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

## Mark scheme (4 marks)

1. The black cat must be crossed with a homozygous recessive (ginger) cat
2. If all offspring show the dominant phenotype (black fur), the black cat is homozygous dominant (BB)
3. If approximately half the offspring show the recessive phenotype (ginger fur), the black cat is heterozygous (Bb)
4. Correct use of genetic notation — parent genotypes correctly stated as BB or Bb for the black cat and bb for the ginger cat, with offspring genotypes consistent with the cross described

## 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-cats-the-allele-for-black-392844c7) · Published by Druglandscape Ltd.