A cat breeder notices that some cats in a litter have short fur and some have long fur. Short fur is controlled by a dominant allele (F) and long fur is controlled by a recessive allele (f). Both parent cats have short fur. Explain why two short-furred parents can produce a long-furred kitten.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Short fur in cats is controlled by a dominant allele (F). Long fur is controlled by a recessive allele (f). A litter of kittens is born to two short-furred parents, yet one kitten has long fur.
Model answer (4 marks)
Both parents are heterozygous (Ff) and therefore carry the recessive allele f.
The recessive allele f is only expressed when two copies are present, so a kitten must inherit one f from each parent.
The long‑furred kitten therefore has the genotype ff.
Although the parents show the dominant short‑fur phenotype, their heterozygous genotype allows them to pass on the recessive allele.
The recessive allele f is only expressed when two copies are present, so a kitten must inherit one f from each parent.
The long‑furred kitten therefore has the genotype ff.
Although the parents show the dominant short‑fur phenotype, their heterozygous genotype allows them to pass on the recessive allele.
Examiner tips
- Show the parents are heterozygous (Ff) first. Explain that f is recessive and only shows when two copies are present. State the kitten’s genotype ff. Mention that the dominant allele masks f in the parents’ phenotype.
Common mistakes
- Assuming the parents are homozygous FF. Forgetting to explain why the recessive allele is not seen in the parents. Not stating the kitten’s genotype ff.
Mark scheme (4 marks)
- Both parents must carry the recessive allele (f) for long fur / both parents are heterozygous (Ff)
- The recessive allele (f) is only expressed when two copies are present / the long-furred kitten must have inherited one recessive allele from each parent
- The long-furred kitten has the genotype ff
- The short-furred parents show the dominant phenotype but their genotype means they can pass on the recessive allele / dominant allele masks the recessive allele in the parents
Key terms in this question
Related
- Revision notes: Inheritance →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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