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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

1. Cross the smooth‑coated dog with a homozygous recessive (rough‑coated) dog (ss). 2. Record the coat colour of all offspring. 3. If every pup has a smooth coat, the unknown dog must be homozygous dominant (SS). 4. If about half the pups have rough coats, the unknown dog is heterozygous (Ss).

Examiner tips

  • Use the word "cross" and specify the partner as ss; show the expected phenotypic ratios; explain the logic of the result; keep the answer concise and to the point.

Common mistakes

  • Failing to state that the test cross uses a homozygous recessive partner; confusing the interpretation of the offspring ratios; not recognising that 100% smooth indicates SS, not Ss.

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 in this question

test cross · homozygous dominant · heterozygous · genotype

Related

More Monohybrid inheritance questions

▶ Try answering this question with AI marking (free) →