Explain why a testcross is used in genetics to determine whether an organism displaying a dominant phenotype is homozygous or heterozygous.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A testcross is performed by crossing the organism of unknown genotype with a homozygous recessive individual. If the unknown organism is homozygous dominant, all offspring will display the dominant phenotype. If the unknown organism is heterozygous, about half the offspring will display the recessive phenotype. The homozygous recessive parent can only contribute recessive alleles, so any recessive phenotype in the offspring must come from a recessive allele in the unknown parent, revealing its genotype.
Examiner tips
- Use the definition of a testcross first, then explain the expected phenotypic ratios for each genotype.
- Show the logic that a recessive phenotype in the progeny can only arise if the unknown parent carries a recessive allele.
Common mistakes
- Confusing a testcross with a dihybrid cross; the testcross is monohybrid.
- Failing to explain why a recessive phenotype in the offspring indicates a heterozygous parent.
Mark scheme (4 marks)
- A testcross involves crossing the organism of unknown genotype with a homozygous recessive individual.
- If the unknown organism is homozygous dominant, all offspring will display the dominant phenotype.
- If the unknown organism is heterozygous, approximately half the offspring will display the recessive phenotype.
- The homozygous recessive parent can only contribute recessive alleles, so any recessive phenotype in the offspring must come from a recessive allele in the unknown parent, thereby revealing its genotype.
Key terms in this question
testcross · homozygous · heterozygous · dominant phenotype
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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