Explain what is meant by the terms homozygous and heterozygous, and describe how a homozygous dominant individual differs in phenotype from a heterozygous individual when the dominant allele is present.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In pea plants, the allele for tall plants (T) is dominant over the allele for short plants (t). A homozygous dominant plant (TT) and a heterozygous plant (Tt) are grown side by side.
Model answer (4 marks)
Homozygous means having two identical alleles of a gene (e.g. TT or tt). Heterozygous means having two different alleles of a gene (e.g. Tt). The dominant allele is always expressed and determines the phenotype in both TT and Tt individuals. Therefore there is no difference in phenotype between TT and Tt – both are tall.
Examiner tips
- Define homozygous and heterozygous clearly. Use the example TT and Tt. State that the dominant allele is expressed in both genotypes. Conclude that phenotype is the same.
Mark scheme (4 marks)
- Homozygous means having two identical alleles of a gene (e.g. TT or tt)
- Heterozygous means having two different alleles of a gene (e.g. Tt)
- The dominant allele is always expressed / determines the phenotype in both TT and Tt individuals
- Therefore there is no difference in phenotype between TT and Tt — both are tall
Key terms in this question
homozygous · heterozygous · allele · dominant · phenotype
Related
- All Pearson Edexcel International GCSE Biology (4BI1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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