A student crosses two true-breeding pea plants. One parent has purple flowers and the other has white flowers. All offspring in the first generation (F1) have purple flowers. When two F1 plants are crossed, the second generation (F2) shows both purple and white flowered plants. Explain why white-flowered plants reappear in the F2 generation, referring to the behaviour of alleles during gamete formation and fertilisation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In pea plants, flower colour is controlled by a single gene with two alleles. The allele for purple flowers (A) is dominant over the allele for white flowers (a). The original true-breeding parents were crossed to produce the F1 generation, and two F1 plants were then crossed with each other to produce the F2 generation.
Model answer (4 marks)
The F1 plants are heterozygous (Aa) because they inherit one dominant allele (A) from the purple parent and one recessive allele (a) from the white parent.
During meiosis the two alleles separate, so each gamete receives only one allele for flower colour.
Consequently, the F1 plants produce two types of gametes – A and a – in equal numbers.
When two F1 plants are crossed, random fertilisation can combine two a gametes, giving aa offspring that are homozygous recessive and therefore white flowers.
During meiosis the two alleles separate, so each gamete receives only one allele for flower colour.
Consequently, the F1 plants produce two types of gametes – A and a – in equal numbers.
When two F1 plants are crossed, random fertilisation can combine two a gametes, giving aa offspring that are homozygous recessive and therefore white flowers.
Examiner tips
- Show the heterozygous nature of F1 (Aa) first. Explain allele separation during meiosis. State the two gamete types and equal proportions. Link the aa genotype to white flowers.
Common mistakes
- Assuming F1 plants are homozygous. Forgetting that gametes carry only one allele. Not recognising that white flowers arise from aa, not Aa.
Mark scheme (4 marks)
- F1 plants are heterozygous, carrying both the dominant and recessive alleles
- During meiosis, alleles separate so each gamete carries only one allele for flower colour
- F1 plants produce two types of gametes in equal proportions: gametes carrying A and gametes carrying a
- Random fertilisation allows two recessive alleles to combine, producing homozygous recessive (aa) offspring with white flowers
Key terms in this question
true-breeding · alleles · gamete
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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