Explain why the frequency of recombinant offspring produced during a dihybrid cross between two genes located on the same chromosome is limited to a maximum of 50%.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Two genes, A and B, are located on the same autosome and are said to be linked. When an organism heterozygous for both genes undergoes meiosis, crossing over may or may not occur between the two loci.
Model answer (4 marks)
Linked genes are on the same chromosome, so they are inherited together more often than not, giving parental combinations a higher frequency than recombinant combinations.
During meiosis I, crossing over can occur between the two loci. A single crossover event only involves two of the four chromatids in the bivalent, producing two recombinant chromatids and leaving two parental chromatids unchanged.
Thus, at most half of the gametes from that meiosis can be recombinant. Even if a crossover occurs in every meiosis between the loci, the non‑crossover chromatids still produce parental gametes, so the total recombinant frequency cannot exceed 50%.
Therefore the maximum recombinant frequency is 50% because only one of the four chromatids can be recombinant in a single crossover event, and non‑crossover chromatids always contribute parental types.
During meiosis I, crossing over can occur between the two loci. A single crossover event only involves two of the four chromatids in the bivalent, producing two recombinant chromatids and leaving two parental chromatids unchanged.
Thus, at most half of the gametes from that meiosis can be recombinant. Even if a crossover occurs in every meiosis between the loci, the non‑crossover chromatids still produce parental gametes, so the total recombinant frequency cannot exceed 50%.
Therefore the maximum recombinant frequency is 50% because only one of the four chromatids can be recombinant in a single crossover event, and non‑crossover chromatids always contribute parental types.
Examiner tips
- Mention that linked genes are on the same chromosome and therefore tend to be inherited together.
- Explain that a single crossover involves only two chromatids, giving a maximum of 50% recombinant gametes.
- Show the logic that non‑crossover chromatids always produce parental types, limiting recombinant frequency.
- Use the phrase "maximum of 50%" to match the mark scheme wording.
Common mistakes
- Confusing the 50% limit with a 100% recombination rate; students sometimes think crossing over can produce all recombinant gametes.
- Failing to explain that only two of the four chromatids are affected by a single crossover, so the limit is 50%.
- Using vague terms like "often" instead of the precise 50% figure.
Mark scheme (4 marks)
- Linked genes tend to be inherited together because they are on the same chromosome, so parental combinations are more frequent than recombinant combinations.
- Crossing over between the two loci during meiosis I (prophase I) can separate the linked alleles, generating recombinant chromosomes.
- A single crossover event between two loci affects only two of the four chromatids of the bivalent, so at most half of the gametes from that meiosis are recombinant.
- Even if crossing over occurs in every meiosis between the two loci, the overall recombinant frequency cannot exceed 50% because the parental-type gametes from the non-crossover chromatids always equal or outnumber recombinant gametes.
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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