# 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%.

> IB DP Biology Higher Level (2023 syllabus) — D3.2 Inheritance · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. Linked genes tend to be inherited together because they are on the same chromosome, so parental combinations are more frequent than recombinant combinations.
2. Crossing over between the two loci during meiosis I (prophase I) can separate the linked alleles, generating recombinant chromosomes.
3. 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.
4. 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.

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