During preparation of gene map, recombination frequencies are additve over short distances but not exactly over long distances due to

Options

A. Synaptinemal complex
B. Inhibitor genes
C. Multiple cross overs
D. Mutations

Answer

### Step-by-Step Solution:

1. **Understanding Gene Mapping**: Gene mapping involves determining the relative positions of genes on a chromosome. The frequency of recombination between genes can be used to infer their distances on the chromosome.

2. **Recombination Frequencies**: Recombination frequency is the percentage of offspring that exhibit recombinant phenotypes. It is directly related to the physical distance between genes on the same chromosome. For genes that are close together, recombination frequencies are additive.

3. **Short vs. Long Distances**: Over short distances, the recombination frequency can be accurately used to create a gene map because the likelihood of multiple crossovers occurring between the two genes is low. However, as the distance between genes increases, the probability of multiple crossovers occurring also increases.

4. **Multiple Crossovers**: When genes are located far apart on the same chromosome, more than one crossover event can occur during meiosis. This phenomenon is known as multiple crossovers. These multiple crossovers can lead to inaccuracies in the gene mapping because they can result in the recombination frequency not reflecting the true physical distance between the genes.

5. **Conclusion**: Therefore, the reason recombination frequencies are not additive over long distances is primarily due to the occurrence of multiple crossovers, which can distort the expected ratios of recombinant offspring.

6. **Correct Answer**: Based on this understanding, the correct answer to the question is option 3: multiple crossovers.