Discussion
Which of the following statements about crossing over is true ?
Answer: A
The true statement is there is a greater probability for a crossover to occur between 2 genes farther apart than between genes nearer to each other.
Crossovers occur at more or less random points along a chromosome, so the longer the stretch between two genes, the more likely a crossover falls within it. This is the basis of Sturtevant's linkage mapping: recombination frequency is used as a measure of distance, 1 percent recombination being one map unit (centimorgan).
The second and third options state the opposite relationship. The fourth is wrong because the maximum recombination frequency between two genes is 50 percent: even with multiple crossovers, at most half the gametes are recombinant, which is the value seen for unlinked genes.
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