What is interference? How does it affect the double cross over recombinants?

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Interference is a genetic feature that controls how crossovers happen during meiosis. When a crossover takes place between two genes on a chromosome, it affects the chances of another crossover happening nearby. Usually, it reduces the possibility of a second crossover in the nearby region. This means crossovers do not occur completely independently. Because of this effect, we see fewer crossovers near each other than what we expect by simple probability. This is called positive interference.

This happens because the chromosome structure becomes less favorable for another crossover after one has already occurred. It is a natural control system to avoid too many crossovers in a small region.

How does interference affect double crossover recombinants?

Double crossovers happen when two separate crossover events occur between three genes. For example, suppose we have three genes A, B and C. A crossover may happen between A and B, and another between B and C. If we know the recombination frequency between A–B and B–C, we can calculate the expected number of double crossover recombinants by multiplying the two frequencies.

But due to interference, the actual number of double crossover recombinants is usually less than expected.

To measure this, we use two terms:

Coefficient of coincidence (c): This is the ratio of the observed number of double crossovers to the expected number.
Interference (I): It is calculated as 1 minus the coefficient of coincidence (I = 1 – c)

If interference is 1, it means no double crossovers occurred at all. If it is 0, there is no interference and crossovers happen freely. So interference directly reduces the number of double crossover gametes and changes the accuracy of genetic distance calculations.

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