Chromosomal crossover, also called genetic crossover, is a normal process by which genes recombine. The recombination increases genetic variation by recombining to produce different traits.

Humans have 23 chromosomes. During meiosis, one cell divides twice to form four daughter cells. These four daughter cells have half the number of chromosomes of the parent cell. They can then join to another chromosomal half from either the egg or the sperm to become a whole pair.

Sometimes, these two parts do not stay together. If the pieces break, they can then join with another broken piece. The two genetic copies that recombine are called chromatids. Problems such as Down’s syndrome or other genetic disorders can be caused when genes combine incorrectly.

Most crossovers occur normally. Occasionally, a double crossover occurs. This is when the chromatids join together at two points instead of one. At the second point of contact, the chromatids can again separate and again exchange genetic information.

Gene interference is a measure of the independence of crossovers from each other. In other words, since you know that double crossovers do occur, you must answer the question of whether crossovers in adjacent chromosome regions are independent or not. If they are not independent, that means a crossover in one region does affect the likelihood of there being a crossover in an adjacent region. If a crossover in one region does affect a crossover in another region, that interaction is called interference.

When considering how to calculate interference, the first step is to calculate the coefficient of coincidence (c.o.c.). The c.o.c. in turn depends on the likelihood of a double crossover, called crossover frequency value, also known as the "frequency of double recombinants."

The coefficient of coincidence is the ratio of the observed to expected double recombinants.

Coefficient of coincidence = frequency observed double recombinants / frequency expected double recombinants

The expected number of double recombinants in a sample of two independent regions is equal to the product of the recombinant frequencies in the adjacent regions.

Interference is then defined as follows:

interference = 1 − c.o.c.

Interference tells you how strongly a crossover in one of the DNA regions interferes with the formation of a crossover in the other region.

If interference is zero, this means that the double crossovers are occurring as predicted and that a crossover in one region occurs independently of a crossover in an adjacent region.

If interference is 1, this means that interference is complete and that no double crossovers are observed because a crossover in one region eliminates the likelihood of a crossover in an adjacent region.

Typically, your data will show an interference of between 0 and 1. Values higher than zero but below one indicate that interference is occurring.