So if two genes are linked, the gametes produced during meiosis (if no crossover occurs) have only two allele combinations. If a crossover occurs between the two linked genes, then the gametes produced have four allele combinations, just as if the genes assorted independently. The important question therefore becomes: how often does crossover occur between two genes?

The frequency of recombination between two points on a chromosome varies directly with the distance between the two points.

In other words, the closer together two genes are on a chromosome, the less likely a crossover is to occur between them. The farther they are apart, the more likely a crossover is to occur between them. This makes sense if you think about it:

If a crossover occurs randomly at one point on a chromosome, it's far more likely to occur between two genes if they're far apart than if they're close together.

This property can be used to get an idea of the distance between two genes on a chromosome. The more frequently a crossover event occurs between two genes, the farther apart they are. This is the basic concept behind gene mapping.

To put this another way, the genetic distance between two points on a chromosome equals the average number of crossovers between them. To express the genetic distance, we need to define a unit of measure:

A map unit is sometimes referred to as a centiMorgan (after T.H. Morgan, who pioneered the use of Drosophila for the study of genetics).

How do we measure the amount of crossing over? We can't actually see the crossover events and tell whether or not they are between two genes, so we have to use an indirect method. By designing crosses carefully, it is possible to see the results of crossover events by assessing individual phenotypes.