The tendency for different genes to segregate together during meiosis because they occur on the same chromosome. For example, if the genes Aa and Bb occur on the same chromosome then a double heterozygote AaBb formed from Ab and aB gametes would yield a significantly larger number of Ab and aB gametes than AB or ab gametes. Similarly, if the parental gametes were AB and ab, the heterozygote would generate AB and ab gametes frequently, and Ab and aB gametes rarely. This is in complete contrast to *independent assortment
where the genes are on different chromosomes and therefore produce all types of gamete equally frequently. When genes are linked, the two less frequent gametes are produced as a result of *crossing over
. As crossing over is relatively rare the genes on one chromosome tend to be inherited together. A group of linked genes is called a linkage group, and in general one chromosome corresponds to one linkage group. However, two widely separated genes on a long chromosome with frequent chiasmata may behave as though they are on different chromosomes.