This complete linkage produces only parental or noncrossover (NCO) gametes in equal proportions. Thus genes on the same chromosome form a linkage group.
"The phenomenon in which linked genes tend to go together in the same gamete as a result of which the parental combinations are obtained in greater frequency than expected in the progeny, is known as linkage or the co–existence of two or more genes or factors in the same chromosome as a result of which they are transmitted from one generation to another in group is known as linkage".
In other words, genes on same chromosome are called linked genes, and the phenomenon of their transmission is termed as linkage.
According to Morgan, the tendency of two genes to remain together in the same chromosome during hereditary transmission is called linkage. Morgan supposed that this tendency of linked genes to remain in their original combinations was due to their location in the same chromosome. Further, he advanced the basic idea that the degree or strength of linkage depends upon the distance between the linked genes in the chromosome.
Linkage groups: All the genes that are linked together form the linkage group. If a certain gene A is linked with B and B with C and C with D and so on, then all the genes AB, CD, etc., are linked together and all the genes that exhibit linkage with one another form a linkage group. In this phenomenon, offsprings exactly resemble parents.