The likelihood of achieving a parental combination if two genes are situated on the same chromosome was relatively higher in the subsequent generation in comparison to the non-parental combination. The physical connection of genes was referred to as linkage.
A linkage is supposed to be complete when two or multiple characteristics are inherited and normally surface in two or further generations in their parental or original combinations, they are known as complete linkage. These selective genes do not generate combinations that are non-parental. The genes that exhibit these linkages are located nearby in the same chromosome. Examples – genes for long wings and grey body in male Drosophila
Crossing over, in different terms, is the exchange of segments observed in homologous chromosomes between non-sister chromatids and takes place during the pachytene stage of the prophase I in the cell division process of meiosis and always takes place within linked genes. The recombination of linked genes that crossing over produces plays a significant role in evolution.