• Linkage has been found to be complete or incomplete depending upon the absence or presence of nonparental or new combinations of linked genes.
• Complete linkage is a linkage or grouping of genes on a chromosome which is not altered and is inherited as such from generation to generation without any cross-over.
• In such cases, linked genes do not separate to form new or non-parental combinations.
• It is rare but has been reported in certain cases. For example a pure breeding red eyed and normal winged female Drosophila (PV/PV) is crossed with pure breeding purple eyed and vestigial winged male fly (pv/pv). F1 generation is red eyed and normal winged showing that both the traits are dominant.
• F1 hybrid males are test crossed with purple eyed and vestigial winged females. The offspring were only of two types, red eyed normal winged and purple eyed vestigial winged in the ratio of 1 : 1. There was no crossing over (non-parental combination) indicating that the linkage in male Drosophila was complete.
Incomplete linkage is the phenomenon of an occasional crossing over between two homologous chromosomes so that one or more alleles present in a linkage group are replaced by other alleles.
• It produces both parental and recombinant individuals.
• The percentage of each parental type is more than 25% while that of each recombinant type is less than 25%, i.e., parental types are more than 50% of population while recombinant types are less than 50%.
• For example, homologous pure breeding red eyed normal winged female Drosophila (PV/PV) is crossed to homozygous pure breeding purple eyed and vestigial winged (double recessive) male Drosophila (pv/pv). F, flies are hybrid red eyed and normal winged.
• A female hybrid or F1 fly (PV/pv) is crossed with double recessive purple eyed and vestigial winged male fly. 90.7% offsprings were found to be parental types (red eyed normal winged, purple eyed vestigial winged) while rest were recombinant types (red eyed vestigial winged, purple eyed normal winged).
• The ratio comes to be 9 :1 :1 :8 while it should have been 1 : 1 : 1 : 1 in case of independent assortment and 1 : 1 in case of complete linkage. This shows that the two genes did not segregate independently of each other.