When both the characters present in F1 generation pass together in F2 generation then the process is known as linkage.
In case of linkage,the two traits /genes are physically associated on the chromosome and do not sort independently but pass together into progeny.Due to this the F2 ratio deviates significantly from the 9:3:3:1 ratio that is expected when two genes are sorted independently. This also results in the low recombination during crossing over.