When both the characters present in F1 generation pass through F2 generation, this 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 in 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 low recombination during crossing over.