Mendel found certain traits not to assort independently. It is due to
When a cross is made between two individuals different from each other in two or more characters, then the inheritance of one character is independent of the inheritance of another character. It is called Law of independent assortment. This is correct if the genes are present on two different chromosomes. But if the genes are present on the same chromosomes, they may or may not show independent assortment, i.e. Linkage.
So, the correct option is ‘Linkage’.