An exception to Mendel's principle/ Mendel did not include in his discoveries
A.Dominance - When two contrasting alleles for a character come together in an organism, only one is expressed and another one is not expressed. Expressed character is called Dominant character and the character which is not expressed is called Recessive character. It is the Mendel’s first principle.
B.Purity of gametes/segregation - In the living organisms, two alleles controlling each character maintains identity in the organism, but during the formation of gametes or spores, by meiosis move apart due to separation of the homologous chromosomes which bear them, so that each gametes or spore receives only one allele of each character on random basis. It is called Law of segregation. It is the Mendel’s second principle.
C.Linkage – The physical association of genes in a chromosome is called Linkgage. It is not the Mendel’s principle.
D.Indepedent assortment - When a cross is made between two individuals different from each other in two or more characters, then the inheritance of one character is independence of the inheritance of another character.
So, the correct option is ‘Linkage’.