Change in the base sequence with a gene is called

The correct answer is A) Mutation. A mutation is a heritable change in the nucleotide sequence of DNA, through substitution, insertion or deletion of bases. An insertion or deletion that shifts the reading frame changes everything downstream, which is why frameshift mutations are often severe.

Why the others fail:

• Cloning produces genetically identical copies of an organism or gene. It copies the sequence faithfully and does not alter it.
• Fusion means joining two cells, nuclei or protoplasts, as in somatic hybridisation. It combines genomes but does not change the base sequence of a gene.
• Breeding is selection and crossing to get desired traits. It recombines existing variation rather than changing bases.

Core concept: mutation is the raw material of evolution and also the cause of hereditary disorders. Sickle cell anaemia is the classic example, where a single base substitution in the beta-globin gene changes one amino acid. For Mains, link this to genetic disorders, mutagens and induced mutation in crop improvement.

Trap: Aspirants confuse mutation with recombination or breeding-based variation. Recombination reshuffles existing sequences; mutation alters the bases themselves.