Sometimes genes, for no known reason, change their form in a process called spontaneous

Mutation.

Explanation:

Mutation may be defined as the sudden heritable change in the DNA sequence of the organisms. Mutation can results in the change in the genotype and phenotype of the organisms.

Two main types of mutation are spontaneous mutation and induced mutation. Induced mutation occurs intentionally with the help of the mutagens. Spontaneous mutation is the natural mutation that occurs for the no known reason.

Thus, the answer is spontaneous mutation.