A mutant gene is a variant form of a gene that produces an abnormal or altered protein, which can lead to changes in the characteristics or traits of an organism. These genetic variations can occur naturally and may be inherited or arise spontaneously during an individual's lifetime due to errors in DNA replication. Mutant genes are often associated with genetic disorders or diseases when they result in non-functional proteins, but they can also contribute to beneficial traits such as resistance to certain environmental conditions or improved ability to survive and reproduce.