Pseudogene

A pseudogene is a nucleotide sequence that is part of the DNA or an organism that appears to have once coded a gene product, such as a protein or transcription factor but no longer does so.

Pseudogenes are believed to have resulted as mutation to an original functioning coding sequence. One scenario for the evolution of a pseudogene is that the environment of an organism changes sufficiently such that the gene product is no longer necessary for the survival of that organism (for example an enzyme may not be required at a different temperature). This means that there is then no longer selection pressure to maintain this sequence, and organisms can continue to survive by accumulating mutations in the sequence. For example, mutations in certain nucleotide positions can result in a premature "stop" codon which leads to a shorter than normal polypeptide chain protein, which is incapable of folding into a functioning protein.