A pseudogene is a DNA sequence that resembles a gene but has mutated in the course of evolutionary history so that it is now inactive. A pseudogene therefore has a common evolutionary history and thus a common DNA sequence with the real gene or the active gene. A pseudogene is often given the symbol ψ as a designation. Another possibility for the formation of a pseudogene is the reverse transcription of mRNA (but also tRNA or rRNA) and the subsequent incorporation of the resulting cDNA into the genome.
Some pseudogenes have the same general structure as functional genes, with exons and introns at the normal positions. The inactivating mutations can, for example, alter the transcription start signals, prevent correct splicing or lead to premature termination of translation. Some pseudogenes have inactivating point mutations in the structural gene region.
A pseudogene usually contains several damaging mutations - probably because the accumulation of further mutations is no longer prevented after inactivation.