pseudogene

A gene that doesn't do anything (like most 'pseudostudents' :-). This might seem odd, and a waste of the organisms time. However, this can be an important evolutionary tool for the genome. By duplicating an existing gene, the pseudogene can mutate as much as it likes while the real copy gets on with doing all the work. In a sense, diploid organisms had(see below) a pseudogenome, since at some stage they duplicated their entire genetic material. Obviously, the genes are now functional - rather than being redundant. This is quite clear in baker's yeast - other creatures (us, for example) have changed so much since then that we cannot survive without both copies (except for diseases that are recessive).

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A duplicated gene that is an exact copy of its duplicate is essentially a "dead" gene from the point of view of information. Although it may increase the copy number of the corresponding mRNA's - this may or may not result in more protein. Copies may be promoterless or close to telomeres (which also down regulates transcription).

A pseudogene is generally not functional, since it lacks the promoter at the beginning, or since it is inserted in a region of the genome which is not transcribed. This is still "useful" in evolutionary terms, since it might be reactivated after some mutations occur. This (both with and without reactivation) is the evolutionary cause of homologous genes.

Some pseudogenes are, however, completely "useless" in this sense -- see processed pseudogene.

I'm not one to halve bunnies, so I'm glad I don't have that problem here. You two are somewhat ambiguous, so I guess I'll just have to wade in here. =o)

The Alchemist: You need to extricate the concept of gene duplication from that of pseudogene formation. And in no sense of the word do diploid/polyploid organisms have pseudogenomes! - (nominally) all the genes on all chromosomes are functional, aren't they?

ariels: I'm assuming you mean 'transcribed' rather than 'functional', no? But pseudogenes may or may not be transcribed; that per sé is not an identification criterion, though the inherent inability of the gene to be transcribed does mark it as such. Short range transcriptional control - ie. promoters - are the order of the day here.

Just so we're absolutely clear: pseudogenes are 'genes that aren't genes'. They are not expressed to give functional protein but they do have sequence characteristics of genes (and are in this manner identified via DNA sequencing). Pseudogenes fall into two classes:

'Dead Genes'

These possess the same intron/exon structure as the related functional gene BUT mutations prevent expression. 'Dead genes' arise as a result of conventional gene duplication, and are thus often physically close to the live gene.
Example: human pseudo-beta globin.

Processed Pseudogenes

These are intron-less and promoter-less versions of functional genes. Processed pseudogenes are thought to arise by integration of the retrotranscribed (by the RT of retrotransposon-type elements) mRNA back into the genome; correspondingly, these can be found anywhere in the genome w.r.t. the progenitor gene - even on a different chromosome.
Example: murine (mouse) pseudo-alpha globin.