Pseudogenes are sections of DNA which look like genes, but do not behave like genes, because they are unable to transcribe. Transcription is the first step in gene expression, and if a segment of DNA is unable to perform transcription, it is non-functional. Because pseudogenes are nonfunctional, some people lump them with so-called “junk DNA” which performs no function, but because they possess some traits associated with genes, they are of great scientific interest. Just because they don't work, in other words, doesn't mean that they have nothing to say.
Several processes can result in the formation of a pseudogene. Sometimes, a mutation causes a gene to become inactive, damaging the DNA so that it is no longer capable of transcription, and the mutation can be passed down and amplified in certain populations. These pseudogenes are known as “disabled.” If enough of a population has the mutation, eventually the gene will stop expressing in that population altogether, causing a particular genetic trait to vanish.
Pseudogenes can also be processed, the result of of mRNA which retrotransposes into DNA and then inserts itself into the DNA. The segment of DNA will be nonfunctional, but it may look like a complete gene. Nonprocessed pseudogenes occur when genes replicate and one of the replicas becomes nonfunctional due to a mutation. Over the course of generations, the DNA will gradually break down even further, eventually turning into true junk DNA.
The primary reason that pseudogenes are of interest is because they can reflect evolutionary shifts and reveal information about the relationships between various organisms. For example, two species of big cat can be traced to common ancestors with the use of pseudogenes, with researchers looking for similar lengths of genetic material to determine when the species started to split off. Pseudogenes can also be used to trade the evolutionary history of an organism, with researchers looking for inactive genes which code for traits which no longer appear. Pseudogenes could be compared to the information encoded in the side of film; it can't be seen in the finished picture, but it provides valuable information for those who know where to look.
By definition, this genetic material is nonfunctional. However, pseudogene activation has been known to occur, most commonly in malignancies where something triggers the activation of the defunct gene. Researchers have noted that pseudogenes are sometimes involved in the development of certain cancers, making the activation process of interest to people working in cancer treatment and prevention.