Pseudogenes

Gene duplication may occur by a direct increase of DNA content (non-processed) or via an RNA intermediate (processed). Nonprocessed pseudo-genes can arise by unequal crossing over in homologous chromosomes (paired meiotic chromosomes containing the same genetic loci) or unequal sister chromatid exchange (crossover at improperly aligned sequences) after replication in a single chromosome (Figure 1A). Replication slippage also increases DNA content by looping of the synthesized strand during DNA replication, but typically involves short sequence stretches such as microsatellites (short repeated sequences).

A nonprocessed duplicated gene contains the introns and regulatory sequences of the original gene. This yields genetic redundancy, which allows Figure 2. Gene duplication yields either the acquisition of a gene with novel function or a pseudogene. X refers to mutations. one of the genes to acquire mutations, becoming a nonfunctional pseudo-gene (Figure 2). Occasionally, the duplicated gene acquires mutations yielding a gain-of-function that differs from the original gene (Figure 2). This may allow the evolution of new capabilities in the organism possessing it.

New genes generated by nonprocessed duplication are generally located in the vicinity of the ancestral (original) gene within the genome. However, it is possible that these genes may become separated from each other as a result of major chromosomal rearrangements (such as translocations). Examples of functional and nonfunctional duplicated genes in adjacent locations and on different chromosomes are exhibited by the globin gene superfamily.