Tandem genes and clustered genes.

Two patterns of gene repetition are described: tandem arraying and clustering. Tandemly arrayed genes reside within segments of DNA that are repeated head-to-tail a number of times. Clustered genes are linked but irregularly spaced, are often mutually inverted in an unpredictable pattern and are connected by non-conserved DNA. Tandem arrays are homogenized by both unequal recombination and gene conversion, are necessary for the maintenance of large gene families, can expand and contract rapidly in response to changing demand, can keep functionally related genes equal in number, and do not engender increased genetic complexity. Gene clusters are homogenized by conversion only, seldom if ever contain more than 50 members, are stable in number, and often engender increased genetic complexity. The interrelationships among these properties are discussed. Tandem gene arrays can evolve into gene clusters. It is suggested that this occurs when some change in the array inhibits unequal recombination but not gene conversion. The most common such change is inversion of part of the tandem array with respect to the rest; however, arrays can evolve into clusters without inversion. Clustered genes are sometimes re-amplified into new tandem arrays. Clustered genes are probably more durable than tandemly arrayed genes during periods of relaxed selection, and in the case of fish antifreeze protein genes, seem to behave as a genetic memory.