Antibody genes and other multigene families.

The multigene family is a unit of chromosomal organization. Its gene members are closely linked, homologous in sequence, and have overlapping functions. Multigene families can be divided into three categories - simple-sequence, multiplicational, and information - by a variety of structural and functional criteria. Multigene families exhibit two novel evolutionary features - coincidental evolution and rapid change in family size - that suggest that they all share one or more evolutionary mechanisms. Natural selection cannot act directly on individual genes in a family because of their identical or overlapping functions; hence selection must operate on the family as a whole or on blocks of genes within the family. The mechanism(s) for coincidental evolution expand out variant genes within a family so they can be acted on by natural selection, and accordingly, permit multigene families to evolve adaptively. The close linkage of the genes in a family appears to be a consequence of the fact that their ocntrol and evolutionary mechanisms may only operate on tandemly linked genes. New multigene families may evolve from a single gene or from other multigene families. In addition to evolving new functions, the latter mode of evolution generate a new multigene family whose members are preadapted to interact with those of the old family. These family interactions can lead to the evolution of more sophisticated molecular machines or to the regulation of one family by the second. Multigene families may be large or small. The three categories of multigene families allow potential multigene families to be identified and they suggest specific experimental approaches for the study of new families. Some of the most interesting genetic systems under investigation today are known or potential informational multigene families. This is not fortuitous in that many of the most interesting aspects of phenotype are complex ones with correspondingly complex genetic, evolutionary, and regulatory requirements. One of the frontiers in modern genetics is the identification characterization, and understanding of informational multigene families.