ABSTRACT
The first step in the establishment of a multigene family is duplication of the original gene (Figure 14.1). This can occur by recombination or during replication by polymerase slippage. There are several possible outcomes. The first is that the presence of two copies is deleterious or lethal to the cell, and therefore, the cell with the duplication dies. The second outcome is that the duplication is advantageous, because it increases the fitness of the organism. The third outcome is that the increase is neutral, such that one of the genes can retain its original function in the organism, whereas the other copy is free to change, so long as it does not produce a protein that is deleterious to the organism. Therefore, this has become a mechanism for relatively rapid change and evolution. Complex genes can be duplicated without reinventing another version. The copy (or copies) changes gradually (or sometimes rapidly) to perform slightly different or sometimes completely new functions. Once an additional copy is made and begins to diverge, there is then a higher likelihood of additional expansion of the family, because there are more homologous regions for crossovers. If two copies exist on a chromosome, misalignment and crossover can then lead to a chromosome with three copies, whereas the other is left with only one copy. There is also a small possibility of generating a chromosome with four copies, with a complete deletion of both copies on the other chromosome. This process can then proceed to make more copies per chromosome. Each copy can change slightly from the previous version until many different versions are present in the genome. Occasionally, some of the copies move to other chromosomes by transposition, splitting of the chromosomes, or chromosomal translocations. Some of the genes may become pseudogenes (Figure 14.2). They are of two types: One type is never expressed, whereas the other type is transcribed and translated, but the proteins are nonfunctional. Some multigene families have evolved into hundreds of different members (Table 14.1).
