Gene families consist of groups of genes proposed to have originated from a common ancestor. Typically these arise through events in which a gene or genes are mistakenly duplicated during cell division. Unlike their parent genes (which are subject to selection pressure to maintain function), these gene copies do not need to preserve their sequences and may evolve at a relatively faster rate.
Occasionally these regions can be adapted to take on new roles within the organism, becoming novel genes in their own right. When this occurs, we refer to these genes as paralogs - two genes within the same species which evolved from a common ancestral gene.
Another common term when referring to members of a gene family is ortholog. Orthologous genes are those which arose from a common ancestral gene but continued to evolve after one or more speciation events. For example, the gene for the mouse enzyme trehalase would have an ortholog in humans that also makes the trehalase enzyme. However, these genes and their products would be at least partly different in sequence due to the years of evolutionary change since the last mouse and human common ancestor. Therefore, they are orthologs in the same gene family.
The third common term used within gene families is homologous genes. This term is broader and applies to all of the related genes within a gene family.
Additionally, the term superfamily is sometimes used to refer to very large groups of genes and proteins which display enough homology to have shared common ancestry. For these large families, the grouping may rely on mechanistic similarities to determine the scope of the group. As a consequence of their shared genetic inception, the genes within a gene family typically perform related functions. The immunoglobulin superfamily, for example, comprises a large number of genes which code for both soluble and cell surface proteins involved in immunological responses like cell binding or adhesion. The key feature of this family is that the members share a common domain called the immunoglobulin fold - which is critical to their function.
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