Overview

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.

Procedure

As cells are frequently dividing, they must also be frequently duplicating their genomes. During this process, mistakes can occur which lead to regions of DNA being duplicated. 

If a duplicated section of DNA contains one or more coding regions, this is referred to as gene duplication. Freed from the constraints placed on the original gene to maintain function, the gene copy can now acquire mutations and may evolve to fulfill a new function within the cell. We refer to genes which have evolved in this manner - through duplication of existing genes in the genome of a single species - as “paralogs”. 

Orthologs, on the other hand, is a term used to describe genes in different species that arose from a common ancestor but continued to evolve after one or more speciation events. 

The term “homolog” may be applied in both instances, and simply describes genes with a common ancestor. Groups of homologous genes are collectively referred to as “gene families”.

Because of their related DNA and resulting protein similarities, genes within one gene family typically produce proteins which perform similar functions. 

For example, hemoglobin and myoglobin are related proteins which both perform oxygen binding in mammals. However, hemoglobin has evolved as a primary oxygen transport molecule, whereas myoglobin’s role is in oxygen storage.

When examining genome sequences, such gene families can often be identified and classified due to their high sequence similarity. 

Additionally, studying gene families can aid in hypothesizing the function of a newly discovered gene even in the absence of a protein.