Gene family: A group of genes that are related in structure and often in function. The genes in a gene family are descended from an ancestral gene. For example, the hemoglobin genes belong to one gene family that was created by gene duplication and divergence.

When examining gene families, there are several basic approaches to seeing how the family has evolved: the presence of related genes in other species, the degree of conservation of sequence or domain structure among the family members, and the dispersion of the family on the chromosomes. There is a considerable degree of local synteny among the mammals. If a large gene family has become largely dispersed among the chromosomes, it may indicate a much earlier origin for those genes. Homologs in invertebrates and microbes provide information about the central role of certain genes and families in the function of cells and in development.