A gene family is a set of related genes occupying various loci in the DNA, almost certainly formed by duplication of an ancestral gene, and having a recognizably similar sequence. The globin gene family is an example.
Figure: the globin gene family consists of a number of clusters of related genes. In humans, illustrated here, there are three clusters. In other species, the globin gene in each cluster and the distribution of the clusters through the chromosomes differs from humans.