Fig. 7.
A hypothetical evolutionary scenario of human OR gene clusters. In the first step, tandem duplications generate OR gene clusters in several chromosomal regions. At this stage, similarity among OR genes in a cluster is always higher than that among OR genes in different clusters. Subsequent recombinations shuffle OR genes located in different clusters. Eventually, OR genes in a cluster become heterogeneous (for example, the cluster 1'' contains black and gray genes), and, at the same time, OR genes located in different clusters are often very similar (for example, both clusters 1'' and 3'' contain black genes).