Weighted pan-genomes. The left panel is a bar-chart showing the number of gene families observed in 1, 2, ..., 32 gene families within the Brucella pan-genome (only genomes from species in genus Brucella are included). The gene families only found in a few genomes (left-wing bars) are called cloud genes, and those found in most genomes (right-wing bars) are called shell genes. The core genes are those that are found in all genomes (rightmost bar). The right panel indicates two different weighting strategies for computing the pan-genome tree. The red bars give more weight to cloud genes when comparing two genomes, while the blue bars emphasize the shell genes.
Bohlin et al. BMC Evolutionary Biology 2010 10:249 doi:10.1186/1471-2148-10-249