Effect of paralogy on probability of essentiality of genes. This graph shows the probability of essentiality for genes with and without paralogues for C. elegans, D. melanogaster, M. musculus and S. cerevisiae. Presence of paralogues means more than one gene from the species is present in an OrthoMCL group. Percentages are numbers of genes with lethal phenotypes expressed as a fraction of genes with available phenotype information for each species. Differences in essentiality of genes with versus without paralogues were highly statistically significant (P < 10-04), as determined using Fisher's exact test (Additional File 2).