Negative correlation between orthology and nonfunctional gene frequencies in Caenorhabditis elegans. Each of the 19 gene families is plotted once. The X-axis is the fraction of C. elegans genes in the family with single orthologs in both C. briggsae and C. remanei. The Y-axis is the fraction of C. elegans genes in the family with probable defective alleles in the N2 reference genome sequence. The correlation shown is much stronger than that between family size and fraction of defective genes (see Additional file 9). R is the Pearson correlation coefficient.