Variance to mean power function. Shown here is a log-log plot of the variance versus the mean number of gene structures per bin, as calculated for a range of bin sizes over chromosome 7. The transformed data points described a straight line on the log-log plot, which implied a power function relationship between the variance and the mean. The solid line represents the theoretical linear relationship determined from the fit of the PG model. A linear model fitted very well to these transformed data as evident from the high value for the correlation coefficient squared r2, and the normal probability plot of the residuals (insert) derived from the differences between the theoretical straight line and the transformed data points. The broken line represents the best fit of a second model, intended for the distribution of genes within conserved segments that was based upon the negative binomial distribution. It did not fit the data as well as did the variance to mean power function.