A theoretical framework for estimating the proportion of donor genome likely to remain in NILs developed by backcross breeding has led to the widespread use of at least six backcross generations to recover an introgressed phenotype in an otherwise recurrent parent background. Our detailed genotypic characterization of over 800 BC1 to BC10 derived inbred lines provides, at an unprecedented scale, an accurate and representative practical appraisal of the reduction in genome complexity achieved by different levels of backcrossing. Using a combination of related parameters (Fig. 3) our results reveal a good concordance with the theoretical predictions