Progressive expansion of RI genetic maps during inbreeding. The middle series of points (red) that start at generation 2 shows the addition of map length - and the proportional increase in the numbers of recombination breakpoints - relative to a standard one meiotic generation F2 map. For example, at generation 7, approximately 2 map lengths have been added to the initial map. By F24 the total RI map is almost precisely 4 times as long as a standard F2 map. This same addition characterizes other diallele crosses that start near Hardy-Weinberg equilibrium, including advanced intercrosses. A two-strain G8 advanced intercross with a 6000 cM map length would ultimately produce a G8 RI set with map length of 6000 + 3 × 1400 cM = 10200 cM. The upper series of points (blue) illustrate the accumulation in map length in a four-strain intercross at Hardy-Weinberg equilibrium at generation 0. This cross will gain up to 3.75 map equivalents. The lowest set of point is the inbreeding coefficient at each generation. For a tabulation of these data and methods for calculating two- and four-strain expansion values see www.nervenet.org/papersBXN.html.