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 two map lengths have been added to the initial map. By F24 the total RI map is almost precisely four 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 6,000 cM map length would ultimately produce a G8 RI set with map length of 6,000 + 3 × 1,400 cM = 10,200 cM. The upper series of points (blue) illustrates 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 points is the inbreeding coefficient at each generation. For a tabulation of these data and methods for calculating two- and four-strain expansion values see .
Williams et al. Genome Biology 2001 2:research0046.1-research0046.18 doi:10.1186/gb-2001-2-11-research0046