(a) Parental chromosomes Region 1 Region 2 bbbb Sister chromatids Home chror vg or Sister chromatids (b) Crossover in region 1 Resultant chromatids b pr bbbb b Crossover in region 2 Resultant chromatids b b-bb+b b pr b (d) Double crossover with one crossover in each region Resultant chromatids pr pr bbb+b