Double recombination

Double crossover among three chromatids

Independent assortment of two loci (A & B) on separate chromosome pairs results in equal proportions of Parental and Recombinant progeny. Mendel crossed AABB with aabb parents, who produce AB and ab gametes, respectively. The F1 generation are all AaBb. Independent assortment of the two loci in gamete formation produces equal frequencies of AB, Ab, aB, and ab gametes: the AB and ab types are called parental, and the Ab and aB are called recombinant. Crossing the F1 with an aabb tester strain [bottom left] allows the genotype of the F2 progeny to be determined directly from inspection of their phenotype [bottom right] since it is determined by the contribution of the F1 parent. The F2 progeny are expected to show equal proportions of the two parental types and of the two recombinant types,

Figure ©2002 by Griffiths et al.; all text material ©2014 by Steven M. Carr