Observing recombination
Realize that to be able to observe recombination, we need progeny in which we can score which of the parental allele(s) each has inherited at each of the loci we are trying to map.
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In this diagram of a cross between 2 inbred lines (ie. all loci are homozygous), there is no opportunity for recombination among the F1 progeny - each parent produces a set of identical gametes, making all the F1 progeny heterozygous at every locus, and all identical to one another. Recombination in the F2 generation, however, shuffles the parental alleles, producing a set of progeny where each has a different assortment of allele combinations at each locus (either blue-blue, red-red, or blue-red/red-blue).
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