In a dihybrid with genes on separate chromosomes,recombinants are produced by independent assortment,as shown
in Figure 1. Let’s reconstruct a standard Mendelian analysis to illustrate how recombinants are produced:
P A/A ; B/B x a/a ; b/b
Gametes A ; B a ; b
F1 A/a ; B/b
Testcross A/a ; B/b x a/a ; b/b
F1 dihybrid Tester
Progeny A/a ; B/b
a/a ; b/b
A/a ; b/b recombinant
a/a ; B/b recombinant
The last two genotypes must be recombinant because they were formed from gametes of the dihybrid (output) that differed from the gametes that formed the F1 (input). Note that the frequency of recombinants from independent assortment must be 50 percent .
A recombinant frequency of 50 percent in a testcross suggests that the two genes under study assort independently.
The simplest and most likely interpretation of independent assortment is that the two genes are on separate chromosome pairs.
(However, we must note that genes that are very far apart on the same chromosome pair can assort virtually independently and produce the same result