Cis Versus Trans
The first difference to note is that the parental gametes are not always the same allele combinations but they are always the most frequently produced gametes. In the smaller experiment above, the parents had either both dominant or both recessive traits together. Therefore the F1 parent was making Cs and cS recombinant gametes. This dihybrid parent was in cis or coupling phase with respect to these genes (Fig. 6). In the larger experiment with 7000 testcross progeny the dihybrid F1 was in trans or repulsion phase for the C,c and S,s genes. In this case the Cs and cS are the parental gametes (Fig. 7). Therefore when genes are linked, they can be arranged in two ways in a dihybrid. This means that a CcSs plant can have it’s genotype in the cis arrangement with both dominant genes on one chromosome and both recessives on the other (written CS / cs ) or can also be in trans (Cs / cS).