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 (Table 2), the parents passed on a chromosome with the Cs genes in one gamete and the cS genes in the other. Therefore, the F₁ parent was making CS and cs recombinant gametes. This dihybrid parent was in trans or repulsion phase with respect to these genes (Fig. 11). In the larger experiment with 7000 testcross progeny (Table 1) 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. 10). Therefore, when genes are linked, they can be arranged in two ways in a dihybrid. This means that a CcSs plant can have its genotype in the cis arrangement with both dominant genes on one chromosome and both recessives on the other (written CS / cs ) or it could be in trans (Cs / cS).