Cis configuration is also known as
Cis configuration termed coupling phase describes linked heterozygote where both wild-type alleles reside on one homologue and both mutant alleles on other, notation + + / a b. Trans or repulsion phase carries wild allele opposite mutant on each homologue, + b / a +. Phase influences which phenotypes appear as parental versus recombinant but does not change map distance between loci measured by recombination frequency. Understanding cis versus trans essential for pedigree risk prediction, DNA diagnostics and interpreting Morgan's Drosophila crosses where parental classes correspond to alleles
Ref: Hartl & Ruvolo, Genetics, 6th ed., Chapter 5: Cis as Coupling Phase; Griffiths et al., Chapter 5: Phase Terminology