Epistasis:

• Some traits are controlled by two or more pairs of genes. Then, the possibility exists for interaction among genes at different loci.
• This kind of interaction is called epistasis which means that gene action at one locus is altered by gene action at another locus.
• The F1 progeny all had round seeds and yellow cotyledons.
• When the F2 progeny were mated among themselves the progeny had a ratio of 9 round seeds and yellow cotyledons: 3 round seeds and green cotyledons: 3 wrinkled seeds and yellow cotyledons: 1 wrinkled seed and green cotyledons.
• This result led to the law of independent assortment. The genes at one locus are segregated independently of the genes at the other locus.
• In this experiment, the parental genotypes could be symbolized: RRYY (round seeds, yellow cotyledons) × rryy (wrinkled seeds, green cotyledons). R and Y are dominant alleles and the F1, with genotype RrYy, all had round seeds and yellow cotyledons.
• During meiosis, the genes at the two loci segregated independently, and F1 plants produced gametes.