There are other genes which work on the threshold principle and these only cause an effect if the number of dominant alleles in any set goes above a certain number.

Take for example the inheritance of polydactyly in guinea pigs. There are four gene loci, and therefore eight alleles controlling the number of fingers or toes. With less than five of these being dominant the guinea pig ends up with 3 toes, but if there are five or more dominants it has 4 toes. So we have a threshold or critical number of dominants to produce an effect.