Expressed epistasis occurs when an expressed gene at one locus on a chromosome interferes with the expression of a non-allelic expressed gene at another locus. Leghorn, Wyndot, Plymouth Rocks, etc., feather colors are the result of pronounced epistasis. Bateson I Punnett (1908) noted this.
Due to pronounced epistasis, the ratio of Mendel’s second law of inheritance is 13 : 3 instead of 9 : 3 : 3 : 1.