Below are the results typical from a cross between a black - usually a Sumatra, but also - and the Red Shouldered Yokohama. This is a good way to revitalize the breed, as the Sumatra has much in common with Yokohama in basic form and direction in feathering that we need in the Yokohama.
The blue that shows up in first generation crosses of Red Shoulder with other breeds (typically Black Sumatra or Phoenix) is due to the homozygous (pure-bred) Red Shoulders carrying the genes for blue (written as Bl). This Bl gene lightens the black areas of the wild Red Junglefowl pattern to a greyish, dilute-black colour. In the case of the Sumatra, the extended black "E" is diluted to a grey.
The blue does not show up in the Red Shoulder Yokohama itself because the Red Shoulder colouration is also pure for dominant white (written as II) which supresses the expression of blue and black but not red. When outcrossed, however, the "blue" shows up because the heterozygote (diluted form, "mixed") only has one dose of red and blue which allows the blue (Bl) to be seen, changing, the black areas of E (extended black) and e+ (all Duckwing patterns with black breasts and tails) to varioous shades of blue.
Birds circulating in America under the name Yokohama but which have blue are not pure for dominant white and need to be bred to back to a pure Red Shoulder to get back to the homozygosity. No true Yokohama has blue or red in the neck, saddle or tail. On occassion, singular black feathers show up, usually a good sign of good pigment reservers.
The breast markings sometime disappear completely in these outcrosses. Because the spangling is a very important trait of the well-bred pure Red Shoulder Yokohama, it must be regained like the pure white hackle, in back-crossing to pure Red Shoulder Yokohamas. - This all seems a bit complicated, but it does sink and make sense after a while of study. - thanks to the work of Brian Reeder on genetics. Below are three examples of outcrosses with the Red Shouldered with Sumatra or E - extended black, showing the effect of the blue gene Bl.