Why does only one copy of the dominant E^D gene cause a dog to appear solid black?
Answer
It overrides the instructions from the Agouti gene governing banded patterns.
The E^D allele, which causes dominant black coloration, operates as a highly effective override switch within the genetic code governing coat color. When this dominant allele is present, even in a single copy (heterozygous state), it directs the entire coat to produce uniform black pigment across all hair shafts. This directive actively suppresses or masks the instructions being sent by the Agouti gene, which is responsible for creating the complex, banded patterns characteristic of sable. Consequently, the visual expression is dictated entirely by the dominant E^D instruction, resulting in a solid black appearance.
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