Slow feathering is caused by a qualitative sex-linked,
Rapid feathering is associated with the recessive allele: k.
The difference in feathering can be easily observed between one to three days from hatching.
At hatching the primary wing feathers are short and the coverts are as long as the primary feathers in the slow feathering male bred to be Kk.
Primary feathers of the rapid feathering k- females are longer than the slow feathering males and the coverts are shorter than the primaries.