The gr locus (Grouse gene) is one of two loci that controls feathering on pigeons' feet (the other one is the Slipper gene).
It is recessive to the wild type (no feathering), and it affects the leg shank featherings.
Therefore, only a homozygous bird will have shank featherings, while a heterozygous bird will not show any shank feathering.
When present homozygously and in combination with the Slipper gene (especially when homozygous too), the bird becomes very densely feather footed with long feathers.
Link: https://learn.genetics.utah.edu/content ... feathering