The Sl locus (Slipper gene) is one of two loci that controls feathering on pigeons' feet (the other one is the Grouse gene).
It has incomplete dominance over the wild type (no feathering), and it affects toe featherings.
Therefore, a homozygous bird will have more/longer/denser toe feathering than a heterozygous bird.
When present (especially in homozygous dose) and combined with the homozygous recessive Grouse gene, the bird becomes very densely feather footed with long feathers.
Link: https://learn.genetics.utah.edu/content ... feathering