Some breeders have concluded that the Grizzling factor present in the Frillback breed may be a different gene to the G//+ found in other pigeons, due to it apparently not creating “stork-marked” birds in the homozygous state G//G. Research, again by Gibson, appears to show that it is in fact G, with a separate (likely recessive) modifier gene inhibiting G//G creating the “stork” phenotype.
