The S locus: white spotting
White markings are governed by the S series, whose alleles appear to be incompletely dominant. Four alleles are generally described for white spotting in dogs:
Research has shown that the white undersides of the Border Collie are actually dictated by a separate gene from the Irish‑spotting collar.
The white factor
The white‑factor gene is a simple autosomal recessive, and it can be carried by a dog that looks like it has ordinary markings. There is no way to be sure a dog carries it until you breed it. To keep white under control:
- Do not breed two known white‑factored dogs together — pair a white‑factor with a non‑white‑factor, or two non‑white‑factored dogs, for the best results.
- Watch for white over the shoulder, back and loin.
- On the back leg, you want the white — inside and out — to stay below the hock (the bend of the back leg). When white climbs past the hock, it signals the white factor.
White and hearing
Excessive white on the head can contribute to deafness. You want the coloured area to cover the ears. Avoid breeding two split‑faced dogs together; two fully masked (coloured‑headed) dogs give the best odds of avoiding too much white on the face. There is more on pigment and deafness on the Nose & Eye Colour page.
Breed two white‑factored mates together and at least some pups will be piebald, extreme white or pattern white. When those white areas later fill with flecks of colour, you are seeing ticking.
