Merle never skips a generation
The single most important rule of the M locus: the only way to produce a merle pup is for at least one parent to be merle. Some breeders believe merle is a recessive quietly carried down the generations — this is not correct. A dog is either a merle or it is not. There are no exceptions to this rule of genetics as it is currently understood.
The blunt version
If someone tells you they have a litter of merle pups with no merles for many generations in the bloodline, those pups were not sired by the dog they think. As the old breeders put it — look for the hole in the fence.
Merle comes in every colour
Merle is not a colour of its own; it is a pattern laid over whatever colour the dog already is. Name a merle by its darkest colour and its pigment:
- Black merle — black nose, eye rims and lips; darkest patches are black with blue and silver‑grey. Commonly (if loosely) called "blue merle."
- Slate merle — slate nose and pigment; darkest is a slate‑blue with lighter blue‑grey and silver.
- You can also have chocolate merle (often called "red merle"), ee red merle, sable merle and lilac merle.
How merle works
The merle gene is an incomplete dominant — a gene with intermediate expression — and it is another dilution gene. Instead of diluting the whole coat evenly, it dilutes in patches: a black coat becomes grey patched with black; a brown coat becomes dilute brown patched with chocolate, sienna, brick and lilac tones. The merling is clearly visible at birth but can fade to little more than mottling on the ear tips in an adult. Merle eyes are sometimes blue or marbled — brown and blue segments in the same eye.
M/m, M/M and double merle
A double merle (M/M) carries far more white than is normal for the breed and may have hearing loss, vision problems including small or missing eyes, and possible infertility. The effects are worse when a gene for white markings is also present — and Border Collies normally already have extensive white — so an M/M white has a strong probability of being deaf or blind. The only mating that produces 100% merles is a double merle (M/M) bred to a non‑merle (m/m).
Cryptic & phantom merles
Cryptic (or phantom) merles carry a merle gene but look tri, bi or self‑coloured. They will have some tiny area of merling somewhere — a patch on an ear, the tail or the top of the head — and it can be as small as a single hair, anywhere on the body. Cryptic merles are very rare, and the rule still holds: even a cryptic merle can only be produced when one or both parents are merle.
