Dun and d1 (nd1) are different versions (alleles) of the same gene.
The coat colour and markings associated with d1 reflect what is commonly referred to as counter-shading.These horses are expected to have a dorsal stripe and possibly other markings usually associated with dun. Horses that are positive for d1 but not dun are NOT dun.
Two copies of d1 (aka nd1)
Here is one example of a black horse with two copies of d1 – the dun gene restricts the distribution of the pigment particles in the hair. Magnus is from Arabec Pintos (arabecpintos.com). As a foal you would be forgiven for thinking he was chestnut but he is actually a black foal! As a yearling you can see he has darkened up considerably but still has a lot of frosting in his tail. Magnus is homozygous for d1.
Two copies of d1 (aka nd1) on Chestnut
Following on from yesterday’s post here is an example of a chestnut horse with two copies of d1 (aka nd1). On this QH mare you can clearly see a dorsal stripe, and there are also faint but visible stripes on her legs. The rump shot also makes her coat look like it’s lighter than usual too. (We did not test this mare for her white markings so unfortunately there is no insight into what is causing her bling!)
Another two copies of d1 (nd1) on black
Just like Magnus, she was a long way from black at birth! There’s some great shots on our FB page of her shoulder bars and dorsal stripe as a foal. She’s still got faint shoulder bars and some frosting as an adult, as well as a dark brown rather than black coat. She’s also got light hairs around her chestnuts which I haven’t seen before – I don’t know if this is a d1 (nd1) effect or just a feature unique to this mare.
Two copies of d1 (nd1) on bay
This is Jubane Nakeeta, an Australian Stock Horse with two copies of d1. As a very adorable foal she looks bay, but you can also see a dorsal strip and dark shoulder bars. As a yearling and as an adult these signs have faded and to me at least she looks like a bay/brown without any remarkable markings or other colour differences. However there must have been a dorsal stripe remaining to make the owner curious enough to test for dun.