Dun is a coat colour dilution that is often accompanied by primitive markings such as a dorsal stripe or leg barring
Symbol
D/nd1/nd2
Breeds known to carry this mutation
Many, including Quarter Horses and related breeds, Paint Horses, Andalusians, Lusitanos, Icelandic Horses, Morgan Horses, Welsh Ponies and Shetland Ponies.
Inheritance pattern
Dun is dominant to nd1 and nd2; nd1 is dominant to nd2.
Description
Dun lightens the body colour, leaving the head, lower legs, mane and tail undiluted. It also can cause a darker dorsal stripe, shoulder stripes, and sometimes leg barring and concentric marks on the forehead (known as 'primitive markings').
There are 3 important variations of the Dun gene. D causes dun dilution and primitive markings, nd1 causes no dilution but there may be some primitive markings present (particularly a dorsal stripe and leg barring), and nd2 causes no dilution and no primitive markings.
Interpretation of results
D/D: Horse carries two copies of dun dilution. It will have a diluted coat and show primitive markings and will pass the D allele to all its offspring.
D/nd1: Horse carries one copy of dun dilution and one copy of non-dun1. It will have a diluted coat and show primitive markings.
D/nd2: Horse carries one copy of dun dilution and one copy of non-dun2. It will have a diluted coat and show primitive markings.
nd1/nd1: Horse carries two copies of non-dun1. It might show primitive markings.
nd1/nd2: Horse carries one copy of non-dun1 and one copy of non-dun2. It might show primitive markings.
nd2/nd2: Horse does not carry either of the dun dilution alleles.
Reference
Imsland et al. Regulatory mutations in TBX3 disrupt asymmetric hair pigmentation that underlies Dun camouflage color in horses. 2015, Nature Genetics; 48:152-8. doi: 10.1038/ng.3475