This page has been sourced from REC.EQUESTRIAN, the body of the text has been unaltered as far as possible. The information is for use at own risk.
8. GREY VS. ROAN, FLEABITTEN GREY
Both grey and roan horses have white hairs mixed in with the base color. However, roan horses are born roan, and the number of white hairs does not change much throughout life (it may change seasonally but after a year should be back where it started). In contrast, grey is progressive. A grey horse typically is born solid-colored, gets more and more white hairs with each coat, and with age will turn completely white. The rate of greying varies a lot; some horses grey out very fast, some slowly. Another way to tell grey from roan is that roan horses usually have dark heads, legs, manes, and tails, while grey horses usually go grey all over (but not always; some grey horses keep a dark mane and tail).
Grey horse owners need to be aware of the increased susceptability grey horses have to "grey horse melanoma", a form of skin cancer.
Grey is caused by a dominant gene, G.
Roan is caused by the gene Rn. It is a homozygous lethal. Foals with one roan allele and one non-roan (Rnrn) live, and are roan. Foals with two roan alleles (RnRn) always die in utero. It is possible to have a horse who is both roan and grey.
Grey horses often show dapples as they grey out. A grey horse without dapples is called "iron grey", especially if the base color is dark.
Roans typically don't have dapples. "Silver dapple" is an unrelated gene that causes dilution of the base color, with dapples, and with a flaxen mane and tail. A silver dapple horse doesn't have a mixture of white and solid hairs, like a grey or roan horse; rather, each hair is lighter colored. It is non-progressive. The silver dapple gene is rare in most breeds, but common in Shetlands.
Roans are given different names based on what the base color is. Some common names are red roan (base color red bay), strawberry roan (base color sorrel), blue roan (base color black), and purple roan (base color mahogany bay). "Rose grey" is used both for roan chestnuts and greying chestnuts. Sometimes roans have "corn" spots of darker color, instead of having the white hairs mixed evenly in. These roans are called red corn, blue corn, etc. A further variant is "silvering" or "varnish roan" in which there are darker areas over the joints and bony prominences.
Question: What color offspring can grey horses have?
Answer from Tracy:
Grey covers up the base color. Let us say that base color is bay, well, bay can hide a recessive black gene (Aa), and the horse might also have a recessive chestnut gene (Ee), so depending on what the grey is bred to you could have a chestnut, a bay, a black, or another grey, or numerous other colors. Remember to look at your horse's breed and parentage -- certain breeds don't have some color genes available to them. For example Arabians don't have Z silver dapple or E-d jet black, and b chocolate brown is a tremendous rarity, so those genes are unnecessary to consider. My [Arabian] mare's line has had nothing but greys, bays, and chestnuts for many generations, black was known eight or ten generations back but hasn't shown up since then so I won't need to bother checking for other colors such as overo genes or cremello genes when I breed her to another of her line. Since I am interested in black I would want to see if the sire had a line that included black genes. One more thing, the likelihood of a recessive gene showing up in subsequent generations decreases with each new generation that does not show it, but as in the case of black coloring, it never completely goes away.
Flea-bitten grey is a version of grey where some pigment cells become reactivated and little dots or speckles or flecks of the original coat color start growing back into the white coat. The fleabitten color can become so total that the horse will begin to resemble the chestnut bay or black that they once were. There is probably a genetic mechanism for flea-bitten grey that has not been discovered yet as some lines of horses, Egyptian Arabians come to mind, are known for producing heavily flea-bitten greys. The extent of the flea-bitten color may also be controlled for genetically, as some lines produce very minimally flea-bitten horses whereas in others the flea-bitten color comes in at an early age and becomes very heavy. Again the mechanism is unknown. It is probably a modifier gene such as the one or ones which control how fast the greying process takes place. I have an entirely unstudied theory that flea-bitten color can at least be limited in extent by keeping the horse out of the sun. However, if a horse is going to get flea-bitten it will get at least some flea-bitten color regardless of where it is kept.
Question: is this horse grey or white?
Quest is silver white with pink skin, but he does have 2 spots, each about the size of a quarter and small varnish marks on the fronts of his foreleg pasterns. He has some of the characteristic mottling here and there under his white hair, but not a lot. He is a few spot leopard, but is he still a true white (I know he's not a grey)? any opinions?
Sue Bishop writes:
No, he's not really a true white. Especially since he has the mottling. I may be wrong though.
I (KH) write:
I agree he isn't a true white. Technically a true white has no color anywhere at all, and has pink skin all over. So Quest's two spots and mottling disqualify him from being totally a true white. But it sounds like he is pretty darn close.