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I have a 3 year old filly from a gray tb and a black shire. The filly is all black with white legs and face. She looks JUST like her dad but not as bulky. Now, if the gray gene is dominant why is she black? And, I can't really tell if she is jet black or faded black. She can become a little on the red but it looks more blue on the purple side in the summer. If you really look at the color. You would say choc or black with the red hairs but it is actually a purple tone. So, is that called black, jet black, or fading black....

You bred a gray mare to a black shire sabino and got a black filly, right?

This is possible because gray IS dominant, but each horse has 2 loci for each gene. A homozygous horse is one that has the same gene at each locus-- so IF your mare were GG, that would mean she was a homozygous gray. Homozgyous horses ALWAYS pass their gene on to their foals, because that is all they have to pass on-- one gene from each locus is transferred to baby, and if the mare is GG, then that means no matter which gene she passes on, it's G, or dominant gray.

Since your mare had a foal that was non-gray, then you know she isn't GG. Instead, she is Gg, meaning she has one dominant gray gene only. Her dominant gene, "G", makes her gray-- but she has that other gene, 'g', which isn't a gray gene. But her G gene is dominant, so she appears gray. In the filly's case, she passed on her 'g' gene.

Gray is called dominant because a horse with one "G" gene will be gray, even if its other genes say it is a black, or a cremello, or a bay. The horse will be born usually non-gray, but then wil turn gray.
So, gray is dominant because if a horse has the G gray gene, it will mask every other color.

Does that make sense to you?

As for the fading/jet black... those are terms that horsemen usually use to describe blacks, but genetically they are the same. There will certainly be wishy-washy cases where you can't really tell, but generally you can use these terms:

Fading black: A black horse that often has reddish/chocolate tones, an off-black
Jet Black: horse is usually very black looking, metallic sheen (in summer coat, good feed), but may sunfade on hairtips.
Since the fading/nonfading really depends on a lot of physical factors, such as the horse's feed, season, care, etc., it's best to just call it "black" if it falls somewhere in the middle wishy-washy area.

See the Horse Color Genetics area for more info!