My brain wants to think that if a horse inherits two grey genes, that it would then quicken the pace to turn nearly white. Maybe this is helped by my experience of growing up with heterozygous greys (my family bred a few grey mares to non grey stallions) and the grey foals took time to get really light, some were teenagers before they turned super light grey.
One of the mares we bred, my mom had bought at auction as a long yearling. No idea of her parentage, but she was a very light grey when we got her. I really think that she is homozygous grey, had three grey foals, all born sorrel/chestnut.
The first two pictures are from when she was about 1 1/2 to 2 years old
Then as a 5 year old with her first foal, she never changed color by this point (couldn't get any lighter) and has only aquired a few flea bitten spots
Now what of her third foal? (also her last foal) This was the only time that the above mare, Misty, was bred to a grey stallion (registered arabian, ee, heterozygous grey). I personally believe that Glory only recieved one grey gene.
Glory on her second birthday. Like her party hat?
She is about 3 years old here, I think...
Riding along the Seattle coast on the other side of Misty, as a four year old
You can barely see her on the other side of Misty.... again
But she is 7 years old here and still has a darker mane and forelock, as well as darker legs than that of her dam (both wet from a bath)
So, I am of the opinion that since it took Glory a lot more time to get nearly white (she is very flea bitten and is turning 18 this spring) that she is a heterozygous grey, where as her dam was white as a ghost at an early age.
Is there any logic behind my thinking?
The reason I am asking is because I am hoping that she is a heterozygous grey and is possibly able to give my sister a non grey this spring -would certainly stand out from her white and nearly white mares (she owns both Misty and Glory)
Just realized that my sister that owns Misty and Glory appears in every picture that has people in it