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Color genetics question

PostPosted: Sat Sep 17, 2016 3:01 pm
by PONYRCR
Hip #25 at the Keeneland sale is cataloged as a chestnut filly. Her sire is Tapit and her dam is Silver Screamer. Both her sire and dam are grey. Shouldn't she be a grey? I thought grey over grey always produced a grey.

Re: Color genetics question

PostPosted: Sat Sep 17, 2016 4:43 pm
by Rainyday
Grey x grey is more likely to produce a grey, but is not guaranteed to, depending on the genetic makeup of the parents.

The two really basic rules of colour genetics, which I believe the Jockey Club uses to do a quick double check of parentage are:

Two chestnut parents will always have a chestnut foal.

A grey horse must have a grey parent. (And this has caused some confusion in the past when grey horses are registered as their birth colour, never get their registration updated, and then have grey foals.)

The chestnut rule is simply because chestnut is the most recessive of horse colours and two chestnut parents are not bringing anything else to the table. (Because equine colour works across multiple genes, they may actually be carrying genes for bay or black, but those can't expressed unless they are bred to a bay or black.)

Grey, on the other hand, is the most dominant of horse colours and it overrides everything (IIRC, Tesio considered it a pigment disorder instead of a colour). It can't 'hide' and pop out later in a line, and a homozygous grey (a horse carrying two copies of the grey gene) will only have grey foals, as every foal it has will be receiving a copy of the grey gene. However, a heterozygous grey (a horse with one grey gene, and one non-grey gene) will pass the non-grey gene down to approximately half its foals.

Tapit is clearly heterozygous, and, from her pedigree, Silver Screamer is as well. (Her dam is chestnut, and you can't inherit a grey gene from a horse that isn't grey.) So when you breed two heterozygous greys together (Gg x Gg), 25% of the foals will be homozygous grey (GG), 50% will be heterozygous grey (Gg), and 25% will not be grey (gg). So this filly is in that 25%; she's inherited both her parents' non-grey genes.

The other possibility is that she is grey, but is greying out slowly and her registration hasn't been updated. However, Tapits tend to go grey unusually fast (I don't know why), and chestnuts tend to grey out faster than black-based horses, so I think it's unlikely in this case. Would have to see her, though.

That was probably more than you wanted to know, but I hope it was useful. :)

Re: Color genetics question

PostPosted: Sat Sep 17, 2016 4:47 pm
by Retrospectiv
Grey is a dominant gene, but it must be passed down to be expressed.

Tapit is obviously G/g, non-homozygous as he sires both grey and solid horses. Assuming the dam is the same as if either had passed the gene the offsping would be guaranteed grey.

G/G horses are Homozygous and will always pass the gene, and their offspring in turn will have the chance to pass it along. G/g gives a 50% chance of passing the gene :)

Re: Color genetics question

PostPosted: Sat Sep 17, 2016 4:51 pm
by ThreeMustangs
I know it's completely irrational, but I always feel cheated when two greys don't produce a grey. :(

Re: Color genetics question

PostPosted: Sat Sep 17, 2016 5:13 pm
by Treve
Well think of it this way... Two Bays can produce a chestnut, a black or a bay. Depends on the genotype!

Also I'll just correct one tiny thing said above chestnut is not the most recessive but it's the basic gene every horse has upon which the others can build so to speak.
For instance Chestnuts can hide Agouti (which is the core gene for bay) because Agouti can't express itself without Extension (Black). The three major base colours are Chestnut (Red) Black and Bay. Agouti on its own is recessive to Chestnut, but Extension is dominant over Chestnut and agouti is dominant over extension.

So basically every horse has the red gene - CC.

The genotype of a chestnut is always going to be one of these:
CC aa ee
CC Aa ee
CC AA ee

A black will always be
CC aa Ee
CC aa EE (homozygous Black - can't produce a chestnut, only bay or black)

A bay will always be
CC Aa Ee
CC AA Ee (can't produce a black - only bay or chestnut)
CC Aa EE (can't produce a chestnut - only bay or black)
CC AA EE (homozygous for both agouti and black - can only produce bay)

Re: Color genetics question

PostPosted: Sat Sep 17, 2016 6:26 pm
by PONYRCR
Thanks guys.

Re: Color genetics question

PostPosted: Sun Sep 18, 2016 8:26 am
by Admin
I'm curious as I can't think of one offhand: which good stallions are GG?

Re: Color genetics question

PostPosted: Sun Sep 18, 2016 8:48 am
by Retrospectiv
Admin wrote:I'm curious as I can't think of one offhand: which good stallions are GG?


Recently deceased French Gr. 1 winner and sire Linamix was.

Re: Color genetics question

PostPosted: Sun Sep 18, 2016 8:52 am
by Retrospectiv
While a more regional type, I believe Wekiva Springs was as well.

Re: Color genetics question

PostPosted: Thu Oct 06, 2016 2:53 am
by Flanders
Gainesway posted a pretty neat article about Tapit and color:
http://gainesway.com/are-gray-tapits-better-than-non-gray-tapits/