Foal Colour – Does it Reflect Bias in the Stallion?

Sometimes assumptions are made about a foal’s potential ability based on the colour of its coat.  We have all heard comments over the years such as, “I would only buy a bay Snitzel,” or, before US Grade 1 winner Outstrip raced, “There hasn’t been a top notch grey by Exceed And Excel”!

The perception that a sire produces more stakes winners of one colour than another can be true, but a cold, hard fact that seems to be overlooked is that various stallions produce more of one colour than others.  This can give a distorted view of the ability of their offspring.

Is colour linked with racing ability?

We are lucky in the Thoroughbred industry to have access to all manner of statistics and records.  It is not too difficult to gather data to assess any theory.

In the case of Snitzel for instance, a look at the colour of all his progeny bred in Australia reveals that 73.8% of them are bay or brown, 24% are chestnut and the remaining 2.2% are grey.  Compare these facts with his current tally of 63 stakes winners and there is a direct correlation.  73% of those stakes winners are bay or brown, while 24% are chestnut and 3% are grey.  As the greys started out life as either bays or chestnuts, they can distort the figures a little.


So the answer to the question of whether colour affect racing ability is a resounding ‘No!

Why is it that some stallions don’t produce chestnut foals and yet there aren’t any stallions that don’t produce bays and browns?

There are two main genetic categories for colour; chestnut and bay (which encompasses variations brown and black).  For the purposes of this article, the term ‘bay’ covers brown and black.  Grey is not a colour, it is a ‘factor’ which overrides the original colour, much like the greying of our hair as we age.  Grey horses are born either chestnut or bay and its variations.

Each horse carries two genes for colour. The bay gene is dominant to the chestnut gene, so if a horse has at least one bay gene, he will be bay.  Having two genes for different colours is termed being ‘heterozygous’ for colour.

Being recessive, it takes two chestnut genes to produce a horse with a chestnut coat.

The reason why some stallions don’t produce chestnut foals is that they haven’t got a chestnut gene to match up with the necessary chestnut gene in a mare to produce the chestnut coat colouration.  A sire or dam having two genes for the same colour is termed being ‘homozygous’ for that trait.

A chestnut horse has two chestnut genes.  When he is mated with a chestnut mare, he will always produce a chestnut. Over a bay mare, he either has a 50% chance of producing a bay (if she has a recessive chestnut gene which she contributes) or a 100% chance of producing a bay (if she has two bay genes).

So, unless a chestnut stallion is only mated with chestnut mares, he will always have a percentage of bay progeny (and vice versa for chestnut mares).

Does this mean that homozygous bay stallions are also more dominant in producing winners?

No, there is no evidence that the inheritance of colour is associated with racing ability.  Although there are some champion homozygous bay stallions such as Danehill, Sadler’s Wells, Fastnet Rock and Zabeel, there are also plenty of champion stallions which are heterozygous bays, such as Snitzel, Street Cry, Nureyev, Storm Cat and Encosta de Lago.

How come even homozygous stallions such as Redoute’s Choice have produced some greys?

The greying factor is dominant, so if the mare contributes the greying factor in a mating, the foal will turn grey.  Like the colour genes, a horse can be a heterozygous grey (ie have one greying factor affected gene and one normal colour gene) or homozygous grey (both genes affected by the greying factor).  A heterozygous mare or stallion will have approximately 50% grey progeny, while a homozygous grey will have 100% grey progeny.  As it is unusual to have both grey parents, homozygous greys are pretty rare.

As most greys are heterozygous, non-grey stallions will generally produce grey foals to about half the grey (heterozygous) mares they serve.

Here are some examples of the various types of colour gene combinations:

HOMOZYGOUS BAYS (can’t throw a chestnut)

Bel Esprit

Charge Forward




Fastnet Rock

High Chaparral



Redoute’s Choice


HETEROZYGOUS BAYS (can throw chestnuts to chestnuts and heterozygous bays)

Danehill Dancer

Elusive Quality

Encosta De Lago

Exceed and Excel

Flying Spur

More Than Ready


Not A Single Doubt


Street Cry

Testa Rossa

CHESTNUTS (throw approx 50% chestnuts to heterozygous bays, 0% to homozygous bays and 100% chestnuts to chestnuts)


General Nediym


Magic Albert


Show A Heart



Written Tycoon

HETEROZYGOUS GREYS (throw approx 50% grey foals as only had one grey parent)


Bradbury’s Luck

Glass Harmonium



Puissance de Lune

The Factor

HOMOZYGOUS GREYS (throw 100% grey foals as had two grey parents)



Written in 2017 and revised 2022 by Jane Henning


  1. Rayma Boyd says:

    Also In the case of snitzel and others they are going to get more bays and browns because the majority of the mare base in the country is bay and brown.

    • Yes, that’s right – bay and brown is the dominant gene and therefore approx 75% of the horse population is in this category. A homozygous bay can’t produce chestnuts and a heterozygous bay (ie with a recessive chestnut gene) has a 25% chance of producing a chestnut with a heterozygous bay mare and a 50% chance with a chestnut mare. It gets complicated!

  2. Question on email: Hi Jane have just read your interesting article on stallions produce and their colour, a subject I could never get my head around when at school. We have just taken on a stallion to stand he is dark bay but by a grey stallion so the question is what chance of grey produce by him ? I presume a grey mare would be a chance but what about others.

    Answer: As the stallion has not inherited the grey gene from his mother (otherwise he would have been grey himself) he has no chance of producing grey progeny unless he covers a grey mare who passes on her greying gene – so nothing to do with him. If the mare is heterozygous grey (most likely) there is a 50% chance she will produce a grey foal to him and if she is a homozygous grey there is a 100% chance.
    Thanks for the great question!

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