Good morning all,

Here's the latest in Caroline's breeding articles, this time looking at the world of genetics. I found it a fascinating read, and I hope you do too.

Where did you get that coat?

This week I thought we’d have a look at one of the most visually obvious ways that genetics work in Thoroughbreds: the transmission of coat colour. Back when I was a teenager, I was fortunate enough to have the great Tony Morris as my tutor for this subject, via one of his weekly Racing Post columns, and I remember being absolutely fascinated. You, my friends, will have to make do with a more potted version from me: I can only apologise!

The perfect prompt for this subject came in the form of a tweet by Coolmore Stud last week, introducing the world to top racemare Alpha Centauri’s first foal. I hope they won’t mind that I’ve borrowed one of the pictures below, showing her with her beautiful firstborn, who is a colt by Galileo.

https://twitter.com/coolmorestud

As is obvious to see here, this superstar pairing of the bay stallion Galileo and the grey mare Alpha Centauri has produced a chestnut foal. It is the perfect illustration of how colour transmission is not always direct. I’ll do my best to explain why not, and how this handsome youngster has turned out looking so different to his parents.

All Thoroughbreds will be born either bay, chestnut or grey in colour. There are no other colours in the breed, except for the occasional, very rare white horse. Any Thoroughbred described as brown or even black is, genetically speaking, simply a variant of bay: they will all have a brown skin colour underneath, with black manes and tails. Let’s leave aside greys and the white horses for a moment and focus on bay and chestnut colouring. All horses inherit two colour genes, one from each of their parents. Bay (B) is the dominant colour and chestnut (Ch) is recessive, so any horse that receives even just one bay gene will be bay. In order for any Thoroughbred to be chestnut, both its parents must pass on a chestnut gene. This is why there will always be more bay/brown racehorses than chestnut ones in the general population. In summary, the three possible combinations are:

B / B = bay foal
B / Ch = bay foal
Ch / Ch = chestnut foal

In this way, it is possible for the recessive chestnut gene to be transmitted to offspring but not to show up for generations, as it will be trumped by a bay gene every time for coat colour. If you’d like a comparison with human genetics, it works exactly the same for us in terms of eye colour, with brown being dominant and blue recessive. Hence more people have brown eyes (including hazel/green etc, which are variations) than blue – but two people with brown eyes can produce a child with blue eyes if they both pass on a recessive blue eye gene.

If you couldn’t already tell on paper from the colour of their own parents, it is soon quite quickly apparent with bay stallions whether or not they have a chestnut colour gene when their first crop are born. If the stallion has two bay genes himself then all his foals will also be bay, regardless of what colour their dams are. However, if he has one bay and one chestnut gene, you can expect a proportion of them (likely 20-30%) to be chestnuts. We knew from the outset that Galileo would be capable of siring chestnut offspring, among the best of whom are Australia, New Approach and Ulysses. However, his own sire, Sadler’s Wells, was a pure-breeding bay who had no chestnut gene to pass on. Galileo’s dam, the Arc winner Urban Sea, was a chestnut so had to give him one of her chestnut genes, thus here it is straightforward to trace the colour legacy.

With Galileo’s best son, Frankel, however, it came as more of a surprise to many that some of his foals have turned out to be chestnut, as both his parents are bay. All seven of his dam Kind’s foals have been bay so far, so the most obvious conclusion to draw is that Frankel inherited Galileo’s chestnut gene. However, go back just two generations in Kind’s family and you will find chestnuts, so it is entirely possible she has a recessive chestnut gene too. I don’t think it is possible to know for certain which of them passed one on to Frankel; all we know is that one of them did.

So where does grey colouring fit into the picture? Well it turns out that grey is not a ‘colour’ at all, per se, it is in fact a genetic ‘mask’ that, when transmitted, literally covers or overlies the bay or chestnut colouring and trumps it. If a Thoroughbred has the grey masking genetic trait then they will always be grey: it dominates all other coat colours. They will still have two bay or chestnut colour genes underneath though, the same as all non-greys. So there are no secrets when it comes to the transmission of grey coat colour and it cannot skip a generation, like chestnut can. In the instance of Alpha Centauri’s foal, she has not passed on her grey masking trait* so it is now lost, genetically-speaking, from this line of her family. However, by the law of averages, we can expect that around 50% of the foals she has will be grey.

*Or at least we think she has not passed it on this time – but sometimes it can be very hard to tell in newborn foals. As we know, greys tend to lighten over the years and some are born so dark, or full-coloured, that the grey mask barely shows to the naked eye at first, if at all. It can take months or even years in some cases to realise that the foal you registered in all good faith as bay or chestnut is in fact a grey. I have seen a number of such apparent errors on racecourses in recent years: listed as bay in the racecard but, now aged four or five and strutting round the parade ring, very clearly greys! I’m surprised connections are not required to correct this with Weatherbys once it becomes apparent that an error has been made. One thing is certain though: in any such instances, one of that horse’s parents will themselves have been grey too.

Either way, in Alpha Centauri’s case, we know now that she carries a chestnut gene, despite also having one grey and one bay parent herself. Her greyness comes from sire Mastercraftsman but he may also have given her a chestnut gene: he has a number of chestnut offspring himself so we know he has one. Equally we know that her dam Alpha Lupi has a chestnut gene, courtesy of her sire Rahy, so it could have come from her. The third possibility is that both of Alpha Centauri’s parents passed her their chestnut genes so that, underneath that grey coat, she has a Ch / Ch colour type and is, herself, a secret chestnut. Courtesy of www.allbreedpedigree.com I’ve reproduced her pedigree below, to four generations, so you can trace the colours back and see the various permutations.

One final note on grey colour transmission, and those rare occurrences where a Thoroughbred has two grey parents and both of them pass on their grey mask trait. He will be grey, of course, but in addition every single one of his offspring would then also be grey. In Alpha Centauri’s fourth generation you will see that Hat Tab Girl was grey. She was by the pure-breeding grey, Al Hattab, who was one such example. It is via this route that, several decades later, Alpha Centauri got her own colour. More recently, the French G1 winner Linamix is the only other pure-breeding grey sire I know of. Given that only around 3% of the total Thoroughbred population is grey, getting 100% grey stock is highly unusual.

I hope that gives you a flavour of how colour genetics work in Thoroughbreds. Personally I love a bit of colour jeopardy in new sires, and for the secret chestnut gene to appear! This is the polar opposite of breeders though; they favour a pure-breeding bay who can more obviously stamp his stock in his own image, for commercial purposes, and preferably nothing too flashy either! I think we’ll have to agree to differ on that one.

Take care everyone, and if you’d like to suggest any subjects for me to look at in future weeks then please do so in the comments below, as there’s no sign of them letting us out any time soon.

Caroline