Horse Genetics 101

January 9, 2009
Steve Thygersen

In order to understand pedigrees, breeding, phenotype and behavior, you really have to understand the fundamentals of
genetics. All mammals have basic two types of cells - the cells that make us who we are and our reproductive cells,
spermatozoa and ovum. What differentiates the reproductive cells are that they contain half of the chromosome sets of a
"standard" cell, which makes sense as it takes the combination of these "half sets" to create a whole set. Horses have 64
chromosomes in 32 pairs, humans have 46 chromosomes in 23 pairs. So any breeding of horses (or humans) will result in
50% of the genetic material coming from each parent. One of the few exceptions to this is a mule - where the donkey
provides 31 chromosomes and the horse 32, leaving a mule with 31.5 chromosome pairs. This is the primary reason that
mules are sterile. There are 8,388,608 possible chromosomal variations in humans and over 12M in horses!

It is these heterozygous pairings that make us all unique (some more than others…). When the genetic pool shrinks, the
bad traits come out as strongly as the weak traits. In fact Cheetahs have now become so inbred that you can swap parts
(heart, liver, skin, etc.) and not evoke an immune response. Are we getting there with horses? It could be, with all of the
Native Dancer, Northern Dancer and Mr. Prospector blood, we have narrowed the gene pool considerably, which is why a
sire like A.P. Indy is so critical, as he has no Northern Dancer, Native Dancer or Mr. Prospector blood. We know that
Boundary progeny have problems with their hooves (Big Brown being the most recent example) and we know that the
Unbridled's are brittle at best. Yet we continue to breed to those bloodlines.

When the genetic material combines at conception there are two types of pairing, homozygous and heterozygous.
Homozygous pairs are identical, where heterozygous are different. If there is a match between genes, that is they are
homozygous, the trait WILL be expressed. In fact ALL recessive traits are homozygous pairings. In heterozygous matches
the expressed gene will usually be the stronger of the two.

In order to explain heterozygous pairings and why certain genes are dominant, I need you to think about the card game
"War". In War, each player gets half the deck and then the players match each card and the higher card wins. In genes,
the exact same thing happens - a Jack beats a Nine, an Ace a Four, etc. The more dominant gene wins. The desirable
sires have very strong gene sets that can override most of the genes coming from the dam. But what happens when the
dams gene set is stronger?

Here is a classic example - Tiznow. His sire, Cee's Tizzy, is by Relaunch, a very light gray with incredible speed (watch
Spectacular Bid's world record 10F and the gray out front is Relaunch). Fully 85% of the progeny of Cee's Tizzy are gray,
as are Relaunch's. But then they bred Cee's Tizzy to Cee's Song (Seattle Song x Lonely Dancer), the classic Northern
Dancer/Bold Ruler mix. The resulting colt was Tiznow, a dark bay/brown that does not look like his sire, does not run like
his sire, and does not behave like his sire. He is long, light framed, runs with his head down and is unflappable. Now his
progeny are 85% brown to dark bay and have the same phenotype and personality as Tiznow. The dam (Cee's Stone) was
holding all of the high cards, and her genes won most of the heterozygous matches. Thus a new branch of the genetic tree
sprouts.

When you discuss the breeding success of a particular stallion, what do you think the percentage of offspring who win
Grade 1 races is? 5% sounds about right for the very best. Now let's look at the "blue hens", those dams whose gene sets
will override almost all of the sires. Toussaud (El Gran Senor x Image of Reality) has five sons - Chester House, Decarchy,
Empire Maker, Chiselling, and Civilisation. All are sires, ALL five (although Chester House died). Even more compelling is
the tale of Hasili, whose 7 children are all Grade 1 winners with the exception of the maiden Raise the Flag. A 100% rate of
Grade 1 winners, and from both Danehill and Green Desert; there has never been a sire that could come within 10 light
years of her record. The mare Special had a bunch of children that changed the landscape, including her son Nureyev, and
daughters Bound (dam of millionaire Archipenko and the filly Liable), Fairy Bridge (the dam of Sadlers Wells), Number
(Chequer, Jade Robbery, Numerous and Swindle). The point is simple, the gene set from the dam is every bit as powerful
as the sire, which is why I have to chuckle when everybody is on a sire rant, you literally only have half the story.

When a strong gene set from the sire meets a strong gene set from the dam, you have the POTENTIAL to create a freak -
as was the case in Sadlers Wells, he seemed to have picked up the best of Northern Dancer and Fairy Bridge (Special)
and the results are an extremely strong gene set which will dominate the expressed gene set of most of his progeny
(Montjeu, Galileo, etc.).

Remember too that the "Wild DNA" is the vast majority of the genes in a horse. Wild DNA are those fundamental traits that
make a horse, well a horse! Four legs, two eyes, a nose, a tail… There is not a lot of genetic material that can be
influenced, probably less than 20%. Which makes Zenyatta and Zarkava about 20% better than Zippy Chippy or Outer
Space.

Any geneticists will tell you I am oversimplifying it and leaving out a ton, and I am, because the rest is about as much fun
as a college Statistics class (now I'll get hate mail).