Sex Linked Chromosomes
Ken Mclean says “The sex strains of a duplicated ancestor are more important than most breeders understand. The collection of dominant genetic strength from famous producing mares is not to be under rated.”
Mares have two X chromosomes – XX and stallions have XY.
Mares inherit one X from each parent. Which X they receive from their dams is not known . Stallions only get their X chromosome from their mothers, and they can only pass this on to their daughters.
The sex chromosomes control sex linked characteristics and are important for many other functions of the body
Ken Mclean " the sex chromosomes hold the key to the hormone system and hormones play a vital role in the development of all athletes "
The sex chromosomes are allowed unique communication with all other 31 chromosome pairs and may have an affect on all other cellular systems
All of the other chromosomes have their influence confined to specific regions of the body only - they cannot communicate with each other like the sex chromosomes can
Sex linked gene groupings may play a critical role in the development of champions with their influence on the neuron and hormone systems - these affect growth rates, maturity, muscle development and fibres, fertility , and nervous energy - along with their interplay with the environment
Hormonal influence can be seen early on - some foals are born very small and are slow to develop while others look like they are 3 weeks old at birth and mature far more quickly - their genetic code has been programmed to react this way depending on the factors driving the foals heredity
Reinforcing superior maternal strains in a pedigree increases the chances for the expression of athletic qualities
Ken Mclean says “ Only mares carry the genetic code for the gene string that is missing in stallions.”
Ken Mclean also says that this is why duplicating a stallion via his sons only in a pedigree is a limiting factor for the transmission of sport talent – as it automatically excludes any valuable sex linked genes located on the X chromosome.
An example would be a horse by De Niro out of a Dimaggio mare – both are sons of Donnerhall and it is duplicating Donnerhall via his sons only.
Ken Mclean says that reinforcing a male ancestor via his sons and daughters is an opportunity for valuable sex-linked gene groupings to be inherited in a foal, because a stallions X chromosome is always given to his daughters - and that this sex balance is required to deploy similar genes on both sides of chromosomal pairings. See more about Sex Balancing in Pedigrees.
This expression of sex linked genes is often seen best when full brother and sister are present in the engine room of the pedigree - and this is one of the most powerful configurations of all to upgrade sport ability
The X chromosome is much larger than the Y and carries far more genetic information. At present it is not known which X the mare gives to her foal, whether this is a random process or ruled by processes we have not yet discovered
One X chromosome could be much larger than the other with many more important sex-linked genes – this could also be a reason explaining the difference in full siblings - where one sibling is a champion and the other a disappointment.
Perhaps the champion inherited the more valuable X chromosome and the unsuccessful brother the smaller X.
Stallions can only transmit sex lined genes through their daughters - so any stallion line in a pedigree should always be paired with a line of his daughter. If you want to increase the chances of setting sport ability and athleticism in your foals add in his full sister or a second progeny line of his dam
Many female family lines have produced stallion sons who succeed at stud whether or not they competed - or even if they had limited sport ability themselves.
“These foundation or tap root mares in families - their progeny have an important role in producing great performers and are selected solely on their ability to transmit superior genes to future generations.” Ken Mclean
Some of these mares may have had special mtDNA or sex-linked genes which they reliably pass on to their daughters who then also produce super stallions.
Madam Vuillier said " It is the mare that makes the stallion and not the other way around"
Ken Mclean says this maternal strength is limited to only a small percentage of stallions and mares in any horse population and may need to be reinforced by bringing in their close relatives in a pedigree, brothers, sisters or progeny from the same female line.
“The biochemistry of some ancestors must have been uniquely different to others. They may have possessed valuable mutated genes or MtDNA from a mix of strains, or sex linked genes on the X chromosome"
We should try to identify some of these mares - where they exist in very prepotent superior stallions that often carry desirable performance traits .
Examples in another section
We should attempt to buy mares, where possible, from successful female families. These mares should be given the opportunity in the breeding shed even if they were never competed or showed limited sport ability themselves, as they may carry valuable sex linked genes
Pedigree expert Anne Peters quoted a study which confirmed that maternal heritability of athletic performance may be stronger than paternal heritability for the same
"If superior mtDNA (and sex linked genes) is the thing that makes the critical difference then we have been putting the emphasis in the wrong place" - meaning breeders rely too heavily on stallions alone to bring all the ability to the foal
"This could explain why so few stallions succeed at stud and why so many top performance stallions fail at stud"
Bill Lathrop said "My own laymans study of genetics, concentrating particularly in areas of research involving the X chromosome and mtDNA have convinced me there is a possibility that aptitudinal traits might be transmitted through multiple generations of the female families, and that selected mares may act as conduits of this genetic material into succeeding generations"
Even if your mares do not come from top female families which have produced international competition horses and licensed stallions - which is most breeders - and most mares -
you can still incorporate important female families in your pedigree designs by building on what is in your mares pedigree.
For instance, if your mare has Pik Bube i in her pedigree you can use a stallion that has Pik Bube 11, or a full sister to these in his pedigree. This has been a very important sport family that has produced different branches of elite mare lines , stallions and top progeny
An example of this is new young stallion Daan G - he is not from a top producing dam line himself - but he is heavily linebred to one of the best mare families of all - Dohlenfuerstin (1.60m jumper ) and her daughter Franka who produced Pik Bube i and ii and the sisters Pik Dame, Pirola, Polljana and Paola - who left a legacy of top horses and breeding stallions
Daan G has Pik bube i and his full sisters Pirola and Paola coming in via their sons - Ragazzo, Bergamon and Donnerschlag
These appear at 4 x 5 x 6 x 6
This depth of linebreeding in the engine room of the pedigree - to 4 different progeny lines of one taproot mare - Franka - happens so rarely by chance - and when it does happen in important superior families such as this - the probability that the foal will inherit the beneficial sex linked genes skyrockets - and it appears clearly that Daan G did inherit something special.
He will also be a good breeding stallion with this tight pedigree design to superior relatives and mare strains
This young stallion will most likely be in heavy demand this 2026 season and i look forward to seeing his progeny
One way to tap into the genetic potency of Daan G would be - if you have a mare with Pik bube i or ii in her pedigree, or a mare by Riverside whose second dam is Pirola - full sister to Pik bube,
or a mare with Sirocco (Sir Oldenburg) in the pedigree - he is out of a Donnershlag mare who is also out of Pirola - this would work
These would be compatible mare strains to add to this combination in Daan G