Are Female Families Really That Important?

female families

female families

Female Families Numbers Explained

To allow this topic to come full circle, please forgive the initial history and science lessons. I promise that I will eventually get to something practical and/or useful!

The Family Numbers commonly used to designate the various Thoroughbred female families were popularized by Bruce Lowe, an Australian pedigree researcher at the end of the nineteenth century. He had traced back the pedigrees of the complete list of winners of the oldest English classics (the St. Leger Stakes, Epsom Derby Stakes and Epsom Oaks) grouping them by direct lines of tail female descent, from dam to grandam and on back until the family was no longer traceable in the General Stud Book. He then tallied the number of classic winners produced by each family and listed them in declining order. The family with the most classic winners, the one descending from Tregonwell’s Natural Barb Mare, was designated Family #1, the Burton Barb Mare second, designated Family #2, and so on. The resulting forty-three numbered families became the core of his study, and while few actually adhere to Lowe’s resulting theory, many still use his family numbers as a convenient way to categorize Thoroughbred families, self included.

Today, the families established by Lowe have been expanded to include families no. 44-74. Other groupings of mares were also added to incorporate the global nature of the Thoroughbred. They are classified as follows:

  • Families 1-50 – Bruce Lowe’s original numbered English families traceable to the earliest volumes of the General Stud book
  • Families 51-74 – Can be additionally traced to General Stud Book mares
  • Families AR1-AR2 – Native to Argentina
  • Families P1-P2 – Native to Poland
  • Families A1-A37 – Native to America
  • Families C1-C16 – “Colonial families” native to Australia
  • Families B1-B26 – Designated as “Half-bred” due to some impure crosses

Whether one agrees or disagrees with all of Lowe’s theories, his family numbers have given researchers the greatest tool possible to seek out similarities in pedigrees. Along with knowledge of the major breeders and their best sires and dams, family numbers give us the best possible information to seek out linebreeding patterns in today’s runner.

Chromosomes and Heart Size

This is a complex subject, so I will just keep it to the basics. Genetic factors and gender biases can be found in the X- and Y-chromosomes. These are the chromosomes most directly involved in gender. A stallion (XY) has one of each chromosome. A mare (XX) has two X-chromosomes and will always pass one of the two on to her progeny. The stallion is always the determinant of the gender of a prospective foal. If he passes his Y-chromosome, the resulting foal is a colt (XY); if he passes his X-chromosome, a filly (XX) will result.

Now let’s add in the X-Factor Theory into the equation.  The X-Factor Theory proposes that a mutation within a gene located on the X-chromosome of females causes a larger-than-average heart. Unlike humans, a large heart in a thoroughbred is a good thing. It gives the horse greater stamina and strength. So, if all if this is true, X-chromosomes, female families and greater heart capacity go hand in hand. They are extremely important in planning matings or analyzing pedigrees of potential racing stock.

One classic illustration of these two points is that of Secretariat. The post-mortem discovery of his huge heart size is legendary. In all his greatness, Secretariat was not an outstanding sire, nor a sire of sires. But, his daughters made him an exceptionally fine broodmare sire producing the likes of Storm Cat, A. P. Indy and Gone West just to name a few. Again, when daughters were produced, he passed his female chromosome.

Racing Families and Sire Families

Lowe’s theory went far beyond identifying female strains. Of these families, he found that nine in particular appeared to be “indispensable” in the breeding of top racehorses and he divided these into two classes, running (family #’s 1, 2, 3, 4, and 5) and sire (family #’s 3, 8, 11, 12, and 14) or as Lowe perceived them, feminine and masculine, respectively. His theory was based on balancing the “feminine and masculine factors” by using these two classes of families as “core” to good matings and essential for successful breeding. Note that Family #3 is in both groups. Lowe considered it able to produce both good runners and good sires. I call it a “dual-qualifier” and can be exemplified by A. P. Indy (3L) and Tapit (3:o).

I have added female family #13 to the sire list in my analyses now making it read 3, 8, 11-14. Family #13C alone has produced the likes of stallions Darshaan (GB), Seattle Slew, Mr. Prospector, Siberian Express, Le Fabuleux (FR), etc.

OPTIMAL Matings and Female Families

An OPTIMAL Mating is a comprehensive snapshot of a seven-generation pedigree that incorporates seven time-tested breeding theories and gives breeders the greatest opportunity to breed a superior racehorse. No single theory is capable of achieving an exceptional runner with every breeding, but occasional success is expected. Therefore, I demand that all seven theories are satisfied within each mating or pedigree of a potential purchase.

My very first step in analyzing a pedigree is to consider female families. I designed the mating of multiple Grade 1 stakes winner Mor Spirit for a long time client. Since I am familiar with the pedigree and for hopefully a better illustration for you, let’s look at his pedigree below:

We all probably have characteristics and/or mannerisms that most reflect one of our four grandparents (2nd generation). Horses are no different. That said, I focus on the two female families located in the 3rd generation that make up each “grandparent”. That creates eight different individuals consisting of eight families. Genetically, we can’t determine the path in advance that nature will take. We can’t even definitively say that the direct female family will come forward even though this is one of our main commercial concerns. But, we can examine and reinforce quality families in hopes that the best ones come forward.

This is where female family numbers, running families, sire families and, to a degree, X-Factor Theory come into play. In analyzing tens of thousands of stakes winning pedigrees, I have determined that the greater majority of stakes winners have at least one sire family and four running families in their 3rd generation. This is my baseline. If I can’t satisfy this first pedigree factor, I’m on to the next stallion for a mating or the next page of a sales catalog.

Let’s look at the individuals and female families in Mor Spirit’s 3rd generation:

  • Storm Cat – female family 8C
  • Mariah’s Storm – female family 11
  • Seattle Slew – female family 13C
  • Altair – female family 2D
  • Dixieland Band – female family 4M
  • She’s Tops – female family 4M
  • Allen’s Prospect – female family 4M
  • Sequins – female family 1N

Based on sire families (3, 8, 11-14) and running families (1-5), Mor Spirit has 3 sire families and 5 running families in his 3rd generation (1 and 4 minimums required). Coincidentally, all 8 families represented here are what Bruce Lowe termed “core” and “indispensable” families.

Female families are just one of seven factors I use in determining an OPTIMAL Mating. All of my pedigree analyses start and most stop with female families. Yes, they are really that important!

Whether you are new to the game, experiencing more losses than wins or just looking for a new approach, I am confident that I can help you breed or buy a better racehorse. To find out more about OPTIMAL Matings and how they can help your program, please subscribe to my Blog/News page HERE to receive future information on complimentary mating recommendations, sales short lists, stakes winner pedigree analysis, news on personal client success and any other random thoughts or ideas that I may have. Also, receive a complimentary copy of my report “Blind Luck or Designed Luck? – How A Kentucky Derby Winner Was Bred” sent directly to your inbox.

For inquiries or more information about Shepherd Equine Advisers, please contact Clark Shepherd at 859-321-6618, or by email at

I would love to hear any comments or questions that you may have. Please submit them below. Maybe we can address them in a future post!

I offer my expertise as a bloodstock agent, sales consultant, and pedigree analyst to assist in breeding or purchasing superior thoroughbred racehorses through sound pedigree principles, horsemanship, and consistent implementation of well-designed plans. To find out more about how my services can help your program, please contact me at 859-321-6618, or by email at

13 thoughts on “Are Female Families Really That Important?

  1. Dear Mr. Shepard

    I found this article very interesting. Do you believe in the large heart theory?
    Do you agree with the 4 sires of the large heart? Princequillo, War Admiral,
    Blues Larkspur and Mahmoud. Do you look for this on the dam side? If the foal fits your criteria on the female side, are there factors that would eliminate the foal, ie inbred in 2nd generation?

    1. Thanks for the feedback William! I believe every theory must be taken with a grain of salt unless verified. But, I guess if it’s verified, it wouldn’t be a theory! My sales analyses create short lists that take me to the horse. If it passes physical and vet inspections, I will also order a cardio scan. Through the process, starting with pedigree analysis, the majority of these individuals are found to have target hearts. Luck? I call it “designed luck”! I love to reinforce those four stallions in doing matings or finding them in existing pedigrees. So, whether you believe X-Factor theory or the power of those stallions, I truly believe that keeping the right female families alive by reinforcing and combining them create superior athletes. I look at all female families involved. Not just the dam side or her direct female family. As stated in the post, we don’t know which path of the four “grandparent” will be taken genetically. The pedigree needs to be structured, balanced and reinforced to accommodate whichever path it takes. As far as elimination goes, I don’t accept anything closer than 3×4 or 4×3. Yes, there outliers to this as there are to any rule or theory. Hybrid vigor is key to superior race horses. The focus may be on the particular inbreeding but what about all the other duplications that occur within the same pedigree? Unfortunately, a lot of inbreedings are done commercially for type for better sales results. Same theory for heavy inbreeding in dogs for type. With that, weaknesses and infirmities are created. Sorry for rambling, but I hope I answered your questions and that it makes some sense. Thanks again and have a blessed day! – Clark

      1. You weren’t rambling, I enjoy reading your blog. Thank you for the reply. I guess I have some homework on pedigree analysis.

        1. A daily effort with me! With new stallions and continually coming on to the scene, it’s ever-changing and I’m always learning. It’s also amazing what one generation removed does to the complexion of an individual’s pedigree and what they have to contribute to their progeny.

  2. Great article on evaluating the true prepotentancy of a stallion! I love your approach! I’m a very small breeder from Ocala, Fl. that has had great success getting horses to the races and into the winners circle but I’m still in pursuit of my first stakes winner. I have always been a strong believer in the power of the female family and I have always believed that the sire acts as a conduit that brings the great female families into the mating! You have really given me something to think about now and I am very grateful for you sharing this information! Just a quick question but how did family #9 get left out of the core and indispensable female familes? Of course one of my broodmares is from 9-f which is loaded with Grade 1 winners and Champions! Thank you so much for sharing your valuable insight and I look forward to reading your blog as you have a wealth of information to offer!😁👍👍🏇🏇🏇

    1. Interesting article as I also give staunch support for breeding to female families. Glad to know there are other professional pedigree researchers doing the same. In regards to family #9 and other important families being left out, you must remember that Bruce Lowe’s work was published in about 1889, when certain families were not as effective as they are today. For example family 9-c gain fanfare with success of Americus Girl’s a foal in 1905. Her production via Mumtaz Mahal led to Nasrullah, Tudor Minstrel, Fair Trial, Mahmoud, etc. Another is family 9-e who produced the great sire Cyllene, foaled in 1895, seven years after Bruce Lowe’s death. And an even more important point to consider is that these family figures were harvested from a bygone era of racing where weather conditions and racing surfaces were different. We all know horses prefer different surfaces, which is why in my opinion family 13-c was not part of that running or siring strain. Family 13-c was doing well in France when used by Marcel Boussac. Anyhow female family numbers are very important and I use it extensively…

  3. Breeders today would like to have a sire with the magnitude of Galileo (9h), Distorted Humor (9f), Medaglia d’Oro (9b), Dubawi (9c), Lookin At Lucky (9f)… Decades ago it was Fair Trial, Mahmoud, Nasrullah, Tudor Minstrel, Aristophanes, Abernant, Royal Charger, Kashmir, Kalamoun, Alydar… all from 9c. Before that it was Cyllene, Fair Play, Friar Rock (9e).. also, Ack Ack, Tom Rolfe, Honeyway, Chieftain (9h), Bull Lea (9f)… and the list goes on and on with A class sires, champion racers, blue hens and classic producing mares. The siring strength of this family (9) is powerful even with the lesser known 9g who gave us Cee’s Tizzy, Bayakoa, etc… Bruce Lowe was a genius considering the enormous work involved tracking and analyzing data without high speed computing systems especially in face of adversity. The idea of labeling families running and siring lines immediately raised skepticism. Generally males are better racers than females plus there’s an added disadvantage of number of foals when sires spread genetics far and wide. A sire success is dependent on breeders choosing the right mare, since not every branch of family 9 or any siring/running lines has produced desired results. Family 9g is questionable in North America, family 9b major contributions were mainly Speightstown and Medaglia d’Oro, There were Kennedy Road, First Landing, Silent Screen and others that failed to make it big. But still we cannot underestimate the power of family 9 while being cautious to avoid a few. In response to the original question, family #9 should be a siring line as they have more collective siring power than 13, but the caveat is family 9 is also effective as a running line. The two major stallions that probably swayed your opinion of family 13 being a siring line are undoubtedly from the branch of 13c. The temptation is understandable as both Seattle Slew and Mr Prospector were amazing, top class sires. However, both their siring power could have came from a combination of other siring lines – Polynesian (14); Myrtlewood’s sire, Blue Larkspur (8)(8); in case of Mr Prospector we can add in Nashua (3); Sunreigh, Raise A Native, John P Grier (8)(8)(8); for Seattle Slew, we can add War Admiral (11); Princequillo also brought forth a cross of 11. Thanks again for the chance to express opinions on my favorite subject breeding.

  4. Thanks Clark!
    As a true novice who started by closing his eyes and throwing a dart I find this short synopsis a great starting point ( since I’m starting all over again). I’ve passed this along to all of the “ dart throwers “ and encouraged them to follow your writing and contact you before moving into buy/ build mode.

    1. Thank you, James! It’s always a good idea to do your research and seek out expert advice before making any big decisions, and I’m happy to hear that you’re encouraging others to do the same. Please don’t hesitate to reach out if you have any further questions or if there’s anything else I can help with. Best of luck in your future endeavors!

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