The Genetics of Nicking
Written by Byron Rogers | Nov 30, 2010 |
I have just finished reading Thoroughbred Breeding: Pedigree Theories and the Science of Genetics by Dr. Matthew Binns and Mr. Tony Morris, and it is a thoroughly interesting and worthwhile read. I can recommend it for anyone involved in breeding racehorses (see it at Amazon.com). The book covers a large range of topics within Thoroughbred breeding as it relates to pedigree theory and does this well, with a healthy dose of skepticism along with the right balance between covering the topics that should be of interest to any Thoroughbred breeder, and covering them in depth enough for the breeder with a little more understanding of the machinations of genetics.
As I said earlier, Morris and Binns cover a lot of topics within Thoroughbred breeding and unsurprisingly see fit to devote one of its 24 chapters to the concept of Thoroughbred nicking, a subject that has attracted its fair set of supporters and detractors alike. In just the second paragraph of this chapter, Morris and Binns cut straight to the heart of what Thoroughbred nicking is truly about, and why to date a lot of popular Thoroughbred nicking services haven’t truly addressed the best way to handle nicking at all. Morris and Binns write:
“When looking at nicks, the questions to be considered include: what proportion of poor horses were produced by the nick, what proportion of top-quality winners were produced by the stallion with mares by other stallions, and what proportion of top-quality races were run by the produce of those mares when bred to other stallions? That last angle is frequently omitted from discussions of nicks, even those that take a statistical approach. It is obviously important, as superior dams will often produce top-class stakes-winners by a number of different stallions, allowing the supposed nick to be seen in its proper perspective.”
While their book was published in July of 2010 Morris and Binns, whether by design or otherwise, have just described the exact computation that the TrueNicks rating is built on. The TrueNicks rating, first established and published in 2007, is calculated by factoring two separate computations:
- 1. The Sire Improvement Index (SII), a comparison of the percentage of stakes winners that a sire (or sire line) has achieved with mares by the specific broodmare sire (or broodmare sire line), with the percentage of stakes winners that the specific sire (or those specific members of the sire line) has achieved with all other mares; and
- 2. The Broodmare Sire Improvement Index (BSII), a comparison of the percentage of stakes winners produced by daughters of a particular broodmare sire (or broodmare sire line) with the percentage of stakes winners from the same mares when bred to all other sires.
The resultant figure shows the stakes winner-to-starter production rate of the nick, compared to the stakes winner-to-starter production rate of the sire/sire line and broodmare sire/broodmare sire line when bred to representatives of all other lines. We here at TrueNicks are obviously glad that Tony Morris and Matthew Binns, both leaders in their respective fields, have pointed out where the severe limitations of other nicking services lie, and have verified that the way that TrueNicks designed its calculation so many years ago is the best way to create a nick rating service.
The chapter goes on to talk about how, from a genetic standpoint, Thoroughbred nicking could be explained in terms of genetics. Binns describes what is possibly occurring when one sire line shows a distinct affinity for daughters of another sire.
“An alternative genetic explanation for nicking is that the daughters of some stallions are able to provide combinations of ‘good’ variants of genes that complement those of the stallion with whom the nick takes place. Not all of the broodmare sire’s daughters would receive the correct combinations of complimentary genes; the shuffling of genetic material that takes place during sperm and egg production would ensure variations. However, those that did would have an increased chance of producing better offspring when mated to that stallion. In this case we might expect that the nick would often be reversible, with the daughters of the stallion also proving complementary with the broodmare sire.”
This is a very interesting point to think about. Let’s put this in a real life example. Sadler’s Wells did very well when mated to Darshaan mares (see High Chaparral). There were 169 foals bred on this cross, from 75 different mares, with 28 stakes winners (17%) coming from 21 different mares. These specific Darshaan mares did not perform as well when bred to all other stallions, nor did Sadler's Wells with mares by all other broodmare sires. So if we can work our way back, lets for a minute presume that of these 75 mares, only 50% of these, or 37 of them, got the “good variants of genes” from their sire Darshaan and presume that these 37 mares had a 50% chance of passing these on to their foals when mated to Sadler’s Wells which would bring us back to 19 mares – close enough to the 21 in reality to show you how nicks really do work from a genetic sense.
Of course all this begs the question, if only 50% of Darshaan mares mated to Sadler’s Wells had the “good variants of genes,” would it not have been better to have known if the Darshaan mare had these genes in the first place, before she was mated to Sadler’s Wells? After all, if the good variants of genes were indeed required to produce a superior runner on the cross, half of the Darshaan mares bred to him may well have had no chance of producing a superior runner by Sadler’s Wells in the first place. Equally, if only 50% of the foals bred on the Sadler’s Wells/Darshaan nick expressed the good variant of genes, would it not have been better to know this if the resultant foal was on their way through the sales ring?
Finally, Binns mentions that “the nick would often be reversible.” In the case of Sadler’s Wells as a broodmare sire, when his daughters have been mated to Darshaan and his sons it has also proved itself a superior nick with a TrueNicks rating of A+ (see Conduit and Hard Top). It seems that in the case of Sadler’s Wells and Darshaan, not only are we talking about a solid nick rating, but indeed a complete genetic affinity between these two stallions.