The point of this little article is to explain some terms commonly used in breeding theory and discussions of breeding.

Note: Some people insist on using the terms differently than described here. The standard definition are stated and explained - this article does not take a position on whether definitions should be different or be changed. It is, however, often confusing and often impossible to discuss with somebody who insist of using a word in a way which is not in accordance with common understanding.

The first definition of inbreeding and linebreeding traces back to James Harrison in the The Care & Training of the Trotter & Pacer.

If an individual is repeated in the pedigree you add up the generations that horse is repeated. If the sum is equal to or under 6 (=less than 7) the horse is said to be inbred and if the sum is 7-9 the horse is linebred. To better understand, consider the pedigree of Roxane Griff:

We see that Chambon P is a common ancestor as he is found both in the sire's and the dam's pedigree. Chambon P is is found in the 3rd generation on the paternal side and in the 3rd generation on the maternal side, so Roxane Griff is inbred 3x3 (or 3+3) on Chambon P.

On the other hand, if we look at the pedigree of Olimede:

We see that Speedy Crown is found in the 4th generation on the paternal side and in the 3rd generation on the maternal side. As a result, Olimede is considered linebred 4x3 on Speedy Crown.

There are some horses where there are several common ancestors. One such example is Napoletano

We see a 2x4 inbreeding on Star's Pride and also a 3x3 inbreeding on Rodney.  There is also a 4x4 cross on Volomite. If we look more closely we see that Volomite appears twice in the sire and three times in the dam. As a result some may also specify the cross on Volomite as being "(4x4) x (4x6x5)" where (4x4) refers to where he appears on the paternal side and "(4x6x5)" on the maternal side (reading from top to bottom).

As we see, a horse can be inbred - or linebred - on more than one individual. It is common, however, to look at the common ancestor that is appears "closest" - if a horse is linebred 3x4 on one horse and 4x5 on another horse, then the horse is generally considered linebred only on the first one.

In Napoletano you also see Victory Song highlighted in yellow. However, Victory Song appears twice on the maternal side but not on the paternal side so he is not a common ancestor. Napoletano's dam Noble Sarah is inbred on Victory Song (in a 2x3 pattern) but Napoletano himself is neither inbred nor linebred on Victory Song.

Be aware that this definition of inbreeding and linebreeding is harness racing specific. In the thoroughbred world the most common definition of an inbred horse is "one that has at least one duplicated ancestor between the parents within four generations,"

The inbreeding coefficient measures the common ancestors of the dam and sire, and indicates the probability of how genetically similar they are.

Technically speaking is it the mathematically which measures the probability that two alleles on a gene pair are identical and from a common ancestor. It is beyond the scope of this summary of definitions to go deeper into this here.

If a horse is not inbred or linebred it is often said to be outcrossed. The term, however, has not been accurately defined in harness racing terminology and a horse can be described as an outcross either if it is neither inbred nor linebred or it if has no common ancestors in the first 5 or 6 generations.

The generation interval indicates the time, usually express in years, between each generation in the pedigree.

There are several ways of measureing the generation interval. The most simple is to look at the parents of a horse and calculate their average age at birth. If we consider Napoletano, he is born 1984 and both his parents are born 1969, so both parents were 15 years when he was born. So by measuring only the age of the parents, Napoletano's generation interval is 15 years. However, doing so ignores the rest of the pedigree: if a foal is born with really young parents but both parents have extremely old parents then on the whole, in a 2-generation perspective, the generation interval isn't that fast after all.

Another approach is a calculate the average birthyear of either grandparents of great grandparents. Again looking at the pedigree of Napoletano, who is born 1984, we see that his grandparents are born 1947 (Star's Pride), 1962 (Pillow Talk), 1962 (Noble Victory) and 1958 (Miss Sarah Rodney). The average of these years is 1957.25. Since Napoletano is born 1984, there are 1984 - 1957.25 = 26.75 years from his grandparents to he was born. Since there are two generations from him to his grandparents, the generation interval, measure in this way, is 26.75 / 2 = 13.375 years.

Another way which tends to look at even more of the pedigree is the Sophia-50 number as generated by the Sophia pedigree program. The Sophia-50 number is found by counting all ancestors in positions in the pedigree born within 50 years of the horse's year of birth (so if the horse is born 2010, then all horses in the pedigree born 1960 and later are counted). Napoletano has a Sophia-50 number of 25, which corresponds to a generation interval of 13.56 years.

Also called the taproot or foundation mare and is the last individual along the female line with a recognized pedigree as either standardbred or thoroughbred

The tail female is the last individual of the same breed (as the current horse) along the female line (i.e. dam, damdam, damdam's dam, damdam's damdam etc). The term "of the same breed" can be somewhat tricky even though it sounds easy. In the earliest US standardbred studbooks the dams were not included and sometimes their names were not even recorded. In a few cases thoroughbred mares have also been commonly regarded as tail females despite been thoroughbreds, Esther (b 1877) is probably the most famous such tail female.

The term bottom line refers to the female line (i.e. dam, damdam, damdam's dam, damdam's damdam etc) found at the bottom of the pedigree. It is common to refer to the damdam's dam (or damdamdam) as 3rd dam, the damdam's damdam as 4th dam etc.

The term top line refers to the sire line (i.e. sire, siresire etc) found at the top of the pedigree. It is the opposite of the bottom line.

The Delta pattern is a pedigree pattern where where a horse on the top line and the bottom line are full siblings. The pedigree of Glee Face illustrates the Delta pattern:

Since Speedy Scot (5th sire) on sireline and Rare Scotch (3rd dam) on damline are full siblings, Glee Face is bred on the Delta pattern.

The Formula One pattern is pedigree pattern where the maternal line of the stallions and the maternal line of the broodmare are identical.