Linebreeding is a form of selective breeding in which animals are bred to retain and concentrate the desirable traits of a specific ancestor or line of ancestors in their pedigree. It involves mating animals that are related to each other to a lesser degree than inbreeding, with the aim of maintaining or enhancing specific genetic qualities in the offspring.

The Linebreeding Report gives complete inbreeding statistics for the number of generations you are currently viewing.

Columns in the Report

Inbreeding Stats

Shows where a particular ancestor is inbred in the pedigree. You'll see notation like 4S x 3d. The numbers (4 and 3) are the generations the inbred ancestor appears in. S refers to the sire's side of the pedigree; D refers to the dam's side. A capital S or D means the ancestor is inbred through its male offspring; a lowercase s or d means the ancestor is inbred through its female offspring.

For breeding planning, the generation numbers tell you how tight each cross is. Close crosses like 3x3 or 4x4 concentrate an ancestor's influence aggressively, which amplifies both the traits you want and any recessive weaknesses the ancestor carried. Distant crosses like 5x6 or 6x7 keep the ancestor present without overexposing the foal to a single bloodline. The case and side pattern matters too: a 4S x 4D cross balances the ancestor through both parents, while 4S x 5S stacks them on the sire's side, which is often used to reinforce a sire-line trait like speed, stamina, or conformation.

Crosses

How many times the inbred ancestor appears in the pedigree.

Higher cross counts signal stronger overall genetic influence from that ancestor, even when each individual cross is distant. When scanning a candidate mate, look for the ancestors you want to reinforce and prefer mates whose pedigrees already carry multiple crosses to them. If an ancestor has an unusually high cross count (7 or 8 in a 5-generation pedigree), the pedigree is already saturated with that line and may benefit from fresh blood rather than additional doubling.

Lines

How many unique offspring the ancestor is inbred through. The numbers in parentheses are the count of males and females it is inbred through. Hover over these numbers to see the full list of animals it is inbred through.

Lines reveal how the ancestor's influence reaches the pedigree. An ancestor with many lines contributes diverse genetic material from different descendants, which carries less inbreeding risk even at high cross counts. An ancestor with few lines is funneling through a narrow channel, which concentrates the specific genes those descendants inherited. The male/female split also matters for sex-linked traits and for breeders building female-family or sire-line strategies: an ancestor inbred through four female descendants behaves differently in the foal than the same ancestor inbred through four sons.

Influence

A simplified estimate of an inbred ancestor's blood percentage, expressed in the same "Generation x Generation" notation as Inbreeding Stats. It always appears as {Number} x {Number}. For example, an ancestor with Inbreeding Stats of 4s x 4S x 6S has a blood percentage of about 14%. The Influence column will show 4x4, because a 4x4 inbred ancestor has a blood percentage of 12.5%. These two percentages aren't equal, but they're close enough to serve as a quick estimate.

Influence is useful as a target when planning a mating. Breeders often aim for a specific cross of a foundation ancestor (a 4x5 of Northern Dancer, a 3x4 of Mr. Prospector) and Influence lets you see at a glance whether a prospective mating lands near that target. Comparing influence values across ancestors also shows you which lines genuinely dominate the pedigree versus which ones look prominent but contribute little.

Blood Percentage

The amount of influence an ancestor has on a descendant. Each parent contributes 50% of their DNA to their offspring. Each grandparent contributes 25%, each great-grandparent contributes 12.5%, and so on. If an ancestor appears in a descendant's pedigree twice, once in the 2nd generation and once in the 3rd, we add both influence values together (25% + 12.5% = 37.5%) to reach the blood percentage.

Blood percentage is the most precise way to quantify how much of a given ancestor sits in the foal. As a rough guide, most effective linebreeding programs keep target-ancestor percentages in the 6% to 18% range. Below that, the ancestor is barely present; above 25%, the pedigree is functionally inbred on that animal and the risks rise sharply. When evaluating a proposed mating, sum the blood percentage of your target ancestors in both parents to predict what the foal will carry, and watch for any single ancestor whose combined percentage crosses into inbreeding territory.

AGR

A value similar to blood percentage, calculated using matrices and linear algebra. It weighs ancestors in younger generations (2, 3) more heavily than older generations (7, 8). For a brief overview, see the forum discussion. For the full mathematical derivation, see these papers:

• Miller, 6WCGALP
• Schaeffer, ANSC637

AGR is the better number to watch when you care about which ancestors will actually express in the foal. Traits from recent ancestors reach the offspring with less genetic dilution, so an ancestor with a modest blood percentage but a high AGR is exerting more practical influence than the raw percentage suggests. Use AGR to separate the ancestors truly driving a pedigree's phenotype from those that are historically prominent but genetically distant.

Unique Ancestor Counts

Alongside the columns, the report shows two summary statistics that describe the overall shape of the pedigree:

Number of unique ancestors in X generations. The count of distinct ancestors within the first X generations of the pedigree.

Number of unique ancestors on generation X. The count of distinct ancestors at exactly that generation depth.

In a completely outcrossed pedigree with no repeated ancestors, generation N contains 2^N ancestors, and the first N generations contain 2^N+1 − 2 ancestors in total. A 5-generation outcrossed pedigree would have 32 ancestors in the 5th generation and 62 unique ancestors across the first five generations. When the actual counts are lower, the gap tells you how much linebreeding is present.

Why it matters for breeding planning

Comparing the actual counts against the outcrossed baseline gives you a quick read on a pedigree's structure, and that read has practical uses:

• Gauge how tight a pedigree is. A pedigree with far fewer unique ancestors than the theoretical maximum is tightly linebred. One at or near the maximum is broadly outcrossed. Knowing where a horse sits on that spectrum frames every mating decision that follows.
• Locate where concentration begins. If the counts track the outcrossed baseline through generation 4 but drop sharply at generations 5 and 6, that's where the linebreeding lives. Useful for deciding whether to reinforce that pattern in the next mating or dilute it.
• Compare prospective mates. Looking at both candidates' unique ancestor counts at matching generations helps you anticipate what their foal's pedigree will look like. Two tight pedigrees produce an even tighter foal; mating a tight one to a diverse one introduces fresh blood without abandoning the foundation.
• Guard against inbreeding depression. When the counts fall well below the outcrossed baseline in the first four or five generations, the pedigree is already concentrated. Introducing unrelated blood in the next mating can restore hybrid vigor before fertility or vitality suffer.
• Consolidate a desired line. If the goal is to concentrate a particular ancestor's influence, choose a mate whose pedigree keeps unique ancestor counts low in the generations where that ancestor appears. The resulting foal will carry more copies of the target bloodline.