The inbreeding coefficient, explained

What the inbreeding coefficient measures

The inbreeding coefficient, written F, is the probability that a chick carries two identical copies of a gene that both come from one and the same ancestor: once through its father and once through its mother. The geneticist Sewall Wright introduced it in 1922, which is why it is often called Wright's F.

F belongs to the chick, but the pair decides it: the coefficient of a pairing is the F their chicks will have. An F of 0 means the parents have no known common ancestor. An F of 25% is what you get when you pair full siblings, or a parent with its own offspring.

The higher F is, the more genes a chick carries in two identical copies. Hidden recessive traits then show up more often. That is useful when you want to fix a colour mutation, and harmful when it brings out weakness, lower fertility or poor hatching results, what breeders call inbreeding depression.

How it is calculated

Wright's path method works straight from the pedigree. Look for every ancestor that appears on both the father's side and the mother's side. For each of those common ancestors, count the generations from the father up to that ancestor (n1) and from the mother up to that ancestor (n2). That ancestor then contributes (½) to the power of (n1 + n2 + 1).

Add up the contributions of all paths and you have F. A path may not pass through the same bird twice, so an ancestor that is only reached through a closer common ancestor adds nothing of its own.

The complete formula also multiplies each contribution by (1 + FA), where FA is the inbreeding of the common ancestor itself. When little is known about that ancestor's own pedigree, FA is usually taken as zero.

Worked examples

Half-siblings share one parent. From each bird to that parent is one generation, so the contribution is (½) to the power (1 + 1 + 1) = 1/8, or 12.5%.

Full siblings share both parents. Each parent contributes 1/8, together 1/4, or 25%.

A parent paired with its own offspring: the parent itself is the common ancestor, zero generations away on one side and one generation on the other. That gives (½) to the power (0 + 1 + 1) = 1/4, or 25%.

First cousins share two grandparents, each two generations away on both sides: (½) to the power (2 + 2 + 1) = 1/32 per grandparent, together 1/16, or 6.25%.

How to read the number

Those reference points are the basis of the risk levels Newbred shows: none (0%), low (under 6.25%, less related than first cousins), moderate (6.25 to 12.5%), high (12.5 to 25%) and very high (25% and above, the level of full siblings).

There is no single safe limit. Many breeders keep general breeding in the low range and only accept more for a deliberate line-breeding step, followed by an outcross to unrelated birds. What matters most is that you know the number before you decide, not after the chicks have hatched.

What the coefficient cannot tell you

F is only as good as the pedigree behind it. Ancestors that were never recorded count as unrelated, so a shallow pedigree underestimates inbreeding. The further back your records go, the more reliable the number.

F is also an expected value, not a DNA test. Full siblings average 25%, but the actual share of identical genes differs from chick to chick.

Let Newbred do the arithmetic

You do not have to trace paths by hand. When you create a new pair in Newbred, it walks both pedigrees up to ten generations deep, calculates Wright's F for their chicks and shows the risk level and every common ancestor with its contribution. Inbreeding analysis is included in the free Starter plan.