Horse Genetics: Breeding Strategies

Inbreeding

• What it is: Breeding very closely related horses (e.g., parent to foal, brother to sister).

• Why is it used: To “fix” or lock in a particular look or ability (called “type”) so offspring look and perform very similarly.

• The risk: Too much inbreeding reduces genetic variety, leading to inbreeding depression — lower fertility, slower growth, more health problems, and higher chance of genetic diseases.

  👉 Think of it like photocopying the same picture again and again: you keep the details, but the copy quality gets worse over time.

Line Breeding

• What it is: Breeding horses of the same breed that look alike but don’t have close relatives in the recent pedigree.

• Why is it used: It’s a compromise — keeps desirable type while avoiding the worst health risks of inbreeding.

• Result: Offspring have roughly the same level of inbreeding as the parents, not much higher.

  👉 Useful if the breed has enough unrelated bloodlines to work with.

Outbreeding

• What it is: Breeding within the same breed (or very similar breeds) but making sure there are no common ancestors.

• Why is it used: To bring in genetic variety, even if type isn’t as consistent.

• Result: Offspring usually healthier, with more variation in looks and abilities, and a lower inbreeding level than their parents.

  👉Think of it as shuffling the deck for a fresh hand of cards

Crossbreeding

• What it is: Breeding horses from completely different breeds.

• Why is it used: To create hybrid vigour (also called heterosis) — stronger, healthier offspring thanks to maximum genetic variety.

• Result: Neither parent’s exact type is preserved, but offspring often show stamina, fertility, and resilience.

  👉 Like mixing two recipes to create a new flavour — not exactly either parent dish, but potentially very good.

In short:

• Use inbreeding cautiously — it fixes traits but carries risks.

• Prefer linebreeding or outbreeding to keep a balance between type and health.

• Remember: Mother Nature loves variety — genetic diversity is the fuel for long-term survival.
  
  

Coefficient of Relation (a)

Definition: A measure of how much DNA two animals share because of common ancestors.

Where:

½ means each parent passes on half its genes.
n = the number of pathways (number of generations) that separate two animals within a pedigree.
∑ = the sum of the calculations for all possible pathways linking the two individuals.

Worked examples:

• Parent ↔ child: one step → (½)^1 = 50% shared DNA

• Full siblings: two paths (via mother and father), each 2 steps → (½)^2 + (½)^2 = ¼ + ¼ = 50%

• Grandparent ↔ grandchild: two steps → (½)^2 = 25%

• First cousins: four steps through a shared grandparent → (½)^4 = 1/16 = 6.25% per path. Two paths (one via each grandparent) = 12.5%.

  👉 The higher the number, the more related two animals are.

Coefficient of Inbreeding (F or COI)

Definition: A measure of how much genetic variety an individual horse has lost because the parents share common ancestors.

Where:

F = the Coefficient of Inbreeding (COI) for the individual in question.
½ = the genetic contribution of a parent towards its offspring.
n = the number of pathways (number of generations) between a common ancestor and the individual in question.
+1 = an additional factor of (1/2) is added to represent the anticipated loss of genetic diversity due to common ancestors on both maternal and paternal sides of the pedigree.
∑ = the sum of the calculations for each individual ancestor in common.
a = the coefficient of relation between the parents.

Worked examples:

• Parent–child mating: Parents share 50% → offspring F = 25% (very high; avoid).

• Full siblings mating: Parents share 50% → offspring F = 25% (very high).

• First cousins mating: Parents share 12.5% → offspring F = 6.25% (borderline).

• Unrelated horses: Parents share 0% → offspring F = 0% (no inbreeding).

  👉 Rule of thumb: Every 1% increase in F = about 1% drop in performance traits like fertility or survival.

• Under 5% = ideal.

• 5–10% = manageable, but watch for problems.

• Above 10% = risky.

• 12.5%+ = incest level; don’t do it.