WikiBreeding & genetics
Inbreeding & kinship
Coefficient of inbreeding, blocked pairings and what it costs.
Breeding two relatives doubles up on the same genes. The game measures that with a coefficient of inbreeding (COI), blocks the closest pairings outright, and makes inbred foals pay for it: weaker legs and nerves, and more mutations that go the wrong way.
What COI means
COI is the chance that a foal gets two copies of the same ancestor’s gene, one through its sire and one through its dam. Two unrelated parents give a COI of zero. The more ancestors the parents share, and the closer those ancestors are, the higher it goes. The game reads the stud book back 10 generations; horses with no recorded parents count as unrelated founders.
The foal keeps its COI for life. You see it as Inbreeding in the horse’s Genetics panel and under its pedigree tree, and the breeding preview warns in words before you pay: “Closely related” over the warning line, “Distantly related” below it.
By the numbers live from the rules
- Stud book generations read
- 10
- Preview says “closely related” from COI
- 6.2%
- Durability and temperament lost
- 80% of the COI (e.g. 12.5% COI costs 10%)
- Extra mutation chance per 10% COI
- 3%
- Extra share of harmful mutations per 10% COI
- 12%
Pairings that are never allowed
Three pairings are blocked outright. The breeding screen won’t let you cover them and tells you why:
- A horse can't breed with itself.
- Parent and child can't be paired.
- Full siblings can't be paired.
Everything else is allowed, however close. Half-siblings, grandparent and grandchild, uncle and niece: you can breed them, and the preview warns you in words.
How related is too related?
A small stud book run through the game’s own COI and block checks. The durability column is a stat whose genes would give 600 with no inbreeding; the last two columns are the mutation odds the rules give.
| Pairing | Foal's COI | Breeding screen | Durability (600 at no COI) | Mutation chance | Mutations that hurt |
|---|---|---|---|---|---|
| The same horse | – | Blocked | – | – | – |
| Parent and child | 25.0% | Blocked | – | – | – |
| Full siblings | 25.0% | Blocked | – | – | – |
| Half-siblings (same sire) | 12.5% | Inbreeding warning | 540 | 9.7% | 65% |
| Grandparent and grandchild | 12.5% | Inbreeding warning | 540 | 9.7% | 65% |
| Uncle and niece | 12.5% | Inbreeding warning | 540 | 9.7% | 65% |
| First cousins | 6.3% | Inbreeding warning | 570 | 7.8% | 58% |
| Half-cousins | 3.1% | Allowed | 586 | 6.9% | 54% |
| Second cousins | 1.6% | Allowed | 593 | 6.4% | 52% |
| Unrelated | None | No shared ancestors | 600 | 6.0% | 50% |
What inbreeding costs
- Inbreeding depression. Durability and temperament potential drop by 80% of the COI. A half-sibling foal loses about 10% of both. Lower durability means more injuries; lower temperament means a horse that wastes energy pulling early.
- More mutations. Each stat gene’s chance to mutate rises from 6.0% by 3 points for every 10% of COI.
- Worse mutations. At no COI half of mutations hurt. Each 10% of COI adds 12 points to that share.
- A lower floor. The family limits drop by a point for every 1% of COI, so an inbred foal can fall further.
Line breeding without paying for it
Breeding within a family has a real upside: it builds an established line with more room above the parents (see Family limits & lineage). The trick is to keep the family close but the parents not too close. Distant shared ancestors cost very little: second cousins come in at 1.6%.
See also Genetics & inheritance for how mutations work, and Injuries & insurance for why durability matters.