F1 hybrids, polyhybrids and inbreeding depression — the differences.
Mr.GramGram · 04 June 2026 · 9 min read
These days anything that is not a landrace or an IBL tends to be called a hybrid. But there is a large difference between a true F1, a polyhybrid and a selfed S1 — and it determines what you will actually grow out of a pack of seeds.
What F1 and heterosis are.
An F1 hybrid is the result of crossing two homozygous strains (landraces or IBLs). The classic case is a sativa landrace crossed with an indica landrace. In most cases an F1 shows heterosis (hybrid vigour): the offspring do not merely combine the parents’ best sides, they exceed them.
Why F2 and S1 lose vigour.
Cross two F1s (producing an F2) or self an F1 (producing an S1) and Mendel’s second law kicks in: traits segregate, hybrid vigour is lost, uniformity disappears. An important detail: an S1 is not the same strain. An S1 from an F1 is practically equivalent to an F2: Aa × Aa → 25 % AA, 50 % Aa, 25 % aa. Instead of a uniform mother plant we get segregation.
The polyhybrid.
Most “strains” on the seed market are polyhybrids. An F1 is a cross of two homozygous parents. As soon as hybrids get crossed with other hybrids, with repeated combinations and messy pedigrees, you have a polyhybrid: richer in genetics, but less predictable in its offspring.
Where the limit lies: inbreeding depression.
When building stable IBL lines breeders almost never go beyond F7–F8. The reason is inbreeding depression, the accumulation of “bad” traits under close-kin crossing. Look at the price of each further step:
| Generation | Heterozygous genes | Depression risk |
|---|---|---|
| F5 | 6.25 % | ×1 |
| F6 | 3.125 % | ×2.5 |
| F7 | 1.5625 % | ×6 |
| F8 | 0.78 % | ×15 |
| F9 | 0.39 % | ×40 |
Going from F7 to F8 raises stability by only ~0.78 %, while the risk of depression grows several times over. Stabilising further is neither profitable nor safe.
What inbreeding depression is
In humans and animals it is devastating — think of the Habsburg dynasty. In plants it is milder, but “bad” traits start piling up after the 7th generation: fasciation (fused stems, often confused with polyploidy, though they are different things), dwarfism, sometimes chlorosis and, at the extreme, non-viable pollen.
Questions and answers.
How does an F1 differ from a polyhybrid?
An F1 is a cross of two homozygous parents, which gives uniformity and hybrid vigour. A polyhybrid results from repeated crossing of hybrids — more varied, but less predictable.
Why is an S1 not the same strain?
Selfing an F1 produces trait segregation just like an F2. The mother phenotype is only partly reproduced and hybrid vigour is lost.
What is inbreeding depression in plants?
The accumulation of harmful traits under close-kin crossing after roughly the 7th generation: fasciation, dwarfism, chlorosis, weakening and pollen sterility.