Double Cross – Four Lines in Two Steps
Double cross is a production route for seed, not a breeding goal in its own right. Four true-breeding lines are first crossed in pairs, A with B and C with D, and the two resulting crosses are then crossed with one another: (A × B) × (C × D). True-breeding here means a line carries the same variant twice at the places in question and therefore passes its traits on unchanged.
The detour has one reason, and it is economic. It does not answer the question of which cross is best, but the question of which plant the seed is harvested from.
Why take a detour at all
Crossing two true-breeding lines is called a single cross, and it is what hybrid breeding is actually after: because both parents pass their traits on unchanged, the offspring of two particular lines always turn out the same. Once the right combination is found, any quantity of identical seed can be produced.
The catch sits in production. During the long narrowing towards true breeding, the lines lose vigour, in maize usually to less than half what open-pollinated varieties manage. Using such a line as the seed parent means harvesting the commercial seed from a weak plant, and getting correspondingly little of it. That is exactly what the double cross avoids: harvesting happens on the vigorous cross A × B rather than on a narrowed line, and seed costs fall.
Anyone offering a double cross as an alternative to backcrossing is therefore comparing two things that answer different questions: one moves a trait into an existing variety, the other produces seed in quantity.
What historically superseded the scheme
The second motive was emasculating the seed parent. In maize, male and female flowers sit on the same plant, so the male part has to be removed to stop the plant pollinating itself; that is done by pulling off the tassel at the top. Later this operation was largely replaced by a heritable factor that renders pollen useless and is passed on exclusively through the mother line – it sits not in the cell nucleus but in the cell fluid, and that comes from the egg cell alone.
With such a factor in place the double cross loses its cost advantage, and single crosses become economic again. The scheme did not turn out wrong, it turned out unnecessary.
Why it does not transfer directly to cannabis
Both motives rest on a plant that carries both sexes on the same individual and therefore has to be separated. Cannabis mostly carries them on separate plants; control over a cross runs simply through keeping male and female plants apart and through chemically induced sex reversion. A heritable factor rendering pollen useless through the mother line is not described for this species in the sources checked.
That removes the maize-specific reason for the detour. What remains is the question of how vigorous the seed parent is, and for cannabis that has been measured: narrowed lines carry increasingly malformed flowers from the fourth selfing generation onward, reaching 13.7 % against 0.4 % at the start in one line followed across seven generations, more than thirtyfold. No published work describes a double cross applied to cannabis.
One note on the scheme’s reach: cannabis carries two of each chromosome, one from each parent. In such species a cross combines exactly two contributions, so a single cross already exhausts the available gain. In species with more than two chromosome sets this does not hold, and the gain keeps rising across more than two parents.