Backcross – Crossing an Offspring Back to a Parent

Backcross means crossing an offspring back to one of its two parent lines. The purpose is always the same: an existing variety is to be kept as it is and gain one single property, say a resistance from an old landrace.

Where the same parent is used again in every round it is called the recurrent parent. The other line is the donor and contributes nothing but the wanted trait. Each round halves whatever is left of the donor, and it does so whether or not any selection is applied.

The sequence of numbers

The share recovered from the recurrent parent can be worked out in advance: after t rounds it averages 1 − (1/2)^(t+1) of the genome. Written out, that is 75.0 % after the first round, 87.5 after the second, 93.8 after the third, 96.9 after the fourth and 98.4 % after the fifth. Moving a single trait by the classical route and recovering 99 % of the original variety accordingly takes five rounds or more.

Markers cut that roughly in half. Markers are places in the genome that can be read unambiguously in the laboratory. Screening each generation not only for the wanted trait but also for how much donor genome an individual still carries gets you there in three generations rather than six.

The two conditions

They appear in the same chapter of the breeding literature and are missing from most short accounts. First, the recurrent parent should itself be a true-breeding line, meaning it carries the same variant twice at the places in question and passes its traits on unchanged. If it does not, but segregates in every round, what returns at the end is not the variety but a random sample of it.

Second, the wanted trait has to depend on one or a few places in the genome. Every additional place lowers the chance of finding them all in the same plant – and turns a manageable programme into one that fails on the sheer number of offspring needed.

German breeding literature calls the method Verdrängungszüchtung, displacement breeding. It suits cases where a simply inherited trait, say the resistance of a landrace, is to be combined with the properties of a high-yielding but susceptible variety.

What does not disappear with the rest

One thing stays behind, and by design: the immediate neighbourhood of the place in question. Because every round selects for exactly that place, its surroundings are unavoidably selected along with it, while the rest of the donor genome halves. That remainder is called linkage drag.

Its size depends not on the number of generations but on how often the chromosomes are cut and rejoined at that place at all. Where the trait sits in a stretch where this rarely happens, a piece of several million base pairs survives even three rounds – against a cannabis chromosome set of roughly 750 million base pairs, a good one percent of the whole genome. Where it sits favourably, the same piece shrinks to a fraction of that.

A backcross is therefore no alternative to a double cross either. The two methods answer different questions: one moves a trait into an existing variety, the other produces commercial seed from four lines.

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