Transgression – Offspring Beyond the Parental Range
Transgression arose, in the case on record, from similar parents rather than distant ones, which runs against the textbook recommendation. Anyone breeding for extremes therefore picks crossing partners differently from what the common rule of thumb suggests. The term describes offspring whose trait values fall outside the range of both parents, above it as well as below, and it arises when polygenes from both parents come together in one descendant.
The case on record
The clearest evidence comes from rice. Among six crosses examined, only the one between two almost identically flowering varieties produced a wide spread: 0.7 days of difference between the parents, 18 days of range in the second filial generation. The crosses with distant varieties produced next to no transgression.
Responsible were not a few large effects but five small loci whose allelic contributions complemented one another, plus an interaction between two of them. The authors attribute the result to positive and negative allelic effects cancelling each other out when parents differ strongly.
Why the rule of thumb persists
Material as heterogeneous as possible is held to favour transgression, for instance crossing partners from geographically distant regions. That recommendation stands in the breeding literature and makes sense for other goals, such as producing novel trait combinations at all. For extremes within a single trait, the measured case says the opposite. For cannabis the question has not been examined; both statements come from other crops and should be treated as expectations rather than rules.
What can be fixed
Unlike the heterosis of the first filial generation, transgression can be fixed genetically from the second onward: the extreme forms in the rice case persisted into the third and fourth generation. It still cannot be predicted, which is why the breeding literature budgets a large number of test crosses for it.
Outliers in both directions
The mechanism does not distinguish between wanted and unwanted. A plant too tall with too short a flowering time arises the same way as the seedling worth keeping: from a combination of small effects that neither parent displayed. In practice both turn up in the same cross and cannot be targeted separately; the only lever is the number of plants among which you look.