Certain inbred lines will display hybrid vigour when crossed. These vigorous lines are said to have favorable combining ability.
Certain inbreds have the ability to combine well with testers--these have general combining ability (GCA). When the inbred combines well only in certain crosses, it has specific combining ability (SCA). The only way to
select for combining ability
is to grow and examine the progeny. An astute breeder can recognize the potentital for
by identifying the dominant traits of the parents and deducing which lines may combine favorably.
Predicting the combining ability of recessive traits can only be determined through
The breeder is interested in single crosses
(also known as F1 generations) that outperform other single crosses. If the breeder has multiple
to work with, she could select first for GCA, then for SCA among the lines with GCA, then identify the best parental gene donors. In most cases with Cannabis you can go directly to selecting for specific combining ability between your IBL and your testers.