A test cross is a cross used to find out a plant's genotype. The unknown plant is crossed with a tester that is homozygous recessive (aa) for the trait. If all offspring show the dominant trait, the unknown is probably AA. If roughly half show the recessive, it is Aa.
In practice
Breeders use test crosses to check whether a plant carries a hidden recessive before using it widely. Example: to learn whether a photoperiod male carries the autoflowering allele, cross him to an autoflowering female (homozygous recessive). Any autoflowering offspring prove he is a carrier.
The term is also used more loosely for any small trial cross made to judge a parent, especially a male, by its offspring. That is closer to a progeny test. Either way, the more offspring, the firmer the answer: in a true test cross of an Aa plant, about 1 in 32 litters of 5 seedlings would show no recessives purely by chance.
AA × aa: 100% dominant phenotype. Aa × aa: 1:1 dominant to recessive. Chance an Aa parent shows no recessive offspring in n seedlings: (1/2)^n.
Don't confuse with
- Test grow: growing out seed to evaluate a line, not a genetic test of one locus.
- Backcross: crossing to a parent to recover its traits, not to diagnose a genotype.
The genotype is the set of alleles a plant actually carries, either at one locus or across its whole genome, as opposed to the traits you can see.A plant is homozygous at a locus when it carries two identical alleles there (AA or aa), so it passes the same allele to every offspring.A recessive allele only shows its effect when a plant carries two copies of it; in a single copy it is masked by a dominant allele and carried silently.A dominant allele is one whose effect shows in the phenotype even when only one copy is present, masking the other (recessive) allele at that locus.The daily length of light versus dark; photoperiod cannabis flowers when nights grow long enough, rather than at a fixed age.A day-neutral cannabis plant that starts flowering by age, usually a few weeks after germination, regardless of light schedule; the trait is mainly recessive.A progeny test judges a parent by growing out and measuring its offspring, the most reliable way to know what a plant, especially a male, actually passes on.A seedling is a young cannabis plant from emergence of the cotyledons until it has several sets of true leaves, typically the first 2-3 weeks.A plant is heterozygous at a locus when it carries two different alleles there (Aa), so its offspring can inherit either one and segregate.A Punnett square is a grid that lists each parent's possible gametes along two sides to predict the expected genotype ratios among offspring of a cross.Male selection is the choice of which male plants become parents, judged by their vigour, structure, early aroma and above all by the quality of their offspring.
