The test cross
by Greg Green
Determining the phenotype of a plant is fairly straightforward. You look at the plant and you see its phenotype. Determining the genotype can not be done through visible observation alone. The genes themselves are somewhat hidden except for their visible related phenotypes.
Again there are three possible genotypes that the plant can create. Let’s say golden bud is dominant and silver bud is recessive. Here is the table.
Homozygous Dominant: BB = Golden Bud.
Heterozygous: Bb = Golden Bud.
Homozygous Recessive: bb = Silver Bud.
(The Golden and Silver bud colours are the phenotype. The b and B are genotype notations.) The reason why Bb is golden and not silver is because B dominates b.
Most phenotypes are observed characteristics but some things like bud taste are also phenotypes that can not be observed. If we look at a Mostly Sativa species like a Haze plant we will notice that it is pale green. Now in a population of Haze plants we may notice that the one or two out of one hundred maybe dark green and not pale green. This suggests that the pale green colour is recessive. We are not totally sure until we have completed the test but the gene frequencies suggest this. We may also notice that the bud is golden on most of the plants so this suggests that the golden bud colour is a dominant trait. Some of the buds on only a few of the plants may be silver. This suggests that silver trait is Recessive (bb in our example).
We know that the only genotype that produces the recessive trait is homozygous recessive (bb). So if a plant shows a recessive trait in its phenotype, its genotype is probably homozygous recessive.
A plant with a recessive trait always has a homozygous recessive genotype.
But this leaves us with a problem. Is the Golden bud or pale green leaf colour a Homozygous Dominant (BB) or is it Heterozygous (Bb).
So now is the time to perform the Test Cross. Any test cross is a cross of an organism with an unknown dominant genotype (like in our case) with an organism that is homozygous recessive for that same trait.
To do this test we need another cannabis plant of the opposite sex that is homozygous recessive (bb) for the same trait. So we will stick with bud colour as our example. Hey, we have a few silver bud plants around that we think are recessive. Let’s use them and see what happens. We pollinate the female plant (Does not matter if the female is dominant or recessive one), and we get our seeds and plant them. 3 – 7 months later we see the results. This brings us to the next important rule that we will learn.
If any of the offspring from a test cross have the recessive trait, the genotype of the parent with the dominant trait must be Heterozygous.
We will explain why in a moment and this will all make sense to you. Also we must mention that we should be talking about a large population here. 1000 plants is a good population to be sure with. 100 plants are good but 20 or less can be dodgy. The more plants we use the more reliable our results will be.