The Cannabis Breeder's Bible by Greg Green Read Free Book Online

this, you will have to perform what is called a test cross.
     
    Determining the phenotype of a plant is fairly straightforward. You look at the plant and you see, smell, feel or taste its phenotype. Determining the genotype cannot be achieved through simple observation alone.
     
    Generally speaking, there are three possible genotypes for each plant trait. For example, if Golden Bud is dominant and Silver Bud is recessive, the possible genotypes are:
     
Homozygous Dominant:
BB
= Golden Bud
Heterozygous:
Bb
= Golden Bud
Homozygous Recessive:
bb
= Silver Bud
     
    The Golden and Silver Bud colors are the phenotypes. BB, Bb and bb denote the genotypes. Because B is the dominant allele, Bb would appear Golden and not Silver. Most phenotypes are visual characteristics but some, like bud taste, are phenotypes that can’t be observed by the naked eye and are experienced instead through the other senses.
     
    For example, looking at a Mostly Sativa species like a Skunk plant you will notice that the leaves are pale green. In a population of these Skunk plants you may notice that a few have dark-green leaves. This suggests that this Skunk strain’s leaf color is not true breeding, meaning that the leaf trait must be heterozygous because homozygous dominant and homozygous recessive traits are true breeding. Some of the Skunk’s pale-green leaf traits will probably be homozygous dominant in this population.
     
    You may also be asking the question: “Could the pale-green trait be the homozygous recessive trait and the dark-green leaf the heterozygous trait?” Since a completely homozygous recessive population (bb) would not contain the allele (B) for heterozygous expression (Bb) or for homozygous dominant expression (BB), it is impossible for the traits for heterozygous (Bb) or homozygous dominant (BB) to exist in a population that is completely homozygous recessive (bb) for that trait. If a population is completely homozygous for that trait (bb or BB), then that specific trait can be considered stable, true breeding or ‘will breed true.’ If a population is heterozygous for that trait (Bb) then that specific trait can be considered unstable, not true breeding or ‘will not breed true.’
     
    If the trait for Bb or BB cannot exist in a bb population for that trait, then bb is the only trait that you will discover in that population. Hence, bb is true breeding. If there is a variation in the trait, and the Hardy-Weinberg law of equilibrium has not been broken, the trait must be heterozygous. In our Skunk example there were only a few dark-green leaves. This means that the dark-green leaves are homozygous recessive and the pale-green leaves are heterozygous and may possibly be homozygous dominant too.
     
    You may also notice that the bud is golden on most of the plants. This also suggests that the Golden Bud color is a dominant trait. If buds on only a few of the plants are Silver, this suggests that the Silver trait is recessive. You know the only genotype that produces the recessive trait is homozygous recessive (bb). So if a plant displays a recessive trait in its phenotype, its genotype must be homozygous recessive. A plant that displays a recessive trait in its phenotype always has a homozygous recessive genotype. But this leaves you with an additional question to answer as well: Are the Golden Bud or pale-green leaf color traits homozygous dominant (BB) or heterozygous (Bb)? You cannot be completely certain of any of your inferences until you have completed a test cross.
     
    A test cross is a performed by breeding a plant with an unknown dominant genotype (BB or Bb) with a plant that is homozygous recessive (bb) for the same trait. For this test you will need another cannabis plant of the opposite sex that is homozygous recessive (bb) for the same trait.
     
    This brings us to an important rule: If any offspring from a test cross display the recessive trait, the genotype of the parent with the dominant trait must be