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

Bud.
     
    Understanding how recessive and dominant traits are passed down through the phenotype and genotype so that you can predict the outcome of a cross and lock down traits in future generations is really what breeding is all about.
     
    When you breed a strain, how do you know that the traits you want to keep will actually be retained in the breeding process? This is where the test cross comes in. If you create seeds from a strain that you bought from a seed bank, how can you be sure that the offspring will exhibit the characteristics that you like? If the trait you wish to continue is homozygous dominant (BB) in both parent plants then there’s no way that you can produce a recessive genotype for that trait in the offspring, as illustrated in the Punnett square below.
     
B
B
B
BB
BB
B
BB
BB
    It is impossible for the recessive trait to appear. And if both parents contain the recessive trait then they cannot produce the dominant trait.
     
b
b
b
bb
bb
B
bb
bb
    In order to breed a trait properly you must know if it is homozygous, heterozygous or homozygous recessive so that you can predict the results before they happen.

    MENDEL AND THE PEA EXPERIMENTS
    Gregor Mendel (1822-1884) was an Austrian monk who discovered the basic rules of inheritance by analyzing the results from his plant breeding research programs. He noticed that two types of pea plants gave very uniform results when bred within their own gene pools and not with one another. The traits he noticed were:
     
PEA PLANT #1
PEA PLANT #2
Solid seed shells
Wrinkled seed shells
Green seeds
Yellow seeds
White flowers
Purple flowers
Tall plants
Short plants
    He noticed that the offspring all carried the same traits when they bred with the same population or gene pool. Since there were no variations within each strain he guessed that both strains were homozygous for these traits. Because the pea plants were from the same species, Mendel guessed that either the solid seed shells were recessive or the wrinkled seed shells were recessive. Using the genotype notations SS for solid seed shells and ss for wrinkled seed shells, he knew that they couldn’t be Ss because one lot didn’t exhibit any of the other strain’s phenotypes when bred within its own gene pool.
     
    Let’s illustrate this using two basic Punnett squares where SS is pea plant #1 with the trait for solid seed shells and ss is pea plant #2 with the trait for wrinkled seed shells.
     
    Pea plant #1 results:
     
S
S
S
SS
SS
S
SS
SS
    All the offspring will be SS.
     
    Pea plant #2 results:
     
s
s
s
ss
ss
s
ss
ss
    All the offspring will be ss.
    The First Hybrid Cross (the F1 Generation)
    Mendel made his first hybrid cross between the two strains and the results were all solid seeds as seen in the chart below.
     
F1 Cross
S
s
S
Ss
Ss
S
Ss
Ss
    Up until this point, he didn’t know which trait was recessive and which was dominant. Since all the seeds shells were solid, he now knows with certainty that pea plant #1 contained the dominant genotype for solid seed shells and pea plant #2 contained the recessive genotype for wrinkled seed shells. This meant that in future test crosses with other pea strains, he could determine if a particular seed shell trait was homozygous or heterozygous because he had identified the recessive trait (ss).
    The Second Hybrid Cross (the F2 Generation)
    The offspring in the F1 cross were all Ss. When Mendel crossed these offspring he got the following results:
     
F2 Cross
S
s
S
SS
Ss
s
Ss
ss*
*Take special note of this offspring and compare with parents.
    Mendel had mated two pea plants that were heterozygous (e.g., Ss) for a seed shell trait. In this group, the resulting offspring were:
25% of the offspring were homozygous for the dominant allele (SS)
50% were heterozygous, like their parents (Ss)
25% were homozygous for the recessive allele (ss)
    In his first cross to create the hybrid plant, Mendel ended up with no recessive traits for seed shape. But when he crossed the