Originally Posted by R.C. Clarke in "Marijuana Botany"
A common technique used to deduce the genotype of
the parents is the back-cross. This is done by crossing one
of the F1 progeny back to one of the true-breeding P1
parents. If the resulting ratio of phenotypes is 1:1 (one
heterozygous to one homozygous) it proves that the
parents were indeed homozygous dominant WW and
homozygous-recessive ww.
The 1:1 ratio observed when back-crossing F1 to P1
and the 1:2:1 ratio observed in F1 to F1 crosses are the two
basic Mendelian ratios for the inheritance of one character
controlled by one pair of genes. The astute breeder uses
these ratios to determine the genotype of the parental
plants and the relevance of genotype to further breeding.
This simple example may be extended to include the
inheritance of two or more unrelated pairs of genes at a
time. For instance we might consider the simultaneous
inheritance of the gene pairs T (tall)/t (short) and M (early
maturation)/m (late maturation). This is termed a poly-
hybrid instead of monohybrid cross. Mendel's second law
allows us to predict the outcome of polyhybrid crosses
also:
II. Unrelated pairs of genes are inherited indepen-
dently of each other.
If complete dominance is assumed for both pairs of
genes, then the 16 possible F2 genotype combinations will
form 4 F2 phenotypes in a 9:3:3:1 ratio, the most frequent
of which is the double-dominant tall/early condition. In-
complete dominance for both gene pairs would result in 9
F2 phenotypes in a 1:2:1:2:4:2:1:2:1 ratio, directly re-
flecting the genotype ratio. A mixed dominance condition
would result in 6 F2 phenotypes in a 6:3:3:2:1:1 ratio.
Thus, we see that a cross involving two independently
assorting pairs of genes results in a 9:3:3:1 Mendelian
phenotype ratio only if dominance is complete. This ratio
may differ, depending on the dominance conditions present
in the original gene pairs. Also, two new phenotypes,
tall/late and short/early, have been created in the F2 genera-
tion; these phenotypes differ from both parents and grand-
parents. This phenomenon is termed recombination and
explains the frequent observation that like begets like, but
not exactly like.
A polyhybrid back-cross with two unrelated gene
pairs exhibits a 1:1 ratio of phenotypes as in the mono-
hybrid back-cross. It should be noted that despite domi-
nance influence, an F1 back-cross with the P1 homozygous-
recessive yields the homozygous-recessive phenotype
short/late 25% of the time, and by the same logic, a back-
cross with the homozygous-dominant parent will yield the
homozygous dominant phenotype tall/early 25% of the
time. Again, the back-cross proves invaluable in determin-
ing the F1 and P1 genotypes. Since all four phenotypes of
the back-cross progeny contain at least one each of both
recessive genes or one each of both dominant genes, the
back-cross phenotype is a direct representation of the four
possible gametes produced by the F1 hybrid.
So far we have discussed inheritance of traits con-
trolled by discrete pairs of unrelated genes. Gene inter-
action is the control of a trait by two or more gene pairs.
In this case genotype ratios will remain the same but
phenotype ratios may be altered. Consider a hypothetical
example where 2 dominant gene pairs Pp and Cc control
late-season anthocyanin pigmentation (purple color) in
Cannabis. If P is present alone, only the leaves of the plant
(under the proper environmental stimulus) will exhibit
accumulated anthocyanin pigment and turn a purple color.
If C is present alone, the plant will remain green through-
out its life cycle despite environmental conditions. If both
are present, however, the calyxes of the plant will also ex-
hibit accumulated anthocyanin and turn purple as the
leaves do. Let us assume for now that this may be a desir-
able trait in Cannabis flowers. What breeding techniques
can be used to produce this trait?
First, two homozygous true-breeding ~1 types are
crossed and the phenotype ratio of the F1 offspring is
observed.
The phenotypes of the F2 progeny show a slightly
altered phenotype ratio of 9:3:4 instead of the expected
9:3:3:1 for independently assorting traits. If P and C must
both be present for any anthocyanin pigmentation in leaves
or calyxes, then an even more distorted phenotype ratio of
9:7 will appear.
Two gene pairs may interact in varying ways to pro-
duce varying phenotype ratios. Suddenly, the simple laws
of inheritance have become more complex, but the data
may still be interpreted.
Summary of Essential Points of Breeding
1 - The genotypes of plants are controlled by genes
which are passed on unchanged from generation to
generation.
2 - Genes occur in pairs, one from the gamete of the
staminate parent and one from the gamete of the pistillate
parent.
3 - When the members of a gene pair differ in their
effect upon phenotype, the plant is termed hybrid or
heterozygous.
4 - When the members of a pair of genes are equal in
their effect upon phenotype, then they are termed true-
breeding or homozygous.
5 - Pairs of genes controlling different phenotypic
traits are (usually) inherited independently.
6 - Dominance relations and gene interaction can
alter the phenotypic ratios of the F1, F2, and subsequent
generations.
