Simple Breeding
Your approach to breeding will depend on what you ultimately hope to achieve.
Do you want to create a new strain; create seeds that are similar to the
parents; or cross two plants to create a simple hybrid strain?
Continuing a Strain through Seeds
Say you purchased $120 worth of Silver Haze seeds and you want to make
more seeds without any interference from another strain. That's easy.
Just make sure that the male and female plants you breed with are from
the same strain batch. In this instance the same strain batch would be
Silver Haze from the same breeder. If you use Silver Haze from different
breeders then the offspring may express a great deal of variation.This
is because most breeders create their own versions of a popular strain.
Their variety may have dissimilar characteristics from those of other
breeders who have bred the same strain.
If you only have Silver Haze from the same breeder in your grow room,
then all you need are a group of males and a group of females. Let the
males pollinate the females and you will get more Silver Haze seeds, but
you will loose some of the features of the original parent plants unless
the strain you have is an IBL or from a very stable inbred pure line.
Making a Simple Hybrid
Again, making a simple hybrid is easy. Just take a male plant from one
strain and a female plant from another, for example Big Bud and Skunk.
The result will be vBig Bud x Skunk', but there will be differences in
the offspring. Some of the plants will exhibit more Big Bud traits and
some will exhibit more Skunk traits. Genes not expressed by each of the
parents may also appear in the offspring.
If you want to breed for specific traits by eliminating variations, ultimately
creating uniform plants or even an IBL, then you should start with a basic
knowledge of plant genetics.
INTRODUCTION TO PLANT GENETICS
Genetics can be somewhat difficult to understand at first so we'll start
by explaining a few rudimentary concepts and the basic terminology. The
explanations for the words below can be treated as a glossary for your
benefit.
Genes
Genes are the units of heredity transmitted from parent to offspring,
usually as part of a chromosome. Genes usually control or determine a
single characteristic in the offspring. There are genes responsible for
each feature of your plant to be inherited, including leaf color, stem
structure, texture, smell, potency, etc.
Gene Pairs
All of life is made up of a pattern of genes. You can think of this pattern
as being similar to the two sides of a zipper. One side is inherited from
the mother and the other from the father. Each gene occupies a specific
locus, or particular space on the chain, and controls information about
the eventual characteristics of the plant. So each gene locus contains
two genes, one from the mother and one from the father. These gene pairs
are usually denoted by a pair of letters, such as BB, Bb, Pp, pp, etc.
Capital letters refer to dominant genes while lower case letters refer
to recessive genes. By way of example, B can represent Big Bud while b
can represent small bud. Any letter can be assigned to any trait or gene
pair when you are working out your own breeding program.
Chromosome
A threadlike structure of nucleic acids and proteins in the cell nuclei
of higher
organisms that carries a set of linked genes, usually paired.
Locus
A position on a chromosome where a particular gene pair is located.
Allele
Alleles are any of a number of alternative forms of one gene. For example
the gene for purple bud color may have two forms, or alleles, one for
purple and one for dark red.
Homozygous
Having identical alleles at one or more genetic loci, which is not a heterozygote
(see below) and breeds true. Your plant is said to be homozygous for one
feature when it carries the same gene twice in the responsible gene pair,
which means both genes of the gene pair are identical.
Heterozygous
Having different alleles at one or more genetic loci. Your plant is said
to be heterozygous for one feature when the genes of the responsible gene
pair are unequal, or dissimilar.
Phenotype
The phenotype is the summary of all of the features you can detect or
recognize
on the outside of your plant, including color, smell and taste.
Genotype
The genotype is the genetic constitution of your plant, as distinguished
from the phenotype.The genotype characterizes how your plant looks from
the inside. It is the summary of all the genetic information that your
plant carries and passes on to its offspring.
Dominant
Dominant is used to describe a gene or allele that is expressed even when
inherited from only one parent. It is also used to describe a hereditary
trait controlled by a gene and appearing in an individual to the exclusion
of its counterpart, when alleles for both are present. Only one dominant
allele in the gene pair must be present to become the expressed genotype
and eventually the expressed pheno-type of your plant.
Recessive
Recessive describes a gene, allele or hereditary trait perceptibly expressed
only in homozygotes, being masked in heterozygotes by a dominant allele
or trait. A gene is called recessive when its effect cannot be seen in
the phenotype of your plant when only one allele is present.The same allele
must be present twice in the gene pair in order for you to see it expressed
in the phenotype of your plant.
Dominant/Recessive and Genetic Notation
Assume that the dominant'B'allele carries the hereditary trait for Big
Bud, while the recessive xb' allele carries the hereditary trait for small
bud. Since B is dominant, a plant with a Bb genotype will always produce
Big Bud.The B is dominant over the b. In order for a recessive gene to
be displayed in the phenotype, both genes in the gene pair must be recessive.
So a plant with the BB or Bb gene will always produce Big Bud. Only a
plant with the bb gene will produce small bud.
Now that we have explained the basic terminology of plant genetics, we
can move on to the next step: rudimentary breeding concepts as laid out
in the Hardy-Weinberg law of genetic equilibrium.
THE HARDY-WEINBERG MODEL OF GENETIC EQUILIBRIUM
An understanding of plant breeding requires a basic understanding of the
Hardy-Weinberg law.To illustrate the value of the Hardy-Weinberg law,
ask yourself a question, like: "If purple bud color is a dominant
trait, why do some of the offspring of my purple bud strain have green
buds?" or "I have been selecting Indica mothers and cross-breeding
them with mostly Indica male plants but I have some Sativa leaves. Why?"
These questions can be easily answered by developing an understanding
of the Hardy-Weinberg law and the factors that can disrupt genetic equilibrium.
The first of these questions, reflects a very common misconception: that
the dominant allele of a trait will always have the highest frequency
in a population and the recessive allele will always have the lowest frequency.This
is not always the case. A dominant trait will not necessarily spread to
a whole population, nor will a recessive trait always eventually die out.
Gene frequencies can occur in high or low ratios, regardless of how the
allele is expressed.The allele can also change, depending on certain conditions.
It is these changes in gene frequencies over time that result in different
plant characteristics.
A genetic population is basically a group of individuals of the same species
(cannabis Indica or cannabis Sativa) or strain (Skunk#l or Master Kush)
in a given area whose members can breed with one another.This means that
they must share a common group of genes.This common group of genes is
locally known as the gene pool.The gene pool contains the alleles for
all of the traits in the entire population. For a step in evolution —
a new plant species, strain or trait — to occur, some of the gene
frequencies must change. The gene frequency of an allele refers to the
number of times an allele for a particular trait occurs compared to the
total number of alleles for that trait in the population. Gene frequency
is calculated by dividing the number of a specific type of allele by the
total number of alleles in the gene pool.
Genetic Equilibrium Theory and Application
The Hardy-Weinberg model of genetic equilibrium describes a theoretical
situation in which there is no change in the gene pool. At equilibrium
there can be no change or evolution.
Let's consider a population whose gene pool contains the alleles B and
b.
Assign the letter p to the frequency of the dominant allele B and the
letter q to the frequency of the recessive allele b. We know that the
sum of all the alleles must equal 100 percent, so:
p + q = 100%
This can also be expressed as:
p + q = l
And all of the random possible combinations of the members of a population
would equal:
p2 + 2pq + q2
WHERE:
p = frequency of the dominant allele in a population q = frequency of
the recessive allele in a population p2 = percentage of homozygous dominant
individuals q2 = percentage of heterozygous recessive individuals 2pq
= percentage of heterozygous individuals
Imagine that you have grown a population of 1,000 'Black Domina' cannabis
plants from seeds obtained from a well known seed bank. In that population,
360 plants emit a skunky smell, while the remaining 640 plants emit a
fruity smell. You contact the seed bank and ask them which smell is dominant
in this particular strain. Hypothetically, they tell you that the breeder
selected for a fruity smell and the skunk smell is a recessive genotype.
You can call this recessive genotype Vv'and use the formula above to answer
the following questions.
QUESTION: According to the Hardy-Weinberg law, what is the frequency of
the Vv'genotype?
ANSWER: Since 360 out of the 1,000 plants have the Vv'genotype, then 36%
is the frequency of Vv' in this population of 'Black Domina'.
QUESTION: According to the Hardy-Weinberg law, what is the frequency of
the Vallele?
ANSWER: The frequency of the Vv'allele is 36%. Since q2 is the percentage
of homozygous recessive individuals, and q is the frequency of the recessive
allele in a population, the following must also be true:
q2 = 0.36
(q x q) = 0.36
q = 0.6
Thus, the frequency of the Vallele is 60%.
QUESTION: According to the Hardy-Weinberg law, what is the frequency
of the NV'allele?
ANSWER: Since q = 0.6, we can solve for p.
p + q = l
p + 0.6 = 1
p = 1 - 0.6
p = 0.4
The frequency of the VV allele is 40%.
QUESTION: According to the Hardy-Weinberg law, what is the frequency of
the genotypesN W and 'Vv'?
ANSWER: Given what we know, the following must be true:
VV = p2
V = 0.4 = p
(p x p) = p2
(0.4 x0.4) = p2
0.16 = p2
VV = 0.16
The frequency of the genotype NVV is 16%
VV = 0.16
vv = 0.36
VV + Vv + vv = 1
0.16 + Vv + 0.36 = 1
0.52 + Vv = 1
Vv = 1 - 0.52
Vv = 0.48 or 48%
Or alternatively, NVv' is 2pq, therefore:
Vv = 2pq
2pq = 2 x p x q
2pq = 2 x 0.4 x 0.6
2pq = 0.48 or 48%
The frequencies of V and v (p and q) will remain unchanged, generation
after generation, as long as the following five statements are true:
1. The population is large enough
2. There are no mutations
3. There are no preferences, for example a VV male does not prefer a vv
female by its nature
4. No other outside population exchanges genes with this population
5. Natural selection does not favor any specific gene
The equation p2 + 2pq + q2 can be used to calculate the different frequencies.
Although this equation is important to know about, we make use of other
more basic calculations when breeding. The important thing to note here
is the five conditions for equilibrium.
Earlier we asked the question: "I have been selecting Indica mothers
and crossbreeding them with mostly Indica male plants but I have some
Sativa leaves. Why?" The Hardy-Weinberg equilibrium tells us that
outside genetics may have been introduced into the breeding program. Since
the mostly Indica male plants are only mostly Indica and not pure Indica,
you can expect to discover some Sativa characteristics in the offspring,
including the Sativa leaf trait.
THE TEST CROSS
Some of you may be asking the question: "How do I know if a trait,
such as bud color is homozygous dominant (BB), heterozygous (Bb) or homozygous
recessive (bb)?"
If you've been given seeds or a clone you may have been told that a trait,
such as potency, is homozygous dominant, heterozygous or homozygous recessive.
However, you will want to establish this yourself, especially if you intend
to use those specific traits in a future breeding plan. To do 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: HETEROZYGOUS: HOMOZYGOUS RECESSIVE:
BB = Golden Bud
Bb = Golden Bud
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 can not 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 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 heterozygous and not homozygous.
In our example, our unknown genotype is either BB or Bb.The Silver Bud
genotype is bb. We'll put this information into a mathematical series
known as Punnett squares.
We start by entering the known genotypes. We do these calculations for
two parents that will breed. We know that our recessive trait is bb and
the other is either BB or Bb, so we'll use B? for the time being. Our
next step is to fill the box in with what we can calculate.
The first row of offspring Bb and Bb will have the dominant trait of
Golden Bud. The second row can either contain Bb or bb offspring.This
will either lead to offspring that will produce more Golden Bud (Bb) or
Silver Bud (bb).The first possible outcome (where ? = B) would give us
Golden Bud (Bb) offspring.The second possible outcome (where ? = b) would
give us Silver Bud (bb) offspring. We can also predict what the frequency
will be.
Outcome 1, where ? = B:
Bb+ Bb+ Bb+ Bb = 4Bb
100% Golden Bud
Outcome 2, where ? = b:
Bb + Bb + bb + bb = 2bb
50% Golden Bud and 50% Silver Bud
RECALL:
Homozygous Dominant: Heterozygous: Homozygous Recessive:
BB = Golden Bud Bb = Golden Bud bb = Silver Bud
To determine the identity of B?, we used another cannabis plant of the
opposite sex that was homozygous recessive (bb) for the same trait.
OUTCOME 2 TELLS US THAT:
• Both parents must have at least one b trait each to exhibit Silver
Bud in the phenotype of the offspring.
• If any Silver Bud is produced in the offspring then the mystery
parent (B?) must be heterozygous (Bb). It cannot be homozygous dominant
(BB).
So, if a Golden Bud parent is crossed with a Silver Bud parent and produces
only Golden Bud, then the Golden Bud parent must be homozygous dominant
for that trait. If any Silver Bud offspring is produced, then the Golden
Bud parent must be heterozygous for that trait.
To summarize, the guidelines for performing a test cross to determine
the genotype of a plant exhibiting a dominant trait are:
1. The plant with the dominant trait should always be crossed with a plant
with the recessive trait.
2. If any offspring display the recessive trait, the unknown genotype
is heterozygous.
3. If all the offspring display the dominant trait, the unknown genotype
is homozygous dominant.
The main reasoning behind performing a test cross are:
1. When you breed plants you want to continue a trait, like height, taste,
smell, etc.
2. When you want to continue that trait you must know if it is homozygous
dominant, heterozygous or homozygous recessive.
3. You can only determine this with certainty by performing a test cross.
We should mention that, as a breeder, you should be dealing with a large
population in order to be certain of the results.The more plants you work
with, the more reliable the results.
Hardy-Weinberg Law, Part 2
The question may arise: "How do I breed for several traits, like
taste, smell, vigor and color?" To answer this question, you will
need to learn more about the Hardy-Weinberg law of genetic equilibrium.
If you breed two plants that are heterozygous (Bb) for a trait, what will
the offspring look like? The Punnett squares can help us determine the
phenotypes, genotypes and gene frequencies of the offspring.
IN THIS GROUP,THE RESULTING OFFSPRING WILL BE:
1 BB - 25% of the offspring will be homozygous for the dominant allele
(BB)
2 Bb - 50% will be heterozygous, like their parents (Bb) 1 bb - 25% will
be homozygous for the recessive allele (bb)
Unlike their parents (Bb and Bb), 25 percent of offspring will express
the recessive phenotype bb. So two parents that display Golden Bud but
are both heterozygous (Bb) for that trait will produce offspring that
exhibit the recessive Silver Bud trait, despite the fact that neither
of the parents displays the phenotype for Silver 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.
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.
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.
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 know 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 Fl cross were all Ss. When Mendel crossed these offspring
he got the following results:
F2 Cross
*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 offspring, because
they were heterozygous for that trait, he ended up with some having the
homozygous recessive trait, some having the homozygous dominant trait
and some continuing the heterozygous trait. In correct breeding terms
his first cross between the plants is called the Fl cross or Fl generation.The
breeding out of those offspring is called the F2 cross or F2 generation.
Now since he has Ss, ss and SS to work with you could use Punnett squares
to determine what the next generations of offspring will look like. Compare
your results with what you have learned about ratios and you'll be able
to see how it all fits together.
More on Genetic Frequencies
Take a look at the cross below between two heterozygous parents. If two
heterozygous parents are crossed, the frequency ratio of the alleles will
be 50% each. Remember the genotype can be Ss; SS or ss, but the allele
is either VS' or Y.
We can see S S S S (4 x S) and s s s s (4 x s). This means that the frequency
of the allele 'S' is 50% and the frequency of the allele V is 50%. See
if you can calculate the frequencies of the alleles *S'and Y in the following
crosses for yourself.
Recall that the Hardy-Weinberg law states that the sum of all the alleles
in a population should equal 100 percent, but the individual alleles may
appear in different ratios. There are five situations that can cause the
law of equilibrium to fail. These are discussed next
1. MUTATION. A mutation is a change in genetic material, which can give
rise to heritable variations in the offspring. Exposure to radiation can
cause genetic mutation, for example. In this case the result would be
a mutation of the plant's genetic code that would be transferred to its
offspring. The effect is equivalent to a migration of foreign genetic
material being introduced into the population. There are other factors
that can cause mutations. Essentially a mutation is the result of DNA
repair failure at the cellular level. Anything that causes DNA repair
to fail can result in a mutation.
2. GENE MIGRATION. Over time, a population will reach equilibrium that
will be maintained as long as no other genetic material migrates into
the population. When new genetic material is introduced from another population,
this is called introgression. During the process of introgression many
new traits can arise in the original population, resulting in a shift
in equilibrium.
3. GENETIC DRIFT. If a population is small, equilibrium is more easily
violated, because a slight change in the number of alleles results in
a significant change in genetic frequency. Even by chance alone certain
traits can be eliminated from the population and the frequency of alleles
can drift toward higher or lower values. Genetic drift is actually an
evolutionary force that alters a population and demonstrates that the
Hardy-Weinberg law of equilibrium cannot hold true over an indefinite
period of time.
4. NON-RANDOM MATING. External or internal factors may influence a population
to a point at which mating is no longer random. For example, if some female
flowers develop earlier than others they will be able to gather pollen
earlier than the rest. If some of the males release pollen early and then
stop producing pollen, the mating between these early males and females
is not random, and could result in late-flowering females ending up as
a sinsemilla crop. This means that these late-flowering females won't
be able to make their contribution to the gene pool in future generations.
Equilibrium will not be maintained.
5. NATURAL SELECTION. With regards to natural selection, the environment
and other factors can cause certain plants to produce a greater or smaller
number of offspring. Some plants may have traits that make them less immune
to disease, for example, meaning that when the population is exposed to
disease, less of their offspring will survive to pass on genetic material,
while others may produce more seeds or exhibit a greater degree of immunity,
resulting in a greater number of offspring surviving to contribute genetic
material to the population.
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