The Cannabis Breeder's Bible (8 page)

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:

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:

All the offspring will be SS.

Pea plant #2 results:

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.

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:

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 F1 cross or F1 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 ‘S’ or ‘s.’

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 ‘s’ is 50%. See if you can calculate the frequencies of the alleles ‘S’ and ‘s’ 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% , 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 below.

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.
Nonrandom 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 earlier than others, 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 regard 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.

HOW TO TRUE BREED A STRAIN

Breeding cannabis strains is all about manipulating gene frequencies. Most strains sold by reputable breeders through seed banks are very uniform in growth. This means that the breeder has attempted to lock certain genes down so that the genotypes of those traits are homozygous.

Imagine that a breeder has two strains: Master Kush and Silver Haze. The breeder lists a few traits that he particularly likes (denoted by *).

This means he wants to create a plant that is homozygous for the following traits and call it something like Silver Kush.

SILVER KUSH
Pale-green leaf
Hashy smell
Silver flowers
Short plants

All the genetics needed are contained in the gene pools for Master Kush and Silver Haze. The breeder could simply mix both populations and hope for the best or try to save time, space and money by calculating the genotype for each trait and using the results to create an IBL.

The first thing the breeder must do is to understand the genotype of each trait that will be featured in the ideal “Silver Kush” strain. In order to do this the genotype of each parent strain for that same trait must be understood. Since there are four traits that the breeder is trying to isolate, and 4 × 2 = 8, eight alleles make up the genotypes for these phenotype expressions and must be made known to the breeder.

Let’s take the pale-green leaf of the Silver Haze for starters. The breeder will grow out as many Silver Haze plants as possible, noting if any plants in the population display other leaf colors. If they do not, the breeder can assume that the trait is either homozygous dominant (SS) or recessive (ss). If other leaf colors appear within the population, the breeder must assume that the trait is heterozygous (Ss) and must be locked down through selective breeding. Let’s look closely at the parents for a moment.

If both parents were SS there wouldn’t be any variation in the population for this trait. It would already be locked-down and would always breed true without any variations.

With one SS parent and one Ss parent, the breeder would produce a 50:50 population—one group being homozygous (SS) and the other heterozygous (Ss).

If both parents were Ss, the breeder would have 25% SS, 50% Ss and 25% ss. Even though gene frequencies can be predicted, the breeder will not know with certainty whether the pale-green leaf trait is dominant or recessive until he performs a test cross. By running several test crosses the breeder can isolate the plant that is either SS or ss and eliminate any Ss from the group. Once the genotype has been isolated and the population reduced to contain only plants with the same genotype, the breeding program can begin in earnest. Remember that the success of any cannabis breeding program hinges on the breeder maintaining accurate records about parent plants and their descendents so that he can control gene frequencies.

Let’s say that you run a seed bank company called PALE-GREEN LEAF ONLY BUT EVERYTHING ELSE IS NOT UNIFORM LTD. The seeds that you create will all breed pale-green leaves and the customer will be happy. In reality, customers want the exact same plant that won the Cannabis Cup last year or at least something very close. So in reality, you will have to isolate all the ‘winning’ traits before customers will be satisfied with what they’re buying.

The number of tests it takes to know any given genotype isn’t certain. You may have to use a wide selection of plants to achieve the goal, but nevertheless it is still achievable. The next step in a breeding program is to lock down other traits in that same population. Here is the hard part.

When you are working on locking down a trait you must not eliminate other desirable traits from the population. It is also possible to accidentally lock down an unwanted trait or eliminate desired traits if you are not careful. If this happens then you’ll have to work harder to explore genotypes through multiple cross tests and lock down the desired traits. Eventually, through careful selection and
record keeping
you’ll end up with a plant that breeds true for all of the features that you want. In essence, you will have your own genetic map of your cannabis plants.

Successful breeders don’t try to map everything at once. Instead, they concentrate on the main phenotypes that will make their plant unique and of a high quality. Once they have locked down four or five traits they can move on. True breeding strains are created slowly, in stages. Well-known true breeding strains like Skunk#1 and Afghani#1 took as long as 20 years to develop. If anyone states that they developed a true breeding strain in 1 or 2 years you can be sure that the genetics they started with were true breeding, homozygous, in the first place.