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The Cannabis Grow Bible

by Greg Green

How to breed a true strain

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

If we can imagine for a moment that a breeder has two strains – Master Kush and Silver haze. The breeder lists a few traits that they like

This means that they want to create a plant with the following features and call it something like Silver Kush.

Now all the genetics that they need are in both of the gene pools for Master Kush and Silver Haze. We could just mix both populations and hope for the best or we could try to save time, space and money by calculating the genotype for each trait and using the results to create a TRUE BREEDING STRAIN (An IBL).

The first thing the breeder must do is to understand the genotype of every trait that is featured in his/her ‘ideal’ strain. In order to do this the genotype of each parent strain or population for that same trait must be understood. Since there are 4 traits that the breeder is trying to isolate then 4 x 2 = 8 Genotypes for these phenotype expressions must be made known to the breeder.

Let us take the Pale Green Leaf of the Silver Haze for starters. The breeder will grow out as many Silver Haze plants as they can find. They will then note down if any of the population have any other leaf colour trait. If not, then the breeder will note that the trait is homozygous (We will call the trait – M). Now it can either be MM or mm. If other coloured leaves appear in with the population then the breeder must assume that the trait is not homozygous, but heterozygous.

If it is heterozygous then we must lock the trait down before we can continue. This is done through selective breeding. Let is look closely at the parents for a moment.

If both parents where MM we would not have seen the variations in the population for this trait. It is a locked down trait. We know that this trait will always breed true in its population without any variations.

If one of the parents were MM and the other Mm we would have ended up with a 50/50 population of both variations. But one is clearly homozygous and the other is heterozygous.

If both where Mm then we would have 25% MM, 50% Mm and 25% mm.

Even though we can see the frequencies we still do not know if the Pale green leaf trait is Dominant or Recessive, but we can find this out by performing A Test Cross.

Now we are not going to go through the Test Cross chapter again but we can show you how to isolate the genotype that you need, which is either MM or mm because we want to breed that trait true. We must also keep track of the parent plants been used.

To keep parent plants alive, clone them! The exact same genetic material will be passed from clone to clone.

So if we ran a seed-bank company called “PALE GREEN LEAF ONLY BUT EVERYTHING ELSE IS NOT UNIFORM LTD” then the seeds that we create will ALL breed PALE GREEN LEAVES and the customer will be happy. In reality though they want the exact same plant that won the cannabis cup last year…..or at least something close to it. So we will have to isolate all the traits that helped that strain of cannabis to win the cup before people are happy with what they are buying. I think you get the point.

How many tests it takes to know the genotype is not certain. You may have to use a wide selection of plants to achieve the goal, but never the less it is still achievable and much more so than non-selective breeding in the wild. Each trait must be locked down in a population, so that the population for that trait is homozygous. The next step is to lock down other traits in that same population.

Now here is the hard part. When you are working on a trait you must keep the other traits that you are looking for in mind.

By breeding alone you may accidentally lock down another trait that you do not want or even remove traits that you want to keep. If this happens then you will just have to work harder at keeping the traits that you want and exploring genotypes through multiple Cross Tests. Eventually through selection and keeping records you will end up with a plant that is true breeding for all the features that you want. The gene pool is there but the objective is to lock down the traits of the pool. Also by keeping your own records you will be building up your own little map of cannabis genes. For instance if someone grows Blueberry from a known breeder and asks what the berry taste genotype is, you might be able to tell them a little bit about your experiences and what you found. This may help them cut corners. Maybe one day we will be able to genetically map cannabis and everything will be much easier.

Also a breeder never sits back and says “Right! I am going to be on the lookout for all 1000 traits that I want.” That is crazy. What they need to do is concentrate on the main phenotypes that will make their plant unique in some way. Once they have locked down 4 or 5 traits they can them move on. Step by step is how True breeding strains are created. If anyone says that they developed a true breeding strain in 1 or 2 years then you can be sure than the genetics they started with where csomewhat true breeding in the first place. (Known true breeding strains like Skunk#1 and Afghani#1 have taken 20 years to get to the stage they are at now.)

Eventually you will have your Silver Kush strain but only with the 4 genotypes that you wanted to keep. You may still have a variety of non-uniform plants in the group. Some may have purple stems, others may have green stems, some might be very potent, and others might not be so potent. By constantly selecting new traits that you want to keep, you can manipulate the strain into a totally true breeding strain for every phenotype. However it is extremely unlikely that such a strain exists on the market that is 100% true breeding for every single phenotype. Such a strain would be calledsingle phenotype. Such a strain would be called ‘A perfect IBL’. If you are able to lock down 90% of the plant’s phenotypes in a population then you can claim that your plant is an IBL. I think in today’s world that this would be an acceptable % to reach.

The core Idea behind this technique is to find what is known as a ‘Donor’ plant. A Donor plant is one that contains a true breeding trait (homozygous Dominant) for that trait. The more lock down traits are homozygous Dominant the better are your chances of developing an IBL.

IBL is short for In Breed Line. This does not mean that the line of genetics will be true breeding for every trait, but in general this terminology (IBL) used by breeders does refer to a strain as being very uniform in growth for a high % of the strains phenotypes.

Let us use the example of hamster. In a litter of hamsters we may find that they all have the same phenotypes. If that population reproduces and no other phenotypes crop up then we can consider the fact that these hamsters come from an In Breed Line. If the hamsters continue to breed and all show the same traits without variation then we know for certain that the gene pool has been locked down.

There are some breeding techniques that you may like to know about. These techniques can seriously breach the law of Hardy-Weinberg’s Equilibrium. Which in our case can be a good thing because it will reduce a trait in a population or promote a trait in a population. The strain MAY not be true breeding for the selected traits, but it will certainly help make the population more uniform for that trait.

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Cannabis Grow Bible

1 comment to How to breed a true strain

  • owz

    i have a seedling that hasn’t grown past the first round leaves and was wondering if anyone knows why this is and what causes could be attributed to this?

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