Cubing and backcrossing
by Greg Green
Our first cross between the Master Kush plant and the Silver Haze is known as the F1 hybrid cross. Let us pretend for the moment that both traits are homozygous for leaf colour. The Haze is pale green the Kush is Dark Green. Which one is MM or mm we do not know? Until we see the offspring.
This F1 cross will result in hybrid seeds. Now since M is dominant over m, then we will know which colour is more dominant and from which parent it came from. In the example let us pretend that the overall results are pale green. This means that the pale green allele is dominant over the dark green.
M = Silver Haze pale green leaf trait is dominant.
m = Master Kush dark green leaf trait is recessive
But we also know that because no variations occurred in the population that both parents where homozygous for that trait. However ALL the offspring are heterozygous.
Now here is where we can take a big short cut in manipulating the gene pool for that population.
By cloning the parent plant MM, we can use this clone in our cross with the Mm offspring. This is known as a BACKCROSS. Obviously if our parent is female then we will have to use males from the Mm selection in out backcross.
Now out first backcross will result in 50% being homozygous for that trait (MM) and 50% of the offspring being heterozygous (Mm) for that trait!
So backcrossing will seriously control the frequencies of a specific trait in the offspring.
The first backcross is simply called A BACKCROSS. Now let us see what happens when we do our second backcross using the same PARENT that we are keeping alive through cloning. Our second backcross is referred too a SQUARING.
Since we are dealing with only 2 types of offspring Mm and MM we will either repeat our results….
All the offspring will be MM and thus true breeding for that trait. Those offspring are the results of squaring. We have not really cubed anything here, but this is a good example to get you started because it shows how we can manipulate a population by backcrossing.
Cubing in reality is less controlled than this. Cubing is a way of increasing the frequency in a population for a certain trait. It MAY not result in true breeding but it will promote a trait in a bunch of plants. Also the actual selection process is somewhat random.
In a population we select a mother plant that we want to keep because of her features. In the same population we gather pollen from 50% of the males that have characteristics similar to the mother plant and 50% that do not. The pollen is mixed is their respective portions. So we have two packets of pollen in the end. We must clone the female to create 2 females. We then use the 2 packets of pollen on each clone separately.
When we grow out the offspring from the two females we will select the population of the offspring that mostly resembles the mother plant traits that we are looking to promote in the population. What happens is that the best male pollen should have been selected by the female as the one that she prefers. The reason for taking the 2 sets of pollen from 2 sets of males is to create a control experiment to show how this method actually interferes with the frequencies of the gene pool. By right, your selection of poor male pollen will only bring about a poorer quality population that do not resemble the female clone. In reality we only select pollen from the best males that most resemble the female when we use this method.
Do you remember one of the laws that breaks equilibrium? Non-random mating and Natural selection. Well that is what we are doing here.
The resulting offspring should have a high frequency for the traits that we like in the mother plant.
Take pollen from the males of that offspring and mix them together. Pollinate a clone of the mother. This step should insure that selection is no longer random and you are promoting the frequency of the mother’s traits in the next offspring.
Repeat the process two more times and you will have effectively CUBED (meaning backcross x 3) this strain. This can push the mother plants traits as high as 90% in a population but we will probably get some non-uniform plants in the offspring too.
Cubing does not really help us to select for traits that we want, like in our Silver Kush experiment. It simply helps us to keep a few traits that a mother plant has. Cubing is a common procedure adopted by breeders who find a good healthy mother plant in a selection of seeds that someone has given them.
This method can also fail very quickly if your selection of males are the wrong choice.