En lisant les différents topics liés au breeding, il apparait souvent le principe de Hardy & Weinberg,
Cela est un extrait du livre de Greg Green.
la traduction viendra, si y'a des bénévoles n'hésitez pas, à vous manifester
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HARDY-WEINBERG EQUILIBRIUM
Introduction:
An understanding of breeding concepts requires a basic
understanding of Hardy-Weinberg’s Equilibrium. Breeding depends
upon the knowledge of population genetics. To understand the value of
the H/W Equilibrium, you may have asked yourself a question like this
before...
“If certain diseases are a dominant trait, then why haven’t
large sections of the population got this disease?”
The same question applies to cannabis breeding. If purple bud
color is a dominant trait then how come my offspring from the purple
bud strain do not have purple buds? Or, I have been selecting Indica
mothers and cross breeding them with mostly Indica male plants but I
have some Sativa leaves. Why does this happen? Hardy-Weinberg’s
Equilibrium will help you to understand these questions and there
answers.
Okay, first of all these questions reflect a very common
misconception. That misconception is 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.
There is no logic behind the idea that a dominant trait should
show a tendency to spread over a whole population. Also there is no
logic behind the idea that a recessive trait should die out.
Gene frequencies can be in high ratios or low ratios no matter
how the allele is expressed. The allele can also change, depending on
certain conditions. It is the changes in gene frequencies over time that
result in different plant characteristics.
Hardy-Weinberg Equilibrium will show us whether or not
gene frequencies have changed in a population.
A population is a group of individuals of the same strain or
species, [such as cannabis Indica or cannabis Sativa (species), or
Skunk#1 and Master Kush (strains of a species)], in a given area
whose members can interbreed with one another. This means that they
share a common group of genes. This common group of genes is
known as the GENE POOL.
Each gene pool contains all the alleles for all the traits of all
the population.
For a step in evolution to occur some of the gene frequencies
must change. That is why we have different types of cannabis plants.
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.
Gene frequency is calculated as follows.
The number of a specific type of allele, divided by, The total number
of alleles in the gene pool.
The Hardy and Weinberg principal describes a theoretical
situation in which there is no change in the gene pool. This means that
there can be no evolution.
For a test example let us consider a population whose gene
pool contains the alleles B and b. Assign the letter c to the frequency
of the dominant allele B and the letter d to the frequency of the
recessive allele b.
[in most cases you will find that c and d are actually notated
as p and q by convention in science, but for this example we will use c
and d.]
The sum of all the alleles must equal 100%.
So c + d = 1.
All the random possible combinations of the members of a
population would equal (c x c) + 2cd + (d x d). Which can also be
expressed as:
(c+d) X (c+d)
We will explain this in detail in moment, but it is best to know it for
now.
The frequencies of B and b will remain unchanged generation after
generation if:
1. The population is large enough.
2. There are no mutations.
3. There are no preferences. For example a BB male does not prefer a
bb female by its nature.
4. No other outside population exchanges genes with this model.
5. Natural selection must not favor any specific individual.
Let us imagine a pool of genes. 12 are B and 18 are b. Now
remember The sum of all the alleles must equal 100%. So this means
that the total in this case is 12 + 18 = 30. So 30 is 100%.
If we want to find the frequencies of B and b and the
genotypic frequencies of B, Bb and b then we will have to apply the
standard formula that we have just been shown.
f (
= 12/30 = 0.4 = 40%
f (
Both add to make 100%. Now we know their ratios.
So,
c + d = 0.4 + 0.6 = 1
We have proven that c + d must equal 1.
Very straightforward, yes.
Remember that all the random possible combinations of the members
of a population would equal (c x c) + 2cd + (d x d), or (c+d) X (c+d)
Then, c + d = 0.4 + 0.6 = 1
And (c x c) + 2cd + (d x d)
= BB + Bb + bb
= .24 + .48 + .30 = 1
This means that the population can increase in size, but the
frequencies of B and b will stay the same.
Now, suppose we break the 4th law about not introducing another
population into this one.
Let us say that we add 4 more b.
b + b + b + b enter the pool. This brings our total up to 34 instead of
30. What will the gene and genotypic frequencies be?
f (b ) = 12/34 = .35 = 35 %
f (b ) = 22/34 = .65 = 65%
f (BB) = .12, f (Bb) = .23 and f (bb) = .42
Oppss, .42 does not equal 1. This means that the Equilibrium law fails
if the 4th law is not met. When the new genes entered the pool it
resulted in a change of the population’s gene frequencies. However if
no other populations where introduced then the frequency of .42 would
be maintained generation after generation.
However we would like to point out that we used a very small
pool in the above example. If the pool were much larger then the
number of changes, even if one or two new genes jumped in, would be
insignificant. You could calculate it, but the change would be on an
extremely low level 0.000000000001 of a difference in reality.
This is just as basic example to get you started. It may not
make complete sense at first but if you read on then it will fall into
place. Some of you may be asking the question.
How do I know if a trait, such as bud color is Homozygous Dominant
(BB), or Heterozygous (Bb) or Homozygous Recessive, (bb)?
If you have been given seeds or a clone you may have been
told that a trait, such potency is Homozygous Dominant, Heterozygous
or Homozygous Recessive. However, you will want to prove this to
yourself. Especially if you are going to use that plant in a future
breeding plan. You will have to do what is called a Test Cross.
Greg Green
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