A) In this first situation, we'll deal with the situation where a plant breeder finds a special individual or clone.
It's a natural thing to be curious and cross a couple of plants that catch your fancy. Grow them out and find a new variation that you like even better. We can preserve the new variation through cloning indefinately, but accidents happen and clones die. They can get viruses or can suffer clonal deprivation from somatic mutations over time. Plus it's harder to share clones with friends through the mail than seeds. So it's only natural that we would want to create seed backups of this special clone.
But before we start breeding this clone, we should try and
figure what exactly it is we want from the seeds we are going to
create. Do we want them to simply be able to reproduce
individuals like the special clone? Simple backcrossing
(cubing) will accomplish this. Or do we want to to
create seeds that will be able to create more seeds like the
special clone, a true breeding strain? These are very different
in nature. You see, chances are that your special clone will be heterozygous for many of traits she
This just means that she will contain genetic information (genes)
for two opposing triats, but you can only see one, the dominant
one. However, her seeds will only get one or the other of the
genes, so her offspring will express all the genetic information
she has, including what you can't see within herself. If you want
to create a true breeding strain, you need to preserve all the
genes you can see, and remove all the genes that you cannot, but
may show up in the offspring. Creating homozygosity.
The only way to accomplish this is through selection
and generational inbreeding
(selecting the homozygous offspring to be parents for the next
BackCrossing and Cubing
Backcrossing is where you breed an individual (your special clone) with it's progeny. Sick in our world, but plants seem to like it
1) Your first backcross is just a backcross.
2) Your second backcross
where you take the progeny from the first backcross and cross
back to the SAME parent (grandparent now) is often called SQUARING by plant breeders.
3) Your third backcross where you take the progency (squared) from the second backcross and cross back to the SAME parent (great grandparent now) is often called CUBING by plant breeders. You can continue the backcrossing but we just call this backcrossing. Cubing is in reference to the number three, as in 3 backcrosses
Cubing works on the basis of mathamatical probabilities with respect to gene frequencies. The more males you use with each cross, the better the chance that your reality matches the theory. In theory, with the first backcross, 75% of your genepool will match the genepool of the P1 parent being cubed. Squaring increases this to 87.5% and cubing increases it to 93.75%. You can arrive at these numbers by taking the average between the two parents making up the cross. For instance, you start by crossing the P1 mom (100%) with and unrelated male (0%) getting 100% + 0% divided by 2 = 50%. Therefore, the offspring of this first cross are loosly thought of as being 50% like the mom. Take these and do your first backcross and you get 100% (mom) + 50% divided by 2 = 75%. And this is where we get the 75% for the first backcross. Same thing applies as you do more backcrosses. As you will see later, you can apply this same probability math to specific genes or traits, and this can have a dramatic effect on your methodology and selection methods.
Your selection of the right males for each backcross are the crucial points for success with this technique. In each case, you could select males that contain the genes you want, or you could inadvertedly pick those individuals that carry the unwanted recessive genes. Or more likely, you could just pick individuals that are heterozygous for both genes like the P1 mom being backcrossed. The easiest way to deal with this is to start by only looking at one gene and one trait, like lets assume that flavour is determined by a single gene (in reality it's probably not). And do some punnet squares to show gene frequencies through 3 generations of backcrossing. Now lets assume that we found a special pineapple flavoured individual in our pine flavoured population that we wanted to keep. The gene causing the pineapple flavour could be dominant or recessive and the selection abilities and cubing outcome will be different in both cases.
a) pineapple flavour is dominant.
P = pineapple flavour and p = pine flavour
Therefore since each individual will have two flavour genes paired up, the possible genotypes are PP, Pp, and pp. Since P is dominant, PP and Pp will express pineapple flavour while pp will exhibit pine flavour, these are their phenotypes. Now since the pineapple is a new flavour, chances are that the special individual will be heterozygous, or more specifically, Pp. Therefore, the only possible parent combination is Pp X pp with the Pp being the parent to be cubed.
Figure 1. The F1 cross
Now most will find it tough to pick males with the gene for pineapple flavour since males don't produce female flowers. Therefore, they will select males randomly and blindly with respect to this trait. The ratio of P to p genes of the male F1 generation to be used in the first backcross will be 2:6. Another way to look at it is to say that the P gene fequency is 25%. This means that one out of four pollen grains will contain the gene for pineapple flavour. Here is how this plays out in the first backcross.
Figure 2. The B1 cross
Now it's this first backcross that first creates an individual that is homozygous (PP) for the pineapple flavour. However, again because of our limited selection abilities, we choose males randomly. From the random males we should expect three out of eight pollen grains to to contain the gene for pineapple flavour. The P1 female will still contribute one P gene for every p gene. I'll spare your computor's memory and and not post the table, feel free to do it yorself though on paper to be sure you understand what happening
The second backcross (Squaring) will produce the following:
3 PP 8 Pp 5 pp
Therefore, 68.75% will have pineapple flavour and 31.25% will have pine flavour. The frequency of the P gene has risen to 7/16 or 43.75%.
And finally, the third backcross (Cubing) will net the following genotypic ratios:
7PP 16Pp 9pp
Therefore, 71.875% will have pineapple flavour after cubing has been completed. Roughly 22% (7/32*100) of the cubed progeny will be true breeding for the pineapple flavour. The frequency of the P gene has risen to roughly 47% (30/64).
In conclusion, if the backcrossing continued indefinately with random selection of males and with large enough of a population size, the frequency of the P gene would max out at 50%. This means that the best that can be expected from cubing is 25% true breeding for pineapple flavour and 75% that will display the pineapple flavour. You would never be rid of the 25% that would maintain the pine flavour. This model would hold true when trying to cube any heterozygous trait.
b) Pineapple flavour is recessive
In this case, P is for the pine flavour and p is for pineapple flavour. Convention is that the capital letter signifies dominance. For the breeder to have noticed the interesting trait, the mom to be cubed would have to be homozygous for the pineapple flavour (pp). Depending where the male came from and whether it was related, it could be Pp or PP, with PP being more likely. It won't make much difference which in the outcome.
F1 cross is pretty basic, we'll skip the diagram. We simply cross the female (pp) with the male (PP) and get offspring that are all Pp. Since the pine flavour is recessive, none of the F1 offspring will have pineapple flavour (hint ). However, the frequency of the gene p will be 50%.
pp X PP = Pp + Pp + Pp + Pp
Since the F1 generation are all the same (Pp), the pollen it donates to the first backcross will contain a p gene for every P gene. The first backcross will be:
B1 = pp X Pp = Pp + Pp + pp + pp
As you can see, 50% of the offspring will be pineapple flavoured and the frequency of the p gene is 6/8 or 75%. This B1 generation will generate pollen containing 6 p genes for every 2 P genes.
Figure 3. The second backcross.
As you can see, the second backcross or squaring produces pineapple flavour in 75% of the offspring. And the p gene frequency within those offspring is roughly 88%. (Remember C88 ). Of the pollen grains from this squaring, 14 out of 16 will carry the p gene for pineapple flavouring. When they are backcrossed to the P1 mom for the third time, they net the following cubed progeny:
Figure 4. The third backcross
After cubing of a homozygous gene pair, we end up with roughly 88% of them displaying the desired trait (pineapple flavour in this case) and also being true breeding for that same trait. The frequency of this desired gene will be roughly 94%. If the backcrossing was to continue indefinately, the gene frequency would continue to approach 100% but never entirely get there.
It should be noted that the above examples assume no selective pressure and large enough population sizes to ensure random matings. As the number of males used in each generation decreases, the greater the selective pressure whether intended or not. The significance of a breeding population size and selective pressure is much greater when the traits to be cubed are heterozygous. And most importantly, the above examples only take into account for a single gene pair.
In reality, most of the traits we select for like potency are
influenced by several traits. Then the math gets more complicated
if you want to figure out the success rate of a cubing project.
Generally speaking, you multiply the probabilities of achieving
each trait against each other. For example, if your pineapple
trait was influenced by 2 seperate recessive genes, then you
would multiply 87.5% * 87.5% (.875 * .875 *100) and get
76.6%. This means that 76.6% of the offspring would be pineapple
flavoured. Now lets say the pineapple trait is influenced by 2
recessive traits and and a heterozygous dominant one. We would
multiply 87.5% by 87.5% by 71.9% (.875*.875*.719*100) and get
55%. Just by increasing to three genes, we have decreased the
number of cubed offspring having pineapple flavouring down to
55%. Therefore, cubing is a good technique where you want to
increase the frequency of a few genes (this is an important point
to remember ),
but as the project increases, the chance of success decreases
.... at least without some level of selective pressure.
Applying the pressure
The best way to significantly increase your chances of success is to apply intended selective pressure and eliminate unintentional selective pressure. Try to find clearcut and efficient ways to isolate and select for and against certain traits. Find ways to be sure your males are passing along the intended traits and remove all males that do not. This includes ALL traits that may be selected for. Some traits you will be able to observe directly in the males. Other traits like flowering duration you may not. If you are selecting for a trait you can't directly observe, you want to do some progeny tests and determine which males pass on the most desireable genes. I'll explain more on progeny tests later.
It's important that when chosing your best males to ignore the superficial traits having nothing to do with the real traits your looking for. You see, cannabis has several thousand genes residing on just 10 chromosome pairs or 20 individual chromosomes. Therefore each chomosome contains hundred of genes. Each gene residing on the same chromosome is said to be linked to each other. Generally speaking, they travel as a group . If you select for one of them, you are actually selecting for all of the traits on the chromosome. There is an exception to this rule refferred to as breaking linked genes via crossing over, but for simplicity sake, we will ignore that for now. Getting back to selection, you could decide to select for a trait such as you like the spikey look of the leaves while really being interested in fixing the grapefruit flavour. But as it may happen, both traits may be on the same chromosome pair but opposite chromosomes. If so, as long as you select the plants with spikey leaves, you will never get the grapefruit flavour you really want. It's good to keep in mind that each time you select for a triat, you are selecting against several hundred genes This is why most serious breeders learn to take small methodical steps and work on one or two traits at a time. Especially with inbreeding projects such as selfing and backcrossing.
Now lets see what kind of improvements we can make in the first example of trying to cube a heterozygous dominant trait using some selective pressure. Lets say that with each generation, we are able to remove the individuals recessive for the pine flavour (pp), but can't remove the heterozygous ones (Pp). If you recall, our P1 mom had the genotype (Pp) in that model and the F1 cross yielded (Pp + Pp + pp + pp) as possible offspring combinations. We remove the two (pp) individuals leaving us with only Pp. Therefore our first backcross will be:
Pp * Pp = PP + Pp + Pp + pp
Again we remove the pp individual leaving us
with PP + 2Pp. Going into the second backcross we have increased
our P gene frequency from 37.5% up to 66.7%. This means that
going into the second backcross 4 of every six pollen grains will
carry the P gene. The outcome is as follows
As you can see, after selecting against the homozygous recessives for 2 backcrosses, we have increased our P gene frequency to 58% from 44% in our squared population. If we again remove the homozygous recessives, our gene frequency increases to 70% (14/20) going into the third backcross, meaning that 7 out of 10 pollen grains will carry the P gene. Again, I'll spare your PC's memory and just give your the results of the third backcross.
B3 cross = 7 PP + 10 Pp + 3 pp
This translates to mean that 95% of the progeny will taste like pineapple after cubing a heterozygous dominant strain if the homozygous pine tasting ones are removed prior to to each backcross. This is an improvent from 72% when no selection occurred. The frequency of individuals true breeding for the pineapple flavour rose to 35%. But more importantly, the P gene frequency improves to 60%. This will be an important consideration when we discuss progeny testing .
But for now lets recap the percentage of individuals true breeding for the pineapple taste in each of the models. In the case where the pineapple flavour trait is heterozygous dominant and no selective pressure is used, cubing produced 22% true breeding individuals. By selecting against the homozygous pine recessive, we were able to increase this too 35%. And finally, when cubing a homozygous recessive gene, we are able to achieve a cubed population that is 87.5% true breeding for the pineapple flavour. And as I pointed out earlier, these numbers only apply to single gene traits. Lets say the pineapple flavour is coded by two seperate genes, one dominant and one recessive, and you are able to select against the homozygous recessive pine flavour while selecting for the dominant pineapple flavour gene. Your cubed population would then contain 87.5% * 35% (.875 * .35 * 100) = 30% true breeding individuals. As you can see, as long as the cubed source is heterozygous, it doesn't matter how many backcrosses you do, you will never achieve a true breeding strain.
That's it for today
I'll proof later, haha.
Re:Gigantor,What's your opinion on......
Posted by gigantor on February 16, 1999 at 01:08:32 PT:
In Reply to: Re:Gigantor,What's your opinion on...... posted
by GOG on February 15, 1999 at 18:09:05
I assume that by ¥cubing¥you are referring to that practice
of selecting a female and a male, then mating
the parent female with a descendent male that is more like the female, then out of that generation another
(grandson) descendent male, etc. Why not raising it to the fifth I ask...?
well...here¥s how I feel. Somehow I don¥t trust the genetic
dynamics...sounds more like a quick fix to
me. Although, cannabis appears to be, as a genus, very good at maintaining strain integrity through
inbreeding than many other genera of plants. However, what¥s all this I hear about STRAIN
DEGENERATION? sounds like a serious problem...hmmm...
For example. Among agriculturists, in order to breed disease
resistance in crops, a WILD species is
found that is resistant naturally by its own virtue, then crossed (say a male) with a cultivar female. then
the above process is repeated etc etc...ad nauseum until the descendants have the traits of the female
and ONLY the disease resistance of the male (I would call this adaptability).
THIS technique is known among the profession, as breeding in VERTICAL RESISTANCE.
however...there is a problem that has RECENTLY been discovered. The strains DEGENERATE.
apparently inbreeding REDUCES the ¥genetic¥background to such a degree that errors are produced. For
example..a copy of a copy of a copy...etc.etc.etc..
NOW they do something called HORIZONTAL RESISTANCE. They grow
LARGE NUMBERS of the
plant, EXPOSE it to the disease and such, and, Guess what! ONLY the RESISTANT ones remain. isn¥t
that something. And they didn¥t have to compromise the background. This is NOW being used in
MExico to produce MUCH better bean crops...wow. literally, through LARGE numbers of seed, the
strain can become VERY comfortable, and very productive.
This ¥cubing¥stuff might be the ¥quick fix¥but, in the
plant world...that shit is JUST like most corporate
production...BUILT IN OBSOLESCENCE!
Posted by Vic High on February 16, 1999 at 09:57:27 PT:
In Reply to: Re:Gigantor,What's your opinion on...... posted
by gigantor on February 16, 1999 at
Wow, you sound like me, or at least say what I feel. I too am
concerned about the lack of vigour in new
stock and the lack of numbers in breeding programs.
My initial reaction to the repeated back crossing (clone
cubing) was much the same as yours. However,
out of respect for a fellow breeder I decided to give it some further thought. One concept that kept
popping up was that with each generation, 1/2 of the parenetal gene pool is coming from the parent that
has the original vigour. This means that 1/2 of the gametes (at least) from this P1 parent will still be
heterozygous on those alleles containing the deleterious recessives. In other words, that P1 mother will
be passing on it's vigour to each inbred generation as much as it would be passing on the genes that
result in loss of vigour. By only selecting the males that have the traits you want and the most vigour,
you may actually be able to retain much of the vigour?
So with this in mind, I've been wondering if maybe the cubing
method may actually be a preferable
method of fixing a clone's traits than line breeding for those breeders with restricted resources and the
inability to put up the numbers. This is a problem common in this field. When I say line breeding I mean
inbreeding where the you only cross the F1s with their siblings, same with the F2s, etc. Strain
degeneration is pretty hard to avoid with this method unless large numbers are used as you said.
Now I'm not saying that cubing is the answer or safe, but I'm
open to considering the idea. My main
understanding of genetics is theoretical or forestry related. I have not been exposed to the areas of
horticultural and agricultural genetic practices in any great detail. Therefore, since my initial debate with
MrSoul on this subject I've been searching for indepth liturature on the subject, well sorta searching I
guess :) I no longer have the literature gathering resources I used to have when I used to work for the
government or University. The University of Victoria is not focussed on genetics or horticulture so it's
library is limited. I can order books to buy but I would be buying them sight unseen. Can you
recommend some good plant breeding resources with specific interest to breeding herbaceous plants?
I'm sure if I took the time on my next trip to Vancouver that I could find what I'm after at UBC, but
there is always so much to do there when I visit :)
Posted by Vic High on July 02, 1998 at 09:27:36:
OK, I was hoping to get some different perspectives on 'cubing the clone' methods.
Method 1: Pick a male closely related to the special
clone that you want to cube to make the first
cross. This way the you are already starting with fairly uniform genetics. Problem is is that it is
hard to pick males that get more of their traits from the mom and not the dad for the
backcrosses. With the close genetic makeup, maybe it doesn't matter. Another concern with this
method is limited diversity in the genetic makup for disease resistance and vigour. (inbreeding
Method 2: Pick a dad whos traits are dramatically
different from the special mom. For future
backcrosses you can then pick males that more closely resemble the mom than the dad. With this
method you can capitalize on some hybrid vigour but after a few backcrosses would all the
hybrid vigour be lost?
Anyone wish to share their methods and why they chose them?
Posted by Lady J on July 04, 1998 at 15:30:23:
In Reply to: Re: cubing posted by webfish179 on July 02, 1998 at 21:47:31:
Nevil backrossed six times to the earliest son before he
called a strain stable. That's what he did
to create nl#5 which was only a clone he got from the U.S.
But more importantly, this was just the starting point.
Once the sixth backcross is complete, the
best indivuals both male and female were selected to start at least two separate lines, neither of
which was the original plant. This was the p1 generation.
Posted by MrSoul on July 08, 1998 at 13:58:38:
In Reply to: Re: cubing posted by MrSoul on July 07, 1998 at 06:55:30:
Just think about what I'm saying. Ask yourself what the
probability would be for selecting all the
vigorous early males of each generation yet somehow "missing" an important gene which was
only found in the scraggly males who's pollen I had eschewed? You eye color example doesn't
logically follow from what I've been trying to explain. It would only be appropriate if I were
limiting my selection of pollen to that of a very small group of males...not when I'm pollinating
with all the best plants based on the only traits that a male can fairly be judged upon. Genetics is
all about probability...you do agree on that?
Re: Getting it?????
Posted by webfish179 on July 08, 1998 at 00:02:59:
In Reply to: Getting it????? posted by Vic High on July 07, 1998 at 22:25:12:
You lost me how after two or three back crosses can the
plant look any thing but the
mother?Plants are not as complicated as people and the genetics are rarely male or female
If you have a yellow plant and a green plant their offspring would be mostly verigated the first
back cross to the green plant would produce mostly green offspring and some verigated mabey
one or two yellow plants even if you used the yellow plant to back cross the third gen would be
mostly green and some verigated and mabey one yellow the wrong selection would slow the
cube but after enough back crosses they would be 99% representitive of the mother.....Do we
know who is responsible for the potency of the plant male or female??I think both...
Hi webfish, your right, plants are simple and their inheritance behaviour is quite basic. Although, genetics is a complex subject, it breaks down to be the adding and subtracting of two genes for each trait (in most cases). Put the calculator away, haha, it'll just confuse the issue. The calculator is only usefull if you are doing ramdon selection in the backcrosses and you are working with very large populations. This is where the rules of probability can be observed. In Soul's situation, he is both not using random selection or large populationbs. His model leaves lots of room for the chance selection of undesireable traits.
And for simplicity, we weren't even taking into account the effect of linked genes. With my romulan, long internodes is linked to bud characteristics and most probably potency. Say if I selected for short internodes in my cubing, I would be selecting against the bud characteristics that we wanted.
Recall the punnet square from grade 12? i'll try to redo the backcroses on screen but it is much easier with pen and paper.
L = long internodes l = short
S = sticky buds s = leafy buds
LS x ls = LlSs for all ofspring
LS x LS = LLSS and LS x ls = LlSs
TOPIC - Genetics
DATE - 08/19/98 07:04:21
FROM - Soul
High ~ Here's the genetics stuff I warned y'all I was gonna post today:
"Following" Sex-Linked Traits - General Rules:
Consider the cross XaXb / XcY, restricting your attention to ONLY
1. Each F1 male carries ONE of the two X chromosomes from his
TWO distinct F1 male genotypes are created. (XaY, and XbY)
2. Each female carries the Xc chromosome from her father AND one
of the two X
chromosomes from her mother. Therefore TWO distinct F1 female genotypes are
created (XaXc and XbXc).
3. Backcrossing to the mother with a SINGLE F1 male creates 50%
with a MATCHING PAIR of X chromosomes from the mother (either XaXa or
XbXb) and 50% female progeny with a pair of X chromosomes identical to the
mother (XaXb and XbXa, which are equivalent). Therefore in order to create females
of ALL possible XX combinations, BOTH types of male genotypes MUST be used
in the backcross. This results in THREE female genotypes. (XaXa, XaXb, and
XbXb). Note that the Xc chromosome is LOST in the backcrossed (0.75) generation,
therefore any X-linked traits from the paternal grandmother are lost as well.
4. Backcrossing to the mother with EITHER male genotype yields
BOTH the male
genotypes obtained in the F1 generation. Therefore all males of backcrosses are
IDENTICAL to the F1 males - regardless of how many backcrosses are performed
or which of the males are used.
I CANNOT EMPHASIZE ENOUGH THAT THESE "RULES" APPLY ONLY
TO THE X AND Y CHROMOSOMES - so don't misconstrue what I've said to
apply to anything more...thanks.
Vic High, Lady_J and anyone else, please feel free to comment and
we'll discuss all
the ramifications these generalities may have in our respective breeding programs.
A Q for Soul pls....
Posted by CUBIST on March 06, 1999 at 03:41:17 PT:
.....I remember reading sometime back you suggested
using the male from a different strain to start
the cubing process.I was wondering your opinion on anything in particular(ie:does it need to be a true
breeding strain or go for the earlier flowerer or does it even matter since you are eventually breeding
out the males genetics anyway) and after the first backcross should I be looking for males that are as
close to the mother in appearence for future b/crosses or maybe something else........TIA L7 man
Re: A (REALLY GOOD) Q for Soul pls....
Posted by MrSoul on March 06, 1999 at 06:53:10 PT:
In Reply to: A Q for Soul pls.... posted by CUBIST on March 06, 1999 at 03:41:17 PT:
This is the most interesting aspect of cubing - choosing
the males. I have learned a lot by going
through the process. Male selection is the only "control" you have in the cubing process; the exact
same female is the mother every time.
You got something I said backward: I said the best
choice of male for starting the process is either the
father or a brother of the female to be cubed. The reason for this is simple. The Y chromosome is
always that of the plant's father - the mother doesnt HAVE ONE to give to the offspring. You'll never
get a cubed seedline that has the Y chromosome of the female's family unless the original male had
THAT Y chromosome.
My philosophy on choosing males from the
"intermediate generations is this: I blend the pollen
ALL the healthy males that exhibit no undesirable traits from each generation and use this mixture of
pollen to pollinate the mother to produce the next generation. This is necessary to preserve all the
genetic diversity the STRAIN is characterized by. Choosing a single male each time to create the next
generation limits variety, but this can purposely be used to control the process IF done wisely.
My method results in males of each generation being
chosen by their individual qualities, and directly
favors the offspring of the best males from the previous generation. That's because the BEST
seedlings came from the BEST fathers (thats the only way to "measure" a male's breeding quality - by
his offspring). I mix all the pollen from this group to create the next generation. Conceivably, if there
were a single best male whose sons were so much superior to his cousins' sons, these would be the
pollen donors to the next generation.
Re: A (Just one more) Q for Soul pls....
Posted by cubist on March 06, 1999 at 14:52:20 PT:
In Reply to: Re: A (REALLY GOOD) Q for Soul pls.... posted by
MrSoul on March 06, 1999 at
...How many males do you grow out then,all of a sudden I
fear I don't have the room to take on a
project like this properly...tia
Re: A (Just one more) Q for Soul pls....
Posted by MrSoul on March 07, 1999 at 06:30:44 PT:
In Reply to: Re: A (Just one more) Q for Soul pls.... posted
by cubist on March 06, 1999 at 14:52:20
Ha! Now you understand. I typically have 6-10 males that
are "keepers" to mix the pollen from.
more breeding ?? (mrsoul))!!!
Posted by no-one on May 07, 1999 at 12:18:10
i fully understand the concepet of cubing now but if i had an
f1 female and an f1 male what would hapen if i followed that
process!! would i still end up with a true breding strain or is
their more to it?? dose the male have to be of a stabilized
strain??? I would be very happy if u could give me some
advice!! or if anyone else reads this comments are welcome!!!
Re: more breeding ?? (mrsoul))!!!
Posted by Vic High on May 08, 1999 at 08:39:35
In Reply to more breeding ?? (mrsoul))!!! posted by no-one.
Your question is actually much more complex then it really looks. The simple and safe answer is that it really doesn't matter as sharrina, myself, and many others would tell you.
However, there is much more to it that requires a fairly good understanding of basic medellian genetics to fully appreciate. First, not all gene pairs operate in a simple dominant/recessive role. Second, there is the effect of linked genes. Third, there is the random linking and unlinking of genes through crossing over. Also, there is the Y chromosome factor.
Basically, when you are cubing a mother plant, you are taking her paired alleles and making them homozygous for each trait that you want to become true breeding. Some paired alleles will already be homozygous but most of the important ones will be heterozygous in the case of an F1 other-to-be-cubed. Mind you this can only be true of those traits that are controlled by basic dominant/recessive genes. This isn't always the case and sometimes genes can be codominant. Here is an example of the implications.
let A & B & C be codominat genes, d being a recessive gene on the same loci. Now for simplicity we will just look at the genotype and ignore the phenotypic effects of each genotype. Lets say our mother-to-be cubed has the genotype AB and the P1 male is Cd (both being F1s).
Notice that you can never really get a completely true breeding situation with this sort of gene. To fully capture the mother's trait you must maintain the heterozygoous AB condition. Crossing two parents with the same characteristic AB will give the following offspring:
AA, AB, AB, BB
Note only 50% of the offspring will ever be able to recreate this mother's genotype (and in this case phenotype)
Ok, now that aside, lets explore the practical issues of trying to cube that mom. Crossing the AB and Cd you the following combinations:
AC, Ad, BC, Bd. You then select from these to do your first backcross to your AB mom (creating the .75 generation)
ABxAC = AA, AC, AB, CA - 25% resemble mom in this case
ABxAd = AA, Ad, AB, Bd - 25% resemble mom again
ABxBC = AB, AC, BB, BC - 25% resemble mom again
ABxBd = AB, Ad, BB, Bd - 25% resemble mom again
As you can see, it really doesn't matter which males you selected for your first backcross as they all brought you equally close to your goal. Notice that it will also take a sharp eye to pick out the special offspring that will take you closer to your goal in the second backcross. Hopefully this shows how difficult it can be to stabililize a trait caused by codominant genes.
This was just the first factor affecting cubing success. Also,
it only dealt with a single genes and you are often trying to
stabilize dozens of gene pairs when cubing. I'll leave the others
for someone else as I have to get to work. Otherwise, remind me
and I'll try to flesh out a couple of the other points when I
have more time.
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