Chimera was wrong with his cubing - a myth posts.
But so was I partly in that backcrossing thread I did, which is a copy of a reply to chimera.
There are multiple possible valid goals of backcrossing. The myth is that you can achieve all those goals simultaneously. For example, you can't backcross a trait into a variety AND make it homozygous for that trait by backcrossing along. You can't save the genes of an elite cut from a hybrid AND at the same time make it homozygous by backcrossing alone.
What you can do through backcrossing is:
- save the genes (but not the exact same genotype and phenotypes) of a hybrid like an elite F1 cut.
- backcross a trait into another variety
- backcross a population to a homozygous plant to make that population more homozygous (this would be cubing).
The grimm bros sort of went for the the first option but got lucky and results more like the third. The key is "population" backcrossing. It only works if the plant you cross back to, the recurrent parent is already homozygous (for the essential and key traits) and only if you cross back many to one like I stressed in that backcrossing thread. The examples in that thread should be split in the last two goals I listed above.
Crossing a trait into another variety and making it true bred requires more than backcrossing alone. Backcrossing to an unstable plant or backcrossing 1 plant to 1 plant and expecting truebreds, that's the myth,
Proper cubing example: (H just being an example, applies to all others too).
Step1– Recurrent Parent [HH] × Donor Parent [hh] = [HHx hh] = F1 Hybrid generation [all Hh]
Step 2 – Cross the best of those F1 (Hh) plants to the recurrent parent [HH]. The generation produced is denoted BC1. [50%HH, 50%Hh, all same phenotype.]
Step 3 – Select plants from BC1 and cross with the recurrent parent; the resulting generation is denoted BC2. [take for example 8 males from BC1. Throw all the BC2 seeds you get from pollinating the recurrent parent together on a pile. Half of it will be HHxHH, half will be HHxHh, which means of 75% of the pile is HH and 25% is Hh. ]
Step 4 – Select plants from BC2 and hybridize with the recurrent parent; the resulting generation is denoted BC3. [select any 8 males from the mixed BC2 pile. 75% of the BC3 will be HHxHH and 25% will be HHxHh, so you get 87.5% HH in the mixed BC3 pile]
Step 5 – Grab 8 males again from BC3 and hybridize with the recurrent parent; the resulting generation is denoted BC4. As you probably guessed by now, the resulting pile of seeds will have 94% HH.
Those percentage are similar to P.50, P.75, P.88 (as in the Cindy example) but represent something different. Those P.xx refer simply to how much of Princess' genes are theoretically in the back cross, like so:
Princess Clone
x (
SSx
JH/?
) =
P.50 (50% Princess)
Princess Clone
x
P.50
=
P.75 (75% Princess)
Princess Clone
x
P.75
=
P.88 (87.5% Princess)
Princess Clone
x
P.88
=
P.99 AKA
Cinderella 99 (94% Princess)
Chimera refers to those percentages in the same context, i.e. how much of the genes of the recurrent parent are in the backcross, which indeed doesn't mean they actually express those genetics in the same way as the recurrent parent as that depends on the genotypes. But once you start using a lot of males (or females) to cross back to the recurrent parent the percentage of homozygous/truebred genotypes/traits increases in the same way in the resulting "combined" offspring. Again b
oth a homozygous (especially for the traits that matter) recurrent parent and using multiple plants from the backcross generations are required.
Chicken and egg situation...