The section caninae is iregular. In the case of Rosa horrida 4+1 chromosome sets. It is a pentaploid rose. One set has 7 chromosomes thus 7x5 = 35; 2n = 35
It's not algebra, it's not meant to balance like math in this case, more of a science short hand.
2n represents somatic number (effectively the number of chromosomes in a cell) n (or 1n) represents gametic chromosome number (eg sex cells, sperm, egg, pollen, etc) x represents genomic chromosome number
Look at rosa canina (just because it's the most studied of these types) it's usually written as 2n = 5x = 35
eg this diagram www.researchgate.net/figure/Diagram-of-canina-meiosis-Dogroses-with-a-pentaploid-somatic-chromosome-number-2n-5x_fig1_49942591
but happens with other things, eg banana 2n = 3x = 33, like www.researchgate.net/figure/In-situ-hybridization-to-banana-chromosomes-2n-3x-33-stained-blue-A-D-with-the_fig3_6074746
It's confusing because we also see things like 4n for tetraploid or 6n for hexaploid but those a different system, somatic number is written as 2n regardless of ploidy.
I agree. I'm sure it made complete sense when it was thought up within the context it was being used, in hindsight probably would have benefitted from a different symbol or something given the overlap that exists now.
Uneven meiosis is really nutty. Even nuttier in the caninae clan than for wheat breeding. And to top it off, roughly half of the chromosomes do not distribute when the caninae type is the seed parent. This is why most caninae hybrids look a lot like their parent. When a caninae types is the pollen donor, there is generally greater distribution and intermix. So, the pretend breeding math looks something like x = (2 + [2])+1, where [2] is genetically immobile.
So, lets say you use Peace. It would look like (2 + [2]) x 2, which theoretically would create a 4+2. Inversely, if used as seed, then it would look like 2 x 1, which would theoretically create a 2+1. When a basic modern tetraploid is used as seed, the immobile set of caninae chromosomes essentially drop. So if you were to do [Peace x (Caninae x Peace)], the likely result would be a typical tetraploid arrangement of chromosomes.
Maybe, but if 2n = 35 then n = 17.5, which is not directly divisible by either 5 or 7. So I'm still not sure what the original comment was trying to convey.
