Barmaley wrote: ↑
Wed May 12, 2021 1:03 am
I understand (it was a big surprise for me to learn it) that the same fruit may be both self pollinated and cross pollinated. This this case if the parent was F1 then F2 grown from its seeds will be different. But in case if it was self-pollinated only - will all F2 tomatoes growing from its seeds identical? If not, if I grow 100 plants will I have 5-6 groups with identical plants or I will get 100 different plants?
A single unit of pollen corresponds to a single seed. It's common for a fruit to self-pollinate some of its seeds and have some of its other seeds be cross-pollinated by another plant. Sometimes the vast majority will be self-pollinated, and you might have one that is a cross. Tomatoes have perfect flowers (with parts of both sexes in the same flower); so, that's why a single flower can pollinate itself.
The F1 is the first generation since the cross. F2 is the second generation (it's not the result of an additional cross; we're only talking about one cross), etc. If it's an F2 then it definitely came from an F1; you can't get an F2 directly from a true-breeding variety, nor can you get an F2 from seed that was just cross-pollinated. It's in the first generation of the cross (the F1) where you might have some siblings that weren't crossed (or that weren't F1s). So, if you're growing F2s, you know they're F2s, because you saved seeds from an F1 (unless you weren't sure whether it was an F1).
F2s aren't stable and if they really are F2s, they were self-pollinated (if it's crossed again, you generally call that an F1 hybrid, again; it's not the same F1 hybrid, however). So, each and every F2 seed (each and every child of an F1) will be genetically different.
The reason commercial F1 hybrids seem to be reliable in the traits that they express in the F1 generation is because both parents have only homozygous traits, and when they cross, you get the same combination every time you make that same cross between those same two varieties. But, the F2s (which are the offspring of the F1) will be an unpredictable mix if they were self-pollinated, and if they were cross-pollinated, then they're not F2s, and they're still unpredictable (because the mother plant wasn't stable).