If you cross an F1 with an F1, that will not have identical offspring as you would get in a cross of two stable lines. Principles of dominance still rule the F1, so you will see more variation in F2, but there will still be genetic variation in that F1.



Every stable line is homozygous for all its genetic traits. Say for example it is tt tangerine (orange fruit). In an F1 cross with wild type TT (non tangerine), every F1 plant from that cross has one t from the orange parent and one T from the non. They are all the same genetically, Tt.



Every hybrid F1 is heterozygous for the different traits of its parents (Not XX or xx but Xx two alleles that are different). When crossing an F1 with another F1, you have potentially four different alleles to segregate as the seed is pollinated, instead of just two. Depending on the parents and how different they are from one another, you will see more variation or less in the F1. Recessive alleles will be masked in the F1 unless both parents were carrying the same recessive.


Supposing you are crossing that F1 in the example with another non tangerine tomato. One F1 is Tt, and one is TT. So in the F1 of that four parent cross you will expect a ratio of two Tt plants to two TT. Because tangerine is only expressed when both alleles agree, none of these plants will have tangerine orange fruit, but one half of them may carry the allele. So they may look the same color, but there will be genetic variation in these F1s.

If the Tt plant self pollinates, your F2 from that plant will have (approximate/expected) ratio of one in four tt tangerine with orange fruit, one stable TT non-tangerine, and two Tt plants which could produce orange fruit in a later generation.



Suppose you have two F1 plants that are red tomatoes, where both had a black parent and are carrying a gf allele which is masked by red dominance. So the two F1's have gf/GF genetics. The cross of these two F1s would produce 1/4 black fruited in the F1.

Thank you very much for the explanation.

I have an f1 tomato seed.
I crossed a regular leaf tomato with a black potato leaf tomato. In the f2 generation if I get a potato leaf black tomato and save seed from it the offsprings will be black and potato leaf right?

Yes that's right. The black and potato leaf are both recessive genes, so they only appear when homozygous gf/gf or pl/pl.