Segregating?

[dld]

Starting with an F-1 about how many grow-outs does it generally take to get the "finished product"?

[jcm05]

It is generally accepted that the F6 generation and beyond is stable enough that it can be considered a "finished product".

[landarc]

I thought F7 was the stage when the math works out that the plant should be stable.

[PVP]

Most people seem to see "stability" generally in terms of when the tomato fruit and the leaves appear to be the same from plant to plant among the variety population. So, when a grower sees all the fruit expressing round, red and 6 - 8 ounces in size, let's say, that grower probably will assume stability has been achieved.

In fact, the plants grown from the F1 seed taking from the original cross will be 100% heterozygotous as described and depicted in the Online Tomatovine tutorial on segregation and recombination of genes following a cross: kdcomm.net/~tomato/gene/genes.html

In the F2 generation, the genes pairs are 50% heterozygotous and 50% homozygotous, so I suppose you might say you are halfway to stability right there in the second season. And let's say the two parents both were roundish, red, 8 oz. tomatoes on regular leaf vines. Well, someone might visualize the process complete and the variety stable ... especially if that someone didn't grow but a couple or three vines and they all coincidentally expressed VISUALLY identical recombinations.

So, I think that's why there are some folks who continuously argue that "hybrids really are stable" and "there is profit for seed companies who sell stable varieties as hybrids," etc. 'Cuz see, many modern hybrids really do have very similar or nearly identical parent lines "VISUALLY" and are only being produced as hybrids to capture certain specific disease resistant traits into a single cultivar.

Fact is, the F2, F3, and so forth generations are far from truly stable because a fair percentage of their gene pair remain heterozygotous and subject to further segregation and recombination as homozygotous:

F2 = 50% homozygotous/50% heterozygotous
F3 = 75% hom. /25% het.
F4 = 87.5% hom. /12.5% het.
F5 = 93.75% hom. /6.25% het.

F6 = Still 3.125% "unstable"
F7 = Still 1.567% "unstable"
etc.

Which means, for example, if one of the original parents is potato leaf (regressive trait) and the other parent is regular leaf (dominant trait), you continue to have a remote chance of a potato leaf vine springing up in subsequent generations for many many years because of genes for PL lurking in the 50% of the population with regressive traits paired heterozygotously to the dominant RL masking the PL even out there in the F6 (3%) or F7 (1.6%) and onward to miniscule percentages of chance.

The same applies for recessive colors like yellow and recessive shapes like pear or strawberry.

You may want to refer to the Online Tomatovine tutorial for a more concise, correct and visual explanation.