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About four years ago I saved some seeds from our favorite grocery store tomato from Trader Joes, Kumato. It is a larger black salad type averaging 1.75-2 inches. I was not scientific in planting but the first year several plants produced fruit almost identical to the TJ variety. I was expecting a more varied result from what I assume are hybrids. The next year I saved seeds from my tomatoes and also saved some from more store bought ones. The effort once again produced some plants with similar fruit as year one, but with my sloppy tracking, I don’t really know which of the seeds gave rise to non Kumato like tomatoes. Then comes this year. The tomatoes are all large fruited black Brandywine looking fruit. Where the [junk] did that come from? I figure that these plants must have crossed with a nearby different variety. In the future, I’ll go back to using seeds saved from the store boughts hoping to get more consistent results. I might even keep all of the seedlings identified to track variability in fruit.

Tomatoes can cross; they self-pollinate, for the most part, but up to 20% can be crossed, by bees and the like, so if you are saving seeds, and don't want a possible hybrid, you need to bag some blossoms, or isolate them some other way. Kumato is supposedly an heirloom, so the storebought should produce the same thing, but who knows where the other varieties start in the fields?

I've learned to save both older seed & newer seed from the same tomato variety.

Using the newest seed is preferred but if it produces suspect fruit, I can toss the new and return to the older seed the following year, with the intent of saving once again. I don't like to use seed that is 5 years old or older, however. It has always germinated but may be slow to start. So, I check supplies each year. I'm sure that this isn't a foolproof method but the odds against valued characteristics being lost are improved. If the variety is widely available, I'm much less than vigilant.

Carol Deppe has written some books, one of which I have used for a little guidance on dominant and recessive tomato characteristics. It provided some assurance that an obvious cross had stabilized. Saving that seed mostly just adds some fun to my gardening.

Steve

I keep our seeds in freezer and add new ones to the pack, never culling older ones. I am sure that I have had good results from seeds that are at least ten years old. Have never had a bad germination problem because of poor seed quality.

You plant so many varieties that I can see how they would cross. This is the first time I have so many tomatoes at once. I rarely have crosses, even though I have lots of volunteers.