The definition of HEIRLOOM, when referring to tomato varieties, is somewhat complicated. I prefer to think of it as a great tasting tomato that has been passed from family member to family member through many generations. Of course, this would be hard to prove or disprove since confirmation via DNA testing is a rather recent science.
Heirlooms must be "open pollinated". This basically means that the seeds are collected from fruit that results from the flowers being pollinated by the same variety. A "purebreed". Basically, as long as there is no other variety of tomato plant within 25 feet or so of the heirloom, there is little chance of significantly different genetics in the resulting fruit's seeds. The heirloom is genetically "stable". Tomato authorities generally agree on four types of heirlooms. (1) open pollinated commercial varieties that have been sold for several generations. (2) Stable varieties that are family heirlooms. (3) Heirlooms that have been hybridized (to improve them) then dehybridized (see below). (4) Mystery varieties that appear by chance.
Hybrids are generally the result of a human conducted cross between two genetically different varieties. The technician doing the hybridizing chooses two different varieties he or she wishes to cross. Since tomato plants are hermaphrodites the technician usually decides which variety he wants to use as the female. On that plant the flower's anthers are removed before any pollen is produced. On the other plant the pollen is taken and applied to the first plant. Some varieties work better as a "female" and others as a "male". The resulting seeds taken from the fruit are then a hybrid of two different parents, a "mutt".
Of course, being a hybrid doesn't insure a good outcome. Hybridizers try many combinations before finding one that is superior. Large seed companies created most of the famous hybrids, but any homeowner or small farmer can hybridize. Whenever they need more seeds of that variety they must do the same cross as the original.
Even though the hybrid plants are nearly identical, and produce nearly identical fruit, the seeds of the hybrid fruits, when germinated, tend to have widely varying offspring that often resemble the two original different parents. If you only keep the seeds of the fruit that resemble the hybrid (discarding the seeds of the fruit that differ from the hybrid), and repeat this generation after generation, it is possible, within 10 generations, to turn what was originally a hybrid into an open pollinated, stable (dehybridized) variety.
Does this make the stabilized hybrid an heirloom?
Most tomato plants are grown from seed. Seeds are always genetically different from each other and the parent, but two seedling plants of the same variety (hybrid or heirloom) are much closer genetically than two plants of different varieties. Grafted tomato plants, and cutting grown tomato plants can be genetically identical (clones), but the vast majority of tomato plants available to homeowners are seedlings and are more like siblings than identical twins.
Do not confuse hybrids with GM (genetically modified) crops. GM crops have genetics that include a foreign gene, generally not from the same species and often not even from a plant. These are introduced in a laboratory using artificial means. Many GM crops can be grown from seed because the gene is passed along as if it has always been there. At this time there supposedly are no GM tomatoes being grown commercially in the US.
Hybridization is one of Nature's processes that insures genetic diversity so that natural selection can continue.
My two kids are certainly hybrids (Scottish-German/Japanese). You can tell they are of mixed heritage, though my daughter looks slightly more Asian and my son slightly more European. My cousin's six kids are hybrids (Italian/Japanese) also. Most are like my kids, but one looks entirely Asian and another totally European.
Maybe this explains why even though you purchase the same tomato variety, some years you have an unexpected result.
CONCLUSION: Always grow a lot of tomato plants with no set expectations.
WHAT TO EXPECT:
In general our inconsistent weather makes it impossible to predict what tomato variety will be outstanding any given year, however there are some generalities:
Heirlooms can provide the best flavor or most unique flavor. You won't forget the rich, creamy flavor of Brandywine or Marianna's Peace. You can fall in love with the smokey, salty flavor of the black tomatoes like Black Krim or Cherokee Purple. You may prefer the zing of the Zebra varieties or the sweet and spicy Aunt Ruby's German Green. However, many heirlooms produce better under the climatic conditions of their origin. Heirlooms also usually require proper crop rotation.
Hybrids can provide the heaviest production, the most vigorous plants and a good flavor. We can count on Big Beef, Better Boy(red) and Lemon Boy (yellow) to make huge crops of good tasting fruit. These and Early Girl, Champion and Celebrity have all been #1 sellers in the last 40 years. Most of these will perform well, even for those of you who don't rotate your crops properly. In my garden, of all the tomato plants that have survived winter to produce for 2 or even 3 years, have been hybrids.
Cherry-types always seem to do well, most will produce more than you'll ever eat.
Determinate or Indeterminate?
This is the growth pattern of the plant.
Determinate tomato plants grow more like a bush and produce flowers on all parts at the same time and stop growing as the fruit is developing. The entire crop ripens during a period of a few weeks.
Indeterminate tomato plants will flower and fruit as the plant grows. This means that the new foliage and flowers are constantly forming so that the oldest section ripens first followed by the younger portions. This means you'll harvest fewer at a time, but constantly over a longer period. The stems grow long like a vine.
