"Traditional Heirloom Seed Varieties"
A Traditional Heirloom Seed variety is any seed currently commonly found and labeled commercially as an "Heirloom Seed Variety".
What this means is that The seeds are not Hybrids of other species which are not known to be found in a natural state and that they have not been genetically modified in any way. These are all good seeds which produce good, healthy, non G.M.O. first Generation Plants which are known to be perfectly safe and nutritious for consumption. Of Course That is under the ASSUMPTION that these seeds are grown to maturity using purely organic methods, maintained with filtered or non polluted water supplies, and are isolated from chemical fertilizers and soil additives and insecticides... as with ANY seed you plant. However if you start adding chemicals to those plants after rooted... the roots will absorb those chemicals and affect the nutritional values of the end plant product.
The only Big downside to Traditional Heritage Seed collections is they are more and more limited in number... which means to maintain production year to year will require harvesting new seed straight from the plants produced by the original heritage seeds. These Seeds are currently only available, at least for agricultural and consumption based purposes, in the common varieties similar to those which are undergoing Genetic Modification in other places.
For example... an Heirloom grown Tomato is a tomato no matter what. Same goes for a Genetically Modified Tomato... it is STILL a tomato. Both will always act Like tomatoes, and like all good tomatoes do... they will pollinate and attempt to reproduce.
When you Grow your heirloom tomatoes outdoors for example... the person down the road from you, or even across the county from you, May be growing THEIR Genetically Modified Tomatoes outdoors as well in THEIR fields. Pollen can Travel extremely Large distances being carried by birds, insects, and winds.
What this means for your heirloom tomatoes is that the first heirloom generation, when it goes to seed, will become pollinated by the Genetically Modified pollen from Plants grown in other places (at least SOME will, a percentage that will increase with each successive generation almost exponentially) and create seeds formed with the genetically modified pollen.
When planted, these seeds create what is known as the second generation, which shares the traits of both parents like all species do. This results in what is know as a hybrid... and the plants from it will start to display Traits that are different from the original heirloom species AND the original Genetically modified species... in essence creating a whole new genetically modified species of that plant type (in this case tomatoes).
In the Long run... if this process repeats through several years of harvests and seed collection, you end up with an entire garden of plants which are no Longer Heirloom... but genetically modified.
Also because the genetically modified traits are also influenced in ways that the engineers of the Genetically modified plant species most commonly have never studied the affects of... and have no knowledge of because of the process of being influenced by the Traits of YOUR heirloom species... the end result in these Hybrids may very well be proven to have even GREATER Long term negative effects on consumers of the products produced by those plants (in our example the people who eat your tomatoes.) than the Original Genetically modified plants did... which you were attempting to avoid by growing Heirloom seed varieties in the FIRST place.