When Greger Mendel did research on pea plants in the 1860’s, he identified a number of concepts that are important to heirloom plant growers, and people wishing to develop their own plant varieties.
His research centered around the concept of pairs of genes. He said all genetic traits were determined by these pairs of genes, and the genes within these pairs were inherited, one from each parent.
If, for example, you had two parents one with gene pair AB and another with gene pair CD, there were 4 possible outcomes for their offspring all with equal statistic probability:
AB x CD
=======
AC . AD
BC . BD
The order of the genes is not important, in other words AC is identical to CA.
Mendel’s research focused on several traits of pea plants, but for the purposes of this blog post we will look at the height of the plant. He noticed two distinct type of pea plants, those which grew tall and those which were short.
Before experimenting with cross pollinating, Mendel developed varieties of plants that only produced the traits he was interested in, in this case the height of the plant. He did this by discarding plants with the unwanted traits, and growing successive generations until the unwanted traits no longer appeared. This is a process called roguing, and it insured he was working with a gene pool that no longer contained the unwanted traits.
In this way he was able to determine that the tall plants has a pair of identical genes which made them tall, and the short plants had a pair of identical genes that made them short. For the purposes of his research, he referred to the tall gene as T, and the short gene as t.
He then took one of each type, cross pollinated them, and produced offspring as follows:
TT x tt
======= . (F1 Hybrid)
Tt . Tt
Tt . Tt
He referred to this generation as an F1 Hybrid. What he observed was that all of the resulting offspring were identical and all were tall. He explained this by saying the tall gene was dominate, and when it was present with a recessive short gene, the plant was always tall. He knew the genetic makeup of each of the plants was identical, with one of each type of gene, because they had to be inherited from parents which he had previously determined to have pairs of identical genes.
He then took pairs of plants from this generation, and crossed them again to create an F2 Hybrid as follows:
Tt x Tt
======= . (F2 Hybrid)
TT . Tt
Tt . tt
What he observed was that 75% of the resulting offspring was tall, and 25% was short. He said this was because only plants with the gene pair tt could produce short plants, otherwise as long as at least one dominate T gene was present, the resulting plant would be tall.
It was not sensible to talk about successive generations of hybrids (F3, F4, etc), because the genetic composition of the tall parents could no longer be determined, they could either be TT or Tt. This is referred to as genetically instability.
He said if it was desired to produce short plants from a genetically unstable variety, you simply needed to select 2 short plants and the resulting offspring would be short. Since then the T gene would no longer be present in the gene pool, all future offspring would also be short and the variety would be genetically stable.
He said if it was desired to produce tall plants from a genetically unstable variety, it was necessary to produce a number of successive generations of offspring and rogue out the short plants. In this way the t genes are gradually removed from the gene pool, and eventually only the T genes would remain, and the variety would become tall and genetically stable.
This turns out to be somewhat of a simplification of a much more complicated process in nature, nevertheless the basic principles here always hold true. In nature some traits are determined by more than one pair of genes, some traits are sex linked, in some pairs of genes one of them is not completely dominant, and so on. This example with pea plants deals with only one pair of genes, and since one particular plant can have hundreds or thousands of pairs of genes producing their own individual traits, a very complex situation can develop when cross pollinating two different plant varieties.
If you are trying to produce a new variety of plant through cross pollination, the basic idea is that you start by selecting two plants with traits you would like to see combined. In the first (F1) generation all the plants will always be identical, and will be an indication of the sort of plant you can hope to develop during the process of genetic stabilisation. During the stabilisation process, you rogue out plants with undesirable traits, and in the end you have a new stable variety. The stabilisation process usually ranges from a few up to 10 years.
Even if as a gardener you are not interested in in developing new strains of plants through cross pollinating, it’s still important to understand the concept of gene pools and roguing. Plants evolve by means of chance genetic variations, and you always want to preserve desirable new traits and rogue undesirable traits as they appear. You always want to select seeds from the best growing and tasting plants in your garden, to insure any genes associated with these traits are preserved. You should never save seeds from plants that bolt (go to seed) prematurely or otherwise don’t develop properly, or you risk preserving these traits.
For more information on Mendel’s work on pea and other plants, see:
http://www.visionlearning.com/library/module_viewer.php?c3=1&mid=129&l=
