A study of heredity in plants almost always begins with a look at the work of Gregory Mendel.
With no knowledge of exactly how it happened within the plant, Mendel was able to reason out the inheritance mechanism by observing and keeping careful records of the behavior of garden peas. A review of some of his work was presented last month.
Sometime after Mendel’s work was completed, men who study the cell behavior of plants (cytologists) and others who specialize in defining life cycles (morphologists) discovered the sequence of events within the plant that proved Mende’s laws and made them easy to understand.
The diagrams show the Menders version of inheritance and the cell picture.
Order Of Events
The order of events within the flower that leads up to pollination, fertilization, and seed production is similar to the same sequence in the animal kingdom.
Briefly, it goes like this: Within the developing pistil and anthers, a special cell divides so that the new cells it produces have just half the number of chromosomes found in all other plant cells.
For instance, a pea plant has 14 chromosomes in each cell of every leaf root, stem, and flower, except in the reproductive organs, the pistil, and anthers, where certain cells contain just seven.
From these develop, in the pistil, the egg cells (one per immature seed), and in the anther, the pollen grains.
Therefore, when a pistil is pollinated, and a seven-chromosome cell from the pollen fuses with the seven-chromosome cell of the pistil, the now-fertilized egg has a total of 11 chromosomes typical of the species.
Cells with a reduced number of chromosomes are called gametes. The fertilized egg is called a zygote.
From the fertilized egg comes the embryonic plant folded up within each seed, and from it comes the mature plant. From this brief review, one can get an idea of the importance of the genetic mechanism.
The genes governing the characteristics of the individual are carried on the chromosomes. The chromosomes are transmitted, half by the mother and half by the father, to each successive generation in an orderly process.
Mender’s Work
Menders’ work reveals the value of deductive thinking. Look at the first generation group of peas, all similar; then at the variation that appeared in the second filial generation.
One realizes that Mendel thought out the inheritance mechanism and expressed it logically in his four laws, working simply from these data.
Here are those laws and the data which supports them:
Law Of Unit Characters
The Law of Unit Characters states that the many characteristics of an individual are controlled by factors that occur in pairs.
Reasoning from the generation backward, Mendel postulated that paired factors account for the hidden dwarf characteristic, which he knew was carried in all plants of the F1 generation.
Law Of Dominance
The Law of Dominance states that one factor may inhibit the expression of its mate. The significance of this law becomes apparent in the make-up of the F1 generation, coming from the pure line but dominant and recessive parents.
From this law, we get the terms “dominant” and “recessive” used in genetics. In the case of pure line plants. Both factors of a pair are the same, both dominant or recessive. The plants would look different.
However, in the first filial generation from such parents, although all offspring resembled the dominant parent exactly, they would carry one dominant factor and one recessive factor to make up the pair.
Law Of Segregation
The Law of Segregation states that when pollen and embryonic seeds are just beginning to develop, each pair factor goes to a different cell.
Applying the information, you will see that the gametes derived from any generation are pure lines in that they contain only one factor for each characteristic. This was caused by the segregation of factors early in spore formation.
Law Of Independent Assortment
The Law of Independent Assortment overlaps the law of segregation by stating that at the time of gamete formation, one factor of each pair is distributed randomly among the gametes.
Still, it further states that at the time of recombination (fertilization), the individual factors regroup in pairs entirely at random.
That is, there was a single factor for each characteristic in the gametes, egg, and sperm cells. The factors are paired again in the fertilized egg, but this pairing is done randomly. No sperm cell fertilizes any particular egg, but happenstance is the rule.
Mendelian Inheritance
By studying these laws and following the diagrams, you will find that Mendelian inheritance is quite complicated. Even so, it represents the simplest scheme in the science of genetics.
We know, for instance, that there is not just one factor on a chromosome but rather a string of them.
We have learned that sometimes, as in the case of human eye color, many sets of factors are involved, giving a wide range of expressions of a particular characteristic.
Geneticists tell us that not only is the sex of an individual plant or animal is decided by a certain pair of chromosomes. But some other factors are carried on these sex chromosomes so that their expression is linked with the sex of the plant or animal.
To complicate matters even more, sometimes the two members of a pair of chromosomes twine about each other, the twisted structure splits down the middle to make the separate chromosomes again, and now these new chromosomes are made up of alternating pieces of the original pair!
A study of genetics is a good background for a well-informed gardener.
44659 by Dr. John P. Baumgardt