Mendel's Discoveries

MENDEL chose a common garden pea (Pisum) for his first experiments in hybridisation. These plants exhibited what are now called "Mendelian Traits" - traits which occur in a very simple form. A simple trait in an organism is one which occurs either in one variation or another, with no in-between.

The seven traits that Mendel studied in the peas were:

Trait: Dominant
Expression:		 Recessive
Expression:
1. Form of ripe seed Smooth Wrinkled
2. Color of seed albumen Yellow Green
3. Color of seed coat Grey White
4. Form of ripe pods Inflated Constricted
5. Color of unripe pods Green Yellow
6. Position of flowers Axial Terminal
7. Length of stem Tall Dwarf

When picking a plant to experiment on, Mendel was also concerned that they must "during the flowering period, be protected from the influence of all foreign pollen, or be easily capable of such protection [because] accidental impregnation by foreign pollen ... would lead to entirely erroneous conclusions." - J. G. Mendel

In the process of experimenting, he ended up making 287 crosses between 70 different purebred plants. Approximately 28,000 pea plants were used! This does not take into account the other species of plants he experimented on!

When Mendel first started his experiments, he immediately noticed that when breeding two peas, a particular variation of a trait in one pea (for example, the greenness of a pea) would not appear in the next generation. However, in the following generation, when breeding the children together, this variation would appear again. He concluded that the traits were being "masked" in the second generation, to be exhibited again in the third.

When two plants breed, the variations of their traits are combined. The combination can only be explained by assuming that, for each trait, there is space for two pieces of "information" describing the variation.

Say we have a pea which is "purebred" to green (that is, when it is bred with itself, it will create only green peas), and another which is "purebred" to yellow. If we breed a purebred green pea and a purebred yellow pea, and our result is all yellow peas, we can say that the "green" variation has been lost. However, since in the next generation, yellow peas appear again, we must instead say that the "green" variation was masked, not lost.

[green-yellow.gif]

Since both the yellow and the green information is being transferred from the parents to the second generation, and the green information is being masked, we say that the yellow variation is dominant, and the green is recessive.

Labeling:

The standard way of labeling the variation information of a trait in a particular organism is using two letters. Capital letters represent information which is dominant. Lowercase letters represent the recessive. The letter being used describes a variation (usually the recessive) of the trait.

GG stands for a plant where both pieces of color information are dominant - yellow. The plant is yellow.
Gg stands for a plant where one piece of color information is dominant - yellow, and the other is recessive - green. The plant is yellow.
Gg stands for a plant where one piece of color information is recessive - green, and the other is dominant - yellow. The plant is yellow.
gg stands for a plant where both pieces of color information are recessive - green. The plant is green.

In the table above, we crossed two peas which contained both green and yellow information. We can draw the combinations created using a "Punnett Square":

Gg
G
GGGg
gGgg
g

As seen in both the table and the Punnett square, we have a ratio of 1:2:1 in the information stored. That is, one plant will have two dominant pieces of information (this is called "homozygous-dominant"), two will have a mix of information (this is called "heterozygous"), and the last will have two recessive pieces of information ("homozygous-recessive.")

However, when you look at the actual pea plants, the ratio of colors is 3:1. Three peas are yellow (the peas containing some dominant information), and one is green (the pea containing no dominant information).

These ratios are called the "genotypic" (because the information is stored on 'genes'), and "phenotypic" (because you 'physically' see the traits exhibited) ratios.

Filials

When creating a diagram showing the relation between generations, parents are labeled "P1," and their children are labeled as "filials" (filial meaning "having or assuming the relation of a child or offspring"). Each generation is numbered: "F1," "F2," and so on.

As mentioned above, Mendel noted the temporary loss of a variation (such as yellow peas) in the first generation of children, "F1" when breeding two purebreds.

[first-filial.gif]

All of the children were heterozygous. But when breeding the "First Filial" generation with itself, the next ("Second Filial") generation showed the variation again, in the 3:1 ratio. Two children were heterozygous, one child was homozygous-dominant, and the other was homozygous-recessive (the one showing the variation).


[second-filial.gif]

The Birth of Genetics

The pieces of information, or "elements," as Mendel called them, was as far as Mendel could go in this science. But even before Mendel's death, the first descriptions of the complex nuclear changes which occur during cell division were being made. In 1873, A. Schneider, while researching flatworms, in 1882 Walther Flemming, using new optical instruments, contributed greatly to the understanding of chromosomal behavior during cell division.

Boveri and Hertwig observed proof of Mendel's "splitting and blending" when they witnessed chromosomes halving. H. Henking furthered Mendel's idea of typical physical traits being exchanged by noticing that chromosomes can also determine the sex of a new individual.

And ever since these first major discoveries, humankind has learned vast amounts of knowledge in the inner workings of life- how some diseases are passed on from generation to generation, how to create new and useful forms of life (like antibiotic-making bacteria), and more every day.

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