MENDEL LIVES!

This basic botany lesson is going to turn into a basic genetics lesson. Why? Because the reasons for doing certain activities in hand-pollination and seed-saving are based in the genetics of the plants you are working with.

GREGOR MENDEL AND HIS PEAS

All of us have heard of Gregor Mendel, the monk who discovered genetics by watching what happened when he crossed purple-flowered, tall, wrinkled-seeded (sweet) peas with dwarf, white-flowered, round-seeded (starchy) peas. First of all, Mendel was lucky. (Not that he would have called it luck …). But his choice of plant species made his life and his discovery much easier. Why? Because the pea is an IB species! He started with very uniform material, because peas inbreed very rapidly because they are virtually 100% self-pollinating. And for that very reason, once he made the original cross, the following generations were "automatically" produced by self-pollination. All that Mendel had to do was to grow them out, watch what happened, collect seeds, and do it again. However, his choice could have been easier to work with. Crossing legumes is not easy because of the way the flower is built: pollen is shed before the flower opens, the filaments are all wrapped around the style, and the flower is relatively small. To successfully hand-pollinate pea flowers as Mendel did without magnification is truly remarkable. But, peas tolerate damage to the style during hand-pollination so that even if Mendel had broken the style and then placed cross-pollen on the "stump", cross-pollination probably would have occurred successfully. Mendel made his crosses, collected seed, and the next season, grew out that seed. What happened? All of the progeny resembled one of the two parents. All of the peas were tall plants, with purple flowers, and having smooth seeds. It was at this point that Mendel made history. He saved seed from this first generation of crosses, his first F1 generation.

Now, let's get some definitions straight. The seed Mendel produced when he made the cross was F1 seed. The "F" stands for "filial generation", and means "generation of siblings". "F1" means "first generation". F1 seed produces a population of F1 plants. Now, if those plants are self-pollinated (and in peas, this was easy), the selfed-seed harvested from the F1s is now second generation seed, or F2 seed. Mendel grew out his F2 seed, and everything went crazy. He got tall plants, he got dwarf plants, some had purple flowers, some had white flowers, round seeds, wrinkled seeds. And the characters switched all around. He got dwarf plants with white flowers and smooth seeds. He got dwarf plants with white flowers --- and wrinkled seeds. All possible combinations of the characters that he looked for occurred in this generation. Including plants that were exactly like the original parents. Why? Well, the best answer needs a little more biology, and we'll get to that next. But Mendel's answer to this question was basically right on the mark. Mendel said that somehow the characters (size, flower color, seed shape) assorted "independently".

What this "independent assortment" means is that in a cross, everything gets mixed up randomly. He also said that some characters are stronger than others, so that in the first generation, the strong characters are visible, and the weak characters are hidden. The independent assortment of these strong and weak characters only becomes visible in the second self-pollinated generation.

MORE BIOLOGY!

Now, we need to have a little more biology. Every organism is made up of many cells. The activity of each cell seems to be controlled by a structure located generally in the center of each cell called a nucleus. Within the cell's nucleus are contained the chromosomes (at least in higher plants and animals), and within every chromosome are the cell's genes. Chromosomes are made up of a chemical known as DNA (deoxyribonucleic acid) and some protein. The chromosomes form a binary (two-part) system, and Mendel was lucky that they do. The characters that Mendel followed are alternative states in this binary system. In pea, Mendel examined tall versus dwarf plant size. He examined purple versus white flower color. Mendel compared smooth versus wrinkled seeds. These characters are called phenotypes, and a plant's phenotype is generally something visible: white flowers, dwarf size, wrinkled seed. In Mendel's system, phenotypes exist as two-part alternative states, and a geneticist would call the gene that controls each alternative state an allele. In this system, there are two alleles possible for any one gene at any one time. When both alleles are the same, the genotype (or genetic combination) is called homozygous ("homo" meaning "same"). When the alleles are different, the genotype is considered heterozygous ("hetero" meaning "different").

Back to Mendel. His original plants were homozygous, and for all of the characters he examined. How could he tell? Because each generation resembled the one before it. Inbred species (or inbred plants) produce progeny that very closely resemble the parents. The more the inbreeding (the more homozygosity), the more close the resemblance. So Mendel crossed two different homozygous genotypes, which appeared to be phenotypically uniform. And he produced F1 seed. So what happened? For every character in which his parental plants differed, the F1 was heterozygous! When you cross different homozygous lines, you get heterozygotes! Even more important, in the F1, every plant is the same. Every plant in the F1 generation is identical to every other plant. Every plant is a heterozygote, and all of the alleles are mixed at random. But every individual plant in the F1 population is uniform, and identical to every other plant. To a geneticist, the F1 generation has zero variability (every plant is the same), but also has the maximum heterozygosity (every possible combination of alleles is present).

As tedious as it was to get to this point, this is probably the most critical point of any discussion about saving seed. F1s are uniform, but in their seed is a tremendous amount of variation. Back to Mendel again. What happened with Mendel's F1? It was uniform, without variation. And "only the strong characters were visible." You just read that the F1 is a heterozygote, with two different alleles present for each gene. Why then is only the strong character visible? The real answer is very complicated, and there are actually many correct real answers. The simplest explanation is that the strong character hides the weak character. A geneticist calls this strong character the dominant character, and the allele causing the dominant character the dominant allele. Alternatively, the weak character is called the recessive character, and the recessive character is caused by the recessive allele.

In Mendel's original cross, tall is dominant over dwarf. Purple flowers are dominant over white flowers. Smooth seeds are dominant over the recessive wrinkled seeds. In Mendel's F1, the dominant characters are the ones that are visible. The dominant characters make up the phenotype of the F1. Mendel created an F2, and found all possibilities in variation. An F2 has the maximum amount of genetic variation possible. How does this occur? Through independent assortment. The alleles in each F1 are mixed randomly during meiosis, so that each sperm or egg cell is different from every other. These highly variable cells fuse during fertilization to produce every possible combination. If the F1 contains every possible combination of alleles, and those combinations are hidden by dominance, then the F2 expresses every possible combination, including those recessive characters previously hidden.

I have asked you to think about only three genes, and only two alleles for each gene. Reality is much more complicated. Other possibilities do exist. Neither allele may be dominant, and a cross between a red flower and a white flower may give you an in-between pink as a result. This is called co-dominance, or non-dominance. In many cases, each allele present gives an incremental response, so that an additive pattern develops. This can be extended across many genes controlling the same phenotype, so that additive effects can be very minor for each allele, but can accumulate and produce major effects. This is considered quantitative inheritance --- but at each gene, the inheritance is precisely the same as that for Mendel's genes. There can be many possible alleles for each gene. Only two at a time in any individual, but many possible combinations do exist.

And finally, three is about as many possibilities as I can handle in my mind at one time. Three genes, and two alleles each, gives a total of 64 different possibilities in an F2. The reality is that there are hundreds of thousands of genes operating in a plant at the same time, and that there may be many possible alleles in a given population of plants. The combinations that are actually possible are truly mind-boggling.

The MendelWeb ( http://netspace.students.brown.edu/MendelWeb/MWtoc.html ) is a Brown University Mendel site which is absolutely awesome. If anyone wants a more involved explanation, or more of the history, it is here. You have to see it!


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