Brother Gregory: Gene One Page 6
Modern growers, less interested in genetics and more interested in the results have three varieties of edible-pod peas to choose from. They are the 2- to 2 1/2-foot Dwarf Gray Sugar, (65 days); and the 41/2-foot Mammoth Melting Sugar, (74 days); and my personal favorite. the 6-foot Sugar Snap peas, 70 days.
If you want to grow Mendel's peas in your garden, the following guideline should help. Modern pea varieties grow best in soil with a pH of 6.0 to 7.5. Prepare the soil by digging down about 8 inches into the soil leaving a flat-bottomed trench about 10 inches wide and 2 inches deep. Put a small amount of low-nitrogen fertilizer at the bottom of each trench (about 2 ounces for every 10 feet of row), and rake it into the soil.
Note: low-nitrogen fertilizer is used because peas, like other legumes, form a partnership with soil bacteria to take nitrogen from the air and fix it into usable fertilizer.
Place your seeds at about 2 inch intervals along the trench and then rake good soil over the top. In cool climates cover with mulch until the seeds start to germinate. Once the shoots begin to appear, protect the delicate plants and later provide them with canes or nets up which they can climb. Stand back, eat, and enjoy.
Think of Mendel.
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Brno
Brno is the capital of Jihomoravsk kraj (region), in the southeastern part of the Czech Republic. It is often considered the capital of Moravia. Germans, like the fictional Grunewald in our story, began to move into the region as early as the 13th century. Their industry and hard work stimulated the growth of the region and Brno grew in importance until it became an incorporated city in 1243.
During the War of the Austrian Succession (1740-45), Brno was invaded and was occupied by the French in 1805. Napoleon led his revolutionary French armies into battle against the combined Austrian and Russian armies and fought was of his most famous battles at Slavkov (which we know as Austerlitz), a place 7 miles southeast of Brno. In Mendel's time there were still old soldiers alive that could remember this battle.
During the height of the Austro-Hungarian empire Spilberk castle in Brno was turned into a political prison and military hospital. A place of unsavory reputation which the poet Silvio Pellico exposed in his book Le mie prigioni ("My Prisons"). He wrote of the horrors he found in the Spilberk dungeons, where Carbonari Italian patriots were imprisoned by the Austrians.
Despite many wars over the centuries, many fine old buildings have survived, including the churches of St. Thomas and St. James and the Gothic church in Mendel's Augustinian monastery.
Mendel would have walked along the narrow streets of the old town that, even today, are enclosed by a belt of open boulevards. A modern tourist, however, can expect to see large, new housing projects where Mendel would have seen fields.
Until World War II Grunewald's descendants were the predominant inhabitants of Brno and the surrounding regions. Mendel gave his talk in German. However, today most of the people walking Brno's streets are mainly Czech. Education is still important in Mendel's town. Brno is the home of Masaryk University, (founded in 1919).
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Transmitting Element
Mendel never knew the nature of his "transmitting element". He suspected that this "elemente" was particulate in nature (more like a pebble than a liquid), but he could not even speculate on its chemical nature.
Twenty years after Mendel's research, and long after everyone had forgotten his seminar in Brno, a few biologists began to turn their rapidly improving light microscopes on the physical structure of living organisms. Their attention was drawn to the strange new world of cells. Among the properties of cells they discovered was the process of cell division and cell reproduction. Watching cells divide was not easy. Seeing inside these tiny structures required new techniques of sample preparation and the staining of the almost invisible contents with special colored dyes. But slowly a visual picture emerged that showed the various stages cells go through as one cell divides into two new cells.
One fact quickly became clear; the cytoplasm and its contents were distributed randomly between the two new daughter cells, whereas the contents of the cell nucleus were elaborately and carefully divided equally between the two offspring. These scientists realized that something in the nucleus was a prime candidate for the "element" that governed and controlled inheritance.
In 1888 staining dyes were applied to dividing cells and showed tiny "threads" of material forming in the nucleus. These "threads" were passed to the daughter cells during cell division and promptly became the candidates for the transmitting element. Because they only became visible after staining with colored dyes, they were called "colored bodies", or chromosomes (see below).
About this time, the cellular nature of the process of fertilization was also being observed through the microscope. Once again cells were involved. The gametes, one male, one female, were highly specialized cells; sperm and eggs. As they were formed, it could be shown that the nucleus and its chromosomal contents were carefully packaged and preserved. At fertilization, a mixture of the chromosomes from both types of cells became the contents of the newly formed zygote that eventually grew into a new organism.
German biologist Theodore Boveri stated that the development of a new individual from a fertilized egg cell was "dependent upon a particular combination of chromosomes ...". His work was the first definitive evidence that chromosomes carried Mendel's "elements". By 1887 another German biologist, August Weismann, was able to speculate that a special kind of cell division, called meiosis, was responsible for reducing the chromosome content of gametes into half of the original cellular number. He called this "reduction division". Slowly the mechanism for inheritance was being elucidated.
Chromosomes
Chromosomes are long threads of material called chromatin. This material is composed of a central core of DNA (making up about 40% of the chromosome), and packaging proteins (about 60%). Depending on the circumstances, some RNA can also be associated with these structures, particularly when they are active in directing protein synthesis.
The central DNA molecule in a typical human chromosome contains about half a billion nucleotides, and, if stretched out to its full length, would be about 2 inches (5 centimeters) end to end. Obviously, in a cell, this DNA has to be coiled and packaged to allow it all to fit inside the nucleus.
Every 200 nucleotides along its length, the thread of DNA is twisted around a complex of eight proteins called histones. This forms a "bead" of material called a nucleosome. Histone proteins carry a net positive electrical charge, and these positive charges neutralize the concentration of negative charges on the phosphate groups of the DNA molecule.
Nucleosomes are further packages into tighter and tighter coils becoming more and more concentrated and highly condensed. Some of this heterochromatin is never uncoiled and the DNA it contains is never used, but the rest, the euchromatin is tightly packaged during cell division, but is present in the nucleus in a much more open form during the rest of the cell cycle.
Chromosomes differ from each other a lot. Some are tiny, some are huge, some have the constricting centromere close to one end, others have their centromere in the middle. Organisms also vary in the number and type of chromosomes they carry.
Mosquito - 6 chromosomes
Housefly - 12 chromosomes
Garden Pea - 14 chromosomes
Tobacco - 48 chromosomes
Toad - 22 chromosomes
Vampire bat - 28 chromosomes
Human - 46 chromosomes
Cow - 60 chromosomes
Duck - 80 chromosomes
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One at a time
Throughout his talk (and his later paper) Mendel discusses his results one character at a time (round peas / wrinkled peas). The concept of studying a single character in this way is so common in genetics and molecular biology today, we
could be forgiven for passing over this aspect of his work without comment. This would be a mistake.
A simple calculation shows that, if Mendel had only planted pea plants that differed in one character at a time, he would have needed a plot of land the size of Brno to grow all the plants he would have needed to study! The number of plants needed for this kind of approach would be impossibly large.
Obviously Mendel used pea plants that showed all seven characters (or more) in every generation. But, when it came time to analyze his results he does so as if he is looking at only one characteristic on each plant. Unfortunately, we don't know exactly how many plants Mendel used, but it is unlikely he studied plants with only one character. It is much more likely that Mendel made yet another breakthrough in the study of genetics by examining single characteristics, or small parts of plants rather than the whole plant at once.
Phenotypes of even the simplest organisms on earth are extremely complicated. It is unlikely that any progress could have been made in the discovery of the mechanism of inheritance without this simple but critical contribution by Mendel. Breaking down the complex phenotype into manageable "phenotypic traits" is taken for granted today, but without Mendel this idea would have taken longer to arrive.
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Versuche uber Pflanzenhybriden
One important source of historical fact used in creating this story is a scientific paper written by Mendel himself which he published in the 1866 issue of the Verhandlungen des naturforschenden Vereins, the Proceedings of the Natural History Society in Brno. As this story tells, Mendel's published paper began as two lectures, one given on February 8 and the second on March 8, 1865, to the Natural History Society of Brno.
Mendel was an enthusiastic teacher (if technically unqualified) who had a real interest in inspiring others to learn and experiment. One of the reasons Mendel gave these lectures, he said, was to try and inspire other botanists to repeat and duplicate his results. In this goal, he was doomed to disappointment, something he realized before his death. Correspondence with a colleague Carl von Nageli (a Professor of Botany at the University in Munich) in 1867, hints at his frustration; "as far as I know," he said, "no one undertook to repeat the experiments."
One reason for the almost total lack of response to the important discoveries in Mendel's paper was the problems of scientific communication in Mendel's day. There was no internet, and all published proceedings had to be printed and mailed to subscribers.
It is known that 115 copies of the Proceedings of the Natural History Society in Brno were sent out in 1866. One even found its way into the library of Charles Darwin. However, the great scientist did not even read it. When Darwin died, his books and papers were examined and the unlucky Mendel's paper was intact. The relevant pages of the Proceedings were uncut! (Unlike modern books, the readers had to cut the pages open for themselves). Darwin was not unique. Most of the other recipients of Mendel's journal seem to have been unimpressed and uninfluenced by the paper.
If you want to read Mendel's own words, a version of the original paper is reprinted in the Journal of Heredity (vol. 42, #1, 1951) and corrected by Blumberg using the copy of Mendel's manuscript reproduced in Gedda [1956].
Or you could just keep reading this continuing story.
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Choosing the right material
One of the reasons why Mendel's work was so successful was his choice of starting material. He realized that his experimental plants must meet special criteria before he could get repeatable results, and before his data could be properly interpreted.
Of great importance was what he called "characteristics". These were highly visible, reproducible and unambiguous differences between one plant and another. It was also important that this "characteristic" should be seen in every generation of the plants he was using. Flower color is a good example of such a characteristic, but only if every plant in every generation bears flowers with a distinct and unambiguous color.
For example, most people would agree that a bright red flower is distinct and different from a yellow flower. Such a characteristic allows for precise, "objective" measurement; there is no disagreement which flower is red and which flower is yellow.
With other characteristics, however, Mendel was aware he was in a gray area. When using the term "long" to describe the stem of a plant, this would only be a "good" characteristic if there was no disagreement about whether a particular stem is long or short. Not always an easy thing on which to agree. Mendel, therefore chose his characteristics with care.
The plants he used must allow for controlled breeding. It was critical that he knew, for any given hybrid, which parent supplied the egg and which the pollen. Mendel solved this problem by his choice of the pea plant. Like many other plants, the flowers of the pea possess both male and female reproductive organs, which, under normal circumstances, will self-fertilize. To Mendel, however, the special advantage with Pisum was that the keel (a part of the flower) covers the reproductive organs. Unless the flower is damaged, there is no possibility of different pollen coming from other flowers and messing up his results.
He could be certain about the origin of both the pollen and egg that produced his hybrids.
Finally, the plants he used were constantly fertile. All the hybrids that came from every cross were all as fertile as their parents. No experiment ever came to a crashing halt because the offspring of a cross was infertile and could not be used again.
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The 3:1 Ratio
At the heart of Mendel's many experiments were two critical results.
When he crossed two plants that differed in one characteristic (e.g. one plant had purple flowers and the other plant had white flowers) all the plants that grew from these seeds (the F1 or first filial, generation) showed a single characteristic (purple flowers in this case). These were the 'hybrids' that he talked about during the early part of his presentation.
Using a term that was already used in the botanical community, he called this trait the dominant form, and the alternative form, the one that had vanished in the F1 offspring, he called the recessive form. For each of the seven traits he reported, each showed a dominant and recessive form. These were;
Flower color : purple - white
Seed color : yellow - green
Seed shape : round - wrinkled
Pod color : green - yellow
Pod shape : round - constricted
Flower position : axial - top
Plant height : tall - dwarf
After these plants had grown to maturity Mendel allowed them to self-pollinate and once again collected the seeds, which he planted the next spring and grew into the F2 or second filial generation.
These F2 plants showed both characteristics; some had, for example, purple flowers and some had white flowers. The same was true for all the characteristics he was studying. But this was where Mendel made a very significant discovery. When he counted up the F2 plants he found that;
Trait - Dominant Form to Recessive Form - -Ratio-
Flower color - 705 to 224 - - 3.15 : 1
Seed color - 6022 to 2001 - - 3.01 : 1
Seed shape - 5474 to 1850 - - 2.96 : 1
Pod color - 428 to 152 - - 2.82 : 1
Pod shape - 882 to 299 - - 2.95 : 1
Flower position - 651 to 207 - - 3.14 : 1
Plant height - 787 to 227 - - 2.84 : 1
No matter what the actual raw numbers he counted, when he divided the number of dominant forms by the number recessive forms, the ratio always came out very close to 3 : 1. Three-quarters of the F2 offspring always showed the dominant trait, and one quarter always showed the recessive trait.
How was he going to explain these results?
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The Species Problem
Men
del was not and is not the only biologist who had, and has, problems with species. Classification of organisms into groups is an inexact science that is constantly changing and always being revised. There is no completely unambiguous classification scheme.
In this story Mendel is challenged on one of the weaker parts of his paper; to what species do his plants belong? In the story he gets out of trouble by giving a fairly straightforward answer, but his vague claim about the ill-defined boundaries between species and varieties is broadly unsatisfactory.
These are still difficult questions today, but in Mendel's time, and for quite some time afterwards, there was an additional complication in that botanists used different criteria for the classification of species (see below) to those used by animal biologists. The definitions of species used by animal biologists frequently used criteria based on two sexes and bisexual reproduction. These were difficult criteria for Mendel to use or to apply to plants like his peas. Thus, Mendel's comment, that two plants can be considered the same species if everyone thinks that they are.
A scientist making such a comment in a modern presentation would not escape unscarred, but it represents a view that plant biology was stuck with for a long time.
Mendel certainly seemed sensitive to the arbitrary nature of species definitions. His mental framework for his thinking and experimentation seem to be based more on physics than botany. Early in his talk he commented on the failure of previous studies to come up with a law for hybrid production. He was clearly disturbed by the lack of clear basic conditions in which and around which these laws could be unambiguously formulated.