Gregor Mendel The Father of Genetics 1823 - 1884 Pisum sativum
As a young boy it was obvious to my disappointed father that I would not be inclined to take over the family farm when he grew incapable. My true passion was with academic pursuits and as a young man I went away to school, supporting myself financially by tutoring other students. This was a very difficult life, but I managed. • Before attending university I had to pass two years at the Philosophical Institute. Again I tutored, but this time found it impossible to keep up with my own courses as well, I always felt tired and cold. I became sick and was sent home. I spent many months ill and depressed in bed and saw no way forward. • My younger sister Theresia became my savior by selling her portion of the family farm to support me at the Philosophical Institute. Many years later I was able to repay her kindness and generosity by helping to raise her 3 sons and support them through a good education. When Theresia’s money ran out I was again stranded from the life of learning that I so much desired. One of my Professors (who was also a priest) stepped in to help by recommending me to a monastery in Brünn where he had actually served for 20 years.
So in 1843 at age 20 I arrived at the St Thomas monastery. I was not particularly religiously inclined, but with the monastery supporting me, it seemed the only way I could continue my education. You see, St Thomas was not a typical monastery. The Augustinian monks that lived there were not deprived certain pleasures of life – such as conversation – but instead were encouraged to learn about the world surrounding them. • The scholars spent more time teaching and undergoing research than they did praying. Science was considered very important and we were encouraged to undertake our own Science projects as well as studying ecclesiastical law, archaeology, moral theology, Hebrew and Greek at the local Theological College. • The library was a beautiful building with over 20,000 books – some dating back many centuries. The supportive Abbot allocated me the use of the glass house and a plot of land in order to carry out some experiments breeding different varieties of common plants such as the common garden pea. I had originally started working on breeding mice with different coloured coats, until I was advised not to by the local Bishop who thought “toying” with the reproduction of animals was vulgar. I don’t think he realized that plants reproduce also!
In 1850 I was sent to gain my formal Science teaching qualifications at the University of Vienna. I had passed the written part and needed to attend the university in person for the oral part. Nerves beat me on the day and the University Professors did not pass me. I returned to the monastery in Brno feeling humiliated. • The Abbot was again very supportive realizing that even though the classes I taught were a great success, I had limited formal study myself. He felt this was the reason I lacked confidence in myself in face of the University. • In 1851 I was sent to complete two years of Science at the Imperial University in Vienna. Amongst my physics teachers was Christian Doppler, famous for his discovery of the Doppler Effect. I enjoyed maths at the university and was able to apply much of what I had learnt when analyzing the results from my experiments breeding pea plants. • In 1856 I made a second attempt at my formal teaching qualifications. Again I was choked with anxiety. One of the examiners did not agree with my theory that both male and female parts of the plant create the zygote in the next generation – he believed that only the male created the offspring.
mendel.imp.univie.ac.at/mendeljsp/ biography/biography.jsp • Depressed about my gaining my qualification I buried myself in breeding my plants. I made many crosses of different kinds of pea plants. • Here is a photo of the garden I used along the wall of the monastery.
Round Seed Green Seed Angular/Wrinkled Seed Smooth/Inflated Pod Constricted/Tight Pod Yellow Seed Green Pod Yellow Pod For my experiments I chose to breed peas using 4 different sets of fruit ‘character traits’. Here they are: OR OR OR OR Two experiments with seed phenotypes Two experiments with pod phenotypes
Short Stem (dwarf) Tall Stem White Flower Violet Flower Axillary Flowers (on the ends of ‘branches’ coming off the stem) Terminal Flowers (on the tip of the stem only) I also crossed peas with 3 different sets of plant ‘character traits’. Here they are: OR OR When scientists studying only one character trait in a breeding experiment, it is known as a monohybrid cross. OR
Like many botanists and naturalists at the time, I was interested in how characteristics were passed on from one generation to the other. Some scientists believed characteristics from the parents ‘blended’ so that the offspring were sort of ‘halfway’ between the two parents. Others thought it was the male that somehow determined what an offspring looked like while the female’s job was to simply ‘grow’ them. • Personally I couldn’t quite go with the blending theory as I had noticed that some character traits (like the ones I have just mentioned to you) occurred in an either/or arrangement and did not blend. For example the plants were either tall or dwarf and not average in height. Even though nothing was known of the function of the cells, let alone DNA and genes, I couldn’t believe that only the male parent was responsible for the characteristics. • So my experiments set out to try to find some answers to how traits were inherited.
http://www.acslp.org/homepages/Seifert/Biology/Mendel.html • Before I even started breeding/crossing my peas, I bred plants with the same characteristics for two years to make sure they were pure breeding. • For example, I crossed yellow pod with yellow pod; green pea with green pea; axillary flowers with axillary flowers and so forth. This meant that I would have plants always showing that particular phenotype and I could be almost certain to have no unusual surprises, showing different characteristics, that would skew or confuse my results. • Once this was done I could commence my hybrid crosses. Hybrid crosses are ones that contain two different types of characteristics.
Round Seed Angular/Wrinkled Seed X Self fertilized All (100%) Round Seeds All (100%) Round Seeds 1/4 Angular/Wrinkled Seed 3/4 Round Seed Firstly I took one plant of each type and cross-bred them to get the first (F1) generation. Then I self-fertilized this F1 plant to produce the second or F2 generation. I shall use seed shape to show you an example of the crosses I made for each variety of pea plant. X F1 is the first familial generation F1 F2 is the second familial generation F2 In the second, F2 generation of this monohybrid cross, the overall ratio of round seed to angular seed was 3:1. The angular seed trait disappeared in F1 as it ‘skipped’ a generation and reappeared in the F2 generation. The round seed type appears to dominate the angular seed type.
X Green Seed X Smooth/Inflated Pod Constricted/Tight Pod Monohybrid F2 ratio 3 yellow:1 green Monohybrid F2 ratio 3 green: 1 yellow X Yellow Seed Green Pod Yellow Pod Monohybrid F2 ratio 3 inflated: 1 tight Each of the 7 different character traits I looked at behaved the same way by showing only the dominant type in F1 and then the 3:1 dominant to recessive ratio in F2. I called this ratio the monohybrid ratio. Here are the other results of the other character traits I used, can you see which ones are dominant?
X X Short Stem (dwarf) Tall Stem White Flower Violet Flower Monohybrid F2 ratio 3 purple: 1 white X Monohybrid F2 ratio 3 tall: 1 short Axillary Flowers (on the ends of ‘branches’ coming off the stem) Terminal Flowers (on the tip of the stem only) Monohybrid F2 ratio 3 axillary: 1 terminal
My critics thought my results were too perfect to believe and actually accused me of faking my results. They said I changed my figures so that they fit a 3:1 ratio every time. My counts were reliable due to the following aspects of experimental technique that I used: • I used hundreds, even thousands of plants in each cross • I used the results from all seven traits rather than just one or two • I had taken the time to prepare pure breeding plants for all my experiments • I was meticulous when recording my data by counting every plant. In the end I had quantifiable data that I could analyse easily with my mathematical skills and passion • Because I studied only one characteristic at a time my results were simple and easy to follow rather than compounded by too much data
X Self fertilized Rr 1 3 • It was painstaking work each year having to count and record all my results. In order to try and understand what was happening in each cross I assigned a letter to each characteristic. It made sense to me to use a capital letter for the dominant trait (R for round seed) in order to symbolize its dominance. I then used a lowercase letter (r for angular seed) for the recessive character. These letters represented the factors that competed with each other to pass on the character traits to the next generation. X rr Parents RR F1 R r R r R r R r • RR x1 • Rr x2 • rr x1 3 F2 R R R r r R r r 1
I developed two Laws to explain how traits were passed on from generation to generation. • The Law of Segregation I developed stated that the two factors (such as R or r) controlled each characteristic. These factors are now called alleles and an allele is a version of a gene. Each factor segregated in the male (sperm) and female (ovum) parts of the plant and recombined at fertilization. So the male had one factor (or allele) and the female had another factor (or allele) that came together when the sperm fertilized the egg. • The Law of Independent Assortment I developed stated that when these factors for a particular characteristic segregated (by a process now know as meiosis) they did so independently of other factors for other characteristics. For example yellow seed and smooth seed were inherited independently of each other. Geneticists now know that this happens in all cases where the alleles are on different chromosomes. I was fortunate that I chose characteristics on different chromosomes so they all sorted themselves independently of each other.
RR R R r rr r An easier way to represent these crosses is with a Punnett Square. Punnett was a biologist who performed similar experiments to mine and several years after I died he invented this easy way to work out the factors of the various character traits. The genes an organism has is known as its genotype. The character traits themselves are now known as the phenotype. Here’s a Punnett square for the F1 cross we did earlier… Phenotype: All round seeds Genotype: All Rr F1 Rr Rr When the two alleles are not the same kind, then the genotype of the individual is said to be heterozygous (hetero is from the Greek for different). These peas are all heterozygous for seed type. Rr Rr
Rr Rr R r R r Here’s a Punnett square for the F2 monohybrid cross Homozygous dominant Phenotype: ¾ round seeds, ¼ angular/wrinkled seeds Genotype: ¼ RR, ½ Rr, ¼ rr F2 RR Rr When the two alleles are the same, the genotype is said to be homozygous (homo comes from the Greek for the same). So here there is 1 homozygous dominant, 2 heterozygous and 1 homozygous recessive for seed coat. Rr rr Heterozygous Homozygous recessive
After several years of experiments I spent most of 1865 writing up my results. My paper was published in the official journal Proceedings that accompanied a series of lectures given at the Brunn society. In 1866 I had to present my findings. • Several people seemed interested in my work but no-one truly understood their value. I sent out many copies of my paper to colleagues and famous Scientists around the world but no-one wrote back to me. I sent one copy to Charles Darwin. After both our deaths it was found unread - the pages were still bound together. I thought he would be interested as he had also carried out similar experiments to myself and obtained the same 3:1 ratio. • Did other Scientists not understand my explanations due to all the mathematics I included? Did they not think I was capable of producing good science simply because I did not have a formal education? After my death when I started to become famous my papers turned up all over the world, so scientists must have been passing them around. One was found in Japan at the National Institute of Genetics, another was found in Indiana, America, and 2 were found in a private collection having been purchased for 4,400and 13,500 German marks.
I think my work was not immediately recognised because: • Most biologist in my day were naturalists and did little more than observe life around them. I presented my paper using mathematics on probability and ratios, this must have confused many naturalists • Perhaps I wasn’t known to a big enough audience for anyone to really notice the importance of my experiments • Perhaps I was not taken seriously as a scientist because I was not only a quiet and shy monk, but I had not actually completed my academic studies • Genetics and inheritance is a very complex field and even though I had found a plant and charateristics that behaved in a classic manner, there were many characteristics on many organisms that did not behave the same way. Scientists used these exceptions to argue against my work. For example: • Some genes do appear to blend due to what was later identified as co-dominance • Some recessive traits are lethal to the individual and therefore the offspring dies before it has chance to show itself • Some genes are actually influenced by environmental conditions • Some genes are linked to the sex chromosomes which means that male and female have different genes because they have different chromosomes. For example human females do not have a Y chromosome so they would not have any genes on that chromosome let alone have them in simple Mendelian ratios
At the turn of the century three biologists(Hugo de Vries, Karl Correns and Tschermak) began to realize that my experiments actually hid the meaning of inheritance. • From their own experiments and the work of scientists such as Boveri and Sutton (who started staining the nucleus of the cell and watched the chromosomes divide into two before passing on half the information to make a new cell) the mechanisms of inheritance began to unravel. • After a few years people began to start rereading my paper and in 1910 my bust was placed on the grounds of the monastery to commemorate my work. Anyone who studied my work was said to study Mendelian inheritance and my ratios became known as Mendelian ratios.
References • Aubusson, P. and Kennedy, E. (2000) Biology in Context. The Spectrum of Life Oxford University Press, Melbourne, Australia. • Board of Studies (2002) STAGE 6 SYLLABUS Biology Board of Studies, NSW, Australia. • Eisenhaber, F. and Schleiffer, A. (2003) The Mendel Site: Gregor Mendel, The Beginning of Biomathematics, IMP Bioinformatics Group, Vienna, Austria. Retrieved from site: http://mendel.imp.univie.ac.at/mendelisp/biography/biography.isp January 2004. • Humphreys, Kerri (2003) Biology. Blueprint of Life. Science Press, Australia. • Kinnear, J and Martin, M (2001) Biology 2 HSC Course: Jacaranda HSC Science John Wiley & Sons, Australia, Ltd. • Marantz Henig, Robin (2000) A MONK AND TWO PEAS: The story of Gregor Mendel and the Discovery of Genetics Houghton Mifflin, New York, USA. • Mudie, K. et.al. (2000) Heinemann Biology Malcom Parsons, National Library of Australia, Australia. • [Author not known] (2002) Gregor MendelThe Moravian Museum, Czec Republic. Retrieved from site: http://www.mzm.cz/mzm/predmety/mendel_fotografie.html January 2004. • Olave, Gabriela and Pacheco, Alejandra (no date) Who is Mendel and what is Meiosis? Retrieved from the site http://www.acslp.org/homepages/Seifert/Biology/Mendel.html February 2004.
X Parents - Phenotype Round Seed Angular Seed X Parents - Genotype RR rr Gametes r r R R Punnett Square Key R R R = Round seed (dominant) r = angular seed (recessive) Rr Rr r Rr Rr r Answer: 100% of offspring are heterozygous (Rr) for the round seed phenotype. How to set out a Mendelian cross for HSC Let’s say you are asked to cross a pure breeding round seed with a pure breeding angular seed and show the percentage of round seeds in the offspring.