Download Gregor Mendel

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.
•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.
mendel.imp.univie.ac.at/mendeljsp/ biography/biography.jsp
For my experiments I chose to breed peas using 4 different sets of fruit
‘character traits’. Here they are:
OR
Round Seed
OR
Angular/Wrinkled Seed
Smooth/Inflated
Pod
OR
Green Seed
Constricted/Tight
Pod
OR
Yellow Seed
Two experiments with
seed phenotypes
Green Pod
Yellow Pod
Two experiments with pod
phenotypes
I also crossed peas with 3 different sets of plant ‘character traits’. Here they
are:
OR
White Flower
OR
Violet Flower
When scientists studying
only one character trait in a
breeding experiment, it is
known as a monohybrid
cross.
OR
Axillary Flowers (on
the ends of ‘branches’
coming off the stem)
Tall Stem
Short Stem
(dwarf)
Terminal Flowers
(on the tip of the
stem only)
•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.
•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.
http://www.acslp.org/homepages/Seifert/Biology/Mendel.html
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
Round Seed
F1 is the first
familial
generation
Angular/Wrinkled Seed
X
F1
Self fertilized
All (100%) Round Seeds
All (100%) Round Seeds
F2 is the second
familial
generation
F2
3/4 Round Seed
1/4 Angular/Wrinkled Seed
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.
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
Green Seed
Yellow Seed
Monohybrid F2 ratio
3 yellow:1 green
Green Pod
Yellow Pod
Monohybrid F2 ratio
3 green: 1 yellow
X
Smooth/Inflated
Pod
X
Constricted/Tight
Pod
Monohybrid F2 ratio
3 inflated: 1 tight
X
White Flower
Violet Flower
Monohybrid F2 ratio
3 purple: 1 white
X
Axillary Flowers (on
the ends of ‘branches’
coming off the stem)
X
Short Stem
(dwarf)
Tall Stem
Monohybrid F2 ratio
3 tall: 1 short
Terminal Flowers
(on the tip of the
stem only)
Monohybrid F2 ratio
3 axillary: 1 terminal
•
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3.
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5.
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
•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.
Parents
F1
F2
X
RR
Rr
3
1
Rr
rr
Rr
Rr
X
Self fertilized
Rr
•RR x1
•Rr x2
•rr x1
RR
Rr
3
rR
rr
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.
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…
RR
Phenotype: All round seeds
F1
R
R
r
Rr
Rr
r
Rr
Rr
rr
Genotype: All 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.
Here’s a Punnett square for the F2 monohybrid cross
Homozygous
dominant
Rr
F2
R
r
Phenotype: ¾ round seeds,
¼ angular/wrinkled seeds
Genotype: ¼ RR, ½ Rr, ¼ rr
R
RR
Rr
Rr
r
Rr
Heterozygous
rr
Homozygous
recessive
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.
•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 codominance
•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
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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 Mendel The 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.
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.
Parents - Phenotype
Round Seed
Parents - Genotype
RR
Gametes
R
R
Punnett Square
r
r
Answer:
X
X
rr
r
R
R
Rr
Rr
Rr
Angular Seed
Rr
r
Key
R = Round seed
(dominant)
r = angular seed
(recessive)
100% of offspring are heterozygous (Rr) for the round seed phenotype.