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A
little history about genetics
(NOTE: it is a good
idea to pass your mouse cursor over every diagram on this page. You may
need to wait a few seconds the first time, but you
never know what you might find)
The
following scene may or may not be an accurate depiction of something
that took place about 10,000 years ago in a cave somewhere
in Europe.
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Okay, so this little
scenario may not have happened exactly as depicted, however you can be
fairly sure that even back in the stone age people were commenting on
family resemblances.
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About 10,000 years
ago people started domesticating animals and cultivating plants. They
may not have known anything about genetics but the evidence we see
today shows that they had the good sense to breed only from certain
plants or animals. By using the individuals with desirable traits, they
were able to produce varieties that better suited their use.
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For example, in
animals they selected more docile animals that were easier to work
with. They chose cows for better milk production and horses that were
faster or stronger. Sheep were bred for better wool or tastier meat.
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Many species of
plants were bred for better or easier food production. For
example, the seeds in the original corn plants were smaller,
fewer on the cob and they fell off easily.
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In the early stages,
they had no idea at all how traits were passed on.
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Over 2,330 years ago,
Aristotle the famous Greek philosopher suggested that offspring
received their "substance" from
the female egg, and "form"
from the male's semen in some mystical way.
He correctly believed that both mother and father
contribute biological material toward the creation of offspring, but he
was mistakenly convinced that a child is the product of his or her
parents' mixing of blood. Aristotle thought semen, was a
man's purified blood, which could start growth of a child when joined
with blood
inside a woman's body.
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Aristotle's ideas
about heredity being influenced by blood are still alive in such terms
as
"bloodline" and "blue blood" (from the color of veins).
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In the 1600s the
microscope was invented and sperm and eggs were seen.
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A Dutch
scientist, Nicolaas
Hartsoeker believed that he
could see tiny people in the tiny "animicules" he found in
semen.
This led to
many people believing that sperm cells held "pre-formed" people
(or animals). The diagram to the left is based on Hartsoeker's original drawing. The miniature person was called a homunculus
and it was believed that all it needed was the right
nourishment to grow into an adult.
Some people objected to
the idea based on the conclusion that a homunculus would therefore contain smaller homunculi, which would contain still smaller ones and so on - where would it all end?!
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In the 1700s,
microscopic studies of growing embryos showed that rather than being
preformed, the different organs and parts of an organism were formed as
cells divided and became more specialised. Of course they still had no
idea what controlled it all or how traits were passed on.
Pangenesis
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In the
1800s, in an
attempt to explain how traits were
passed on, Charles Darwin
came up with an hypothesis he
called Pangenesis.
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This states
that
gametes contained invisibly tiny
"gemmules" which are produced in every part of the body and collect in
the reproductive organs.
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These
gemmules
carried instructions specific about the type
of cell or organ in which they were found. They would not necessarily
be expressed (have an effect) at all in an organism, or might be
expressed only in one gender, or only at a certain age.
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Some
people assumed that these gemmules would travel
through the blood of animals, but Darwin himself did not suggest this.
He suggested they could diffuse from cell to cell. He pointed out that
not all organisms have blood anyway.
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- One
reason why
Darwin's
ideas were reasonably popular among some biologists of the
time was the
fact that they still allowed for Aristotle's ideas about heredity
having something to do with the blood. It also allowed for the
"blending" of characteristics that
many people believed they could
observe when different varieties were
crossed.
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A French
scientist, Lamark,
was enthusiastic about this hypothesis as he could see how it supported
his (now discredited) theory that organisms
could evolve by passing on
traits that they had acquired during their
own lifetime.
- In the late 1880s, A
German Biologist, August Weismann
sought to disprove Pangenesis and
inheritance of acquired characteristics. He cut off the tails of 22
successive generations of mice and showed that young
mice continued to
be born with tails. This showed that physical changes that occur during
the life of an organism are not passed on. There were no gemmules
coming from chopped off tails. Weismann's theory was that at an early
age of embryonic development, "germ" cells separated off from "somatic"
(body) cells. This germ plasm was responsible for sexual reproduction.
This theory proved to be correct. Weismann also correctly suggested
that chromosomes contained genetic material.
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Johann Mendel
was born
in 1822 in a village in what was then Austria and is now in the Czech
Republic. He joined a monastery at Brno (called Brunn at that time) in
1843, changing his name to Gregor.
From 1851 to 1853, Mendel studied zoology, botany, chemistry, and
physics at the University of Vienna. It was unusual at that time to
study both natural science and mathematics.
He returned to Brno in 1854 and became a teacher for a while, but
failed teaching exams.
In 1857 he began what have become very famous experiments, breeding
peas in the monastery gardens. His meticulous methods and the large
numbers of genetic crosses he performed allowed him to succeed where
others had failed to see any patterns. The great Charles Darwin had
experimented with peas for a while and produced no usable results.
Mendel started with pure breeding varieties of peas with contrasting
traits such as yellow-seeded versus green seeded, or tall plants versus
short plants. |
Mendel performed
hundreds of crosses and counted thousands of peas while doing his
experiments
One reason for his success was the fact that he allowed the original
plants to self pollinate for a few generations so he could be sure they
were pure breeding. He then crossed the contrasting varieties and found
that one of the forms disappeared in the offspring (F1). When he
allowed the F1 to self pollinate, the hidden trait reappeared in the
next generation (F2). He said the trait that appeared in the F1 was dominant
while the hidden trait was recessive.
In the F2 he found that three quarters of the plants showed the
dominant trait and one quarter the recessive trait. .
Here Mendel's mathematical training became important. This 3:1 ratio
allowed him to conclude that the traits
were controlled by
separate
"factors" (that we now call genes).
The factors were not changed from generation to generation - there
was no blending.
Mendel's
other important conclusions were:
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The plants had two
factors for each trait.
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Some factors were
dominant over others (recessive ones).
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If a plant had one of
each type of factor, only the dominant would show up.
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When gametes were
produced, each gamete received only one factor each and there was an
equal chance of receiving either factor.
- When a new plant was
produced by fertilisation, it would again have two factors.
Mendel
performed other experiments studying the inheritance of two different
traits at the same time, e.g. crossing tall green-seeded plants with
short yellow seeded plants. He was able to conclude from this that:
- The
inheritance of one
trait was in no way affected by the inheritance of another.
It
should be pointed out here that Mendel's results have been criticised
as being too perfect. It is likely that he ignored some results that
didn't fit with his theory. He was very "lucky" that he studied only
seven traits. Had he studied just one more he would have found that it
was not inherited independently of all other traits. This is because
peas have 7 pairs of chromosomes and genes on the same chromosomes are
inherited together. This does not diminish the importance of his
findings however.
Finally, although Mendel's results were presented at a meeting of
the Natural History Society of Brno, and published in a
widely
distributed Journal in 1866, his findings were largely ignored for 34
years until 1900 - 16 years after he died (in 1884).
Mendel rediscovered
Mendel's work gathered
dust on library shelves for 34 years
while other researchers unknowingly carried out similar, but not as
meticulous experiments. Finally, according to legend, in 1900 three
different men, each
unaware of the others, rediscovered the record of Mendel's work.
Hugo De Vries presented a
paper on his findings on April 24, 1909 in which he acknowledged
Mendel's discoveries. Exactly one month later on May 24, Karl Correns
revealed that he too had just found Mendel's work. Amazingly, exactly
one month later on June 24, Erich von Tchermak also reported that he
had stumbled upon Mendel's work at he same time as
Correns.
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Hugo
De Vries
was
Dutch. He was studying mutation in evening primrose plants when he
concluded that the traits of plants were controlled by discrete units.
While researching other's work on the subject, he found
Mendel's
paper.
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Karl
Correns was
German. He had been studying peas and maize (corn) and had discovered
constant ratios. He too thought he had made an original discovery until
a search of the scientific literature revealed Mendel's work. |
Erich von Tschermak
was
Austrian. He was repeating experiments Charles Darwin had attempted,
rather more successfully when he too read Mendel's work and realised
his results had been already discovered 34 years ago. |
But where
are the genes?
(A brief summary of some genetic discovery land marks.)
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W. S. Sutton
in 1902 noted the similarity in behaviour between chromosomes in
cell division and the the way Mendel's "factors" (genes) behaved.
This included halving of the numbers during meiosis, retaining their individuality over generations, and re-establishing the double number after fertilisation.
Sutton also predicted
that genes would be found in linkage groups
on the same chromosome.
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- William Bateson
and R.
C. Punnett
(yes, the square man) confirmed Sutton's prediction a few
years later, finding linked genes in sweet pea plants.
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Wilhelm Johannsen
coined the word, gene
in 1909 from Greek "to give birth to". He also gave us genotype and
phenotype.
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In 1910, Thomas
Hunt Morgan
found sex linked genes in the fruit fly, Drosophila
and later went on to map several genes onto specific chromosomes.
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Calvin Bridges
is credited with proving
in 1916 that genes are actually
on chromosomes. He did this
by showing that fruit flies which could be proven to have three alleles
for a gene, also had an extra chromosome due to a mistake that must
have
occurred during egg production.
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Herman Muller,
was a student of Morgan (as was Bridges). His main claim to fame was
the
use of x-rays to produce mutations in Drosophila
flys in order to further study and map genes.
Meanwhile,
back in the
lab..... .
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- Real fruit flies, Drosophila
melanogaster
would be not much bigger than this letter o. They have been called the
"geneticists' guinea pig" as they have been used extensively in our
quest to understand inheritance. Not only do they have a large number
of mutant genes whose inheritance can be studied, they have a short
life cycle and also they have huge "polytene chromosomes" in their
saliva glands that greatly helped in the visualisation of gene loci
(where genes are on the chromosomes). No real flies were harmed in
making this web page.
The story
of DNA
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Nucleic acid had been
discovered in 1869 by Friedrich Meischer, but its significance was not
to be realised for many years.
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By the 1940s,
scientists had learned that there were two types of nucleic acid.
Deoxyribose nucleic acid (DNA) was found in the nucleus and ribonucleic
acid
(RNA)mainly in the cytoplasm of cells.
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Once DNA was found to
be associated with chromosomes, it
was believed to be responsible for simply holding the more important
proteins together. It was thought that the relatively simple chemical
structure of DNA was too simple to be able to do the job of storing
genetic information.
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In 1944, Oswald Avery
showed that by transferring DNA from
an infective bacterium to a non-infective one, the harmless bacteria
became infective. This suggested that genes were actually on the DNA.
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In 1950, Erwin
Chargaff reported that in DNA,
the amount of the base, cytosine
always equaled the amount of guanine, and the amount of adenine
equaled the amount of thymine. This became known as Chargaff's Rule.
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Further proof that
DNA was the real genetic material came
in the early 50s when Alfred
Hershey
and Martha Chase
showed that virus DNA
was all that was needed to take over
another cell to start producing viral proteins.
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The big break-through
came in 1953. James
Watson and
Fancis
Crick
finally figured out how the known components fitted
together to make the DNA molecule. The final clue came when Maurice
Wilkins showed Watson and
Crick photographs of crystallised
DNA
made using X-rays. The images had been taken by Rosalind
Franklin,
who
reportedly was not keen to share her findings with others.
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Seeing the images
allowed them to work out it was a double
helix shape. They went on to build a metal model of DNA which was soon
accepted as showing the most likely structure of the molecule.
How
genes work
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To act at the
"genetic material", DNA must be able to carry the genetic
code. This involves not only
storing information, but also being able to use the information to
influence the cell. The code also needs to be duplicated so it
can be passed on every time a cell divides.
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As far as duplication
goes, Watson and Crick's original model allowed for DNA to be able to
be unzipped and each side used to replicate two new copies. This
ability was proven by Meselson and Stahl
in 1958.
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As far back as 1954, George
Gamow suggested that DNA carried
the genetic code in the form of three letter combinations of bases,
each one coding for one amino acid in a protein molecule.
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In the late 50s, Francois
Jacob and Jacques Monod
suggested that RNA was the "go-between" that carried the
code from the nucleus to the cytoplasm. These two French scientists
later (1961) suggested that there must be a method of gene regulation
to turn genes on and off. By 1967 they had shown such a system in
bacteria. They called it the "operon".
- By 1961 Crick and
others showed that it was indeed a three-letter genetic code, and that
because there are more DNA triplets than needed for code for only 20
amino acids, the code is "degenerate", meaning that most amino acids
are coded for be more than one triplet.
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From 1961 to 1967, a
number of researchers, notably Marshall
Nirenberg,
worked out the genetic code dictionary. That is, they discovered
exactly which 3-letter combinations of bases on the DNA coded for each
of the 20 amino acid.
- Many more advances have
been made in the nearly 40 years since the genetic code was worked out,
from the ability to clone genes and even animals, to genetic
fingerprinting at what is perhaps the most outstanding achievement, the
mapping of the human genome. This project was started back in 1989
under the leadership of James Watson. In 2003 the entire base sequence
of all of the chromosomes of a representative human genome was
published two years ahead of schedule.
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