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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.


A bad joke

  • 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.

  • 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.

  • 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.

  • 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.

  • In the early stages, they had no idea at all how traits were passed on.

Some early ideas

  • 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.

  • 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).  

  • In the 1600s the microscope was invented and sperm and eggs were seen. 

picture of homonculus

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?!
  • 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

  • In the 1800s, in an attempt to explain how traits were passed on, Charles Darwin came up with an hypothesis he called Pangenesis

  • This states that gametes contained invisibly tiny "gemmules" which are produced in every part of the body and collect in the reproductive organs. 

  • 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. 

  • 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.

picture of Charles Darwin
  • 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.
  • 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.


The father of genetics
               picture of Gregor Mendel
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:
  • The plants had two factors for each trait.

  • Some factors were dominant over others (recessive ones).

  • If a plant had one of each type of factor, only the dominant would show up.

  • 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.


picture of Hugo Devries
picture of Karl Correns
picture of Erich Von Tschermak
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. 
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.)

picture of Walter Sutton
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.



  • 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.
  • Wilhelm Johannsen coined the word, gene in 1909 from Greek "to give birth to". He also gave us genotype and phenotype.

  • 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.

  • 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.

  • 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..... .
A wild-type Drosophila is amazed at some mutant fruit flies A sepia-eyed Drosophila expresses annoyance at being the result of H.J. Muller's genetic experiment with X-raysAn ebony body Drosophila also expresses indignation at his mutationsA yellow bodied Drosophila is very perturbed with its multiple mutations.

  • 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
  • Nucleic acid had been discovered in 1869 by Friedrich Meischer, but its significance was not to be realised for many years.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

Cartoon of Wilkins, Crick and Watson.


How genes work

  • 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.

  • 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.

  • 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.

  • 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.
  • 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.
Final word
  • Only a few of the many scientists involved in the uncovering of our knowledge of genetics have been mentioned here.  Although some contributions appear much greater than others and have earned people fame - at least in the scientific community, and in some cases Nobel prizes, for every discovery of apparent great importance we must remember that many other researchers have contributed perhaps just a small amount, but those small contributions made the great discoveries possible. I will finish with the still pertinent quotation from sir Isaac Newton, "If I have seen further than others before me, it is because I have stood on The shoulders of giants".

  • A good, brief summary of some major discoveries with photographs of the individuals concerned can be found at Timeline of genetics  For  some more biographies of scientists who contributed to our knowledge of the genetic code go to Biographies. 




Author: R. Wood   contact email.                                                                                                  Return to top

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