GENETIC HISTORY

A summary of the history of genetics (from 1679 to 1958)

Pre-Mendalian era.

In 1679, Swammerdam claimed that he was able to see a miniature man in human sperm cells. He believed that a man’s body is formed in the sperm cell and during the embryonic stage, the growth of different organs occurred and formed a new baby. He named the miniature man in the sperm cell a homunculus.

After some time, another scientist Bonnet proposed his hypothesis which was very closely related to Swammerdam’s hypothesis, the only difference is that he believed more in the egg or ovule of the female than the male’s sperm. He stated his hypothesis as egg encapsulation or box theory.

The hypothesis or ideas of both scientists can be said to Preformation of new individuals in the sex organs.

This preformation idea was replaced by another concept called Epigenesis. According to this concept, both male and female sex cells are required for embryo formation.

Lamarckism.

The Greek philosophers thought that the environment has a great impact on the inherited characteristics of an individual. This philosophy was used by the French biologist Lamarck (1744-1829).

He created many examples to support this idea. According to Lamarck, in the past giraffes had short necks but when there was a shortage of food on the land they started to eat leaves from trees, and with time, the new generations of giraffes had long necks. The giraffes modify their inherited characteristics according to their environment. He gave many such examples that support his idea.

Darwin’s hypothesis of Pangenesis.

Charles Darwin, in the mid of the nineteenth century, proposed another idea about inheritance. According to him, each cell of the body formed its copy and this copy of the cell then enters into the bloodstream of the individual and reached the germinal part (ovary) of that individual. He called this copy of cells Pangene and this hypothesis is called Darwin’s hypothesis of Pangenesis.

But this hypothesis was rejected in Darwin’s life by another scientist, Galton. He transferred the blood of the black rabbit into the blood of the white rabbit and expect a progeny there. But he was not found his expected results and so he rejects Darwin’s hypothesis of pangenesis.

A German biologist, Weismann told that germ tissues are separate from somatic tissues. He took mice and cuts his tail. He cuts the tail of mice for 22 generations and he observed the 23 generations of mice with the normal tail. So he concluded that germ tissues are not affected by somatic tissues.

Mendel’s work (1865).

Gregor Johann Mendel.

In 1865, Gregor Johann Mendel work on pea plants and observed different generations of this pea plant. He told that characters are controlled by two factors (in Mendel’s terms). One factor acts as dominant while the other factor act as a recessive factor. One factor dominates and shows the phenotypic characters in the next generation while the recessive factor shows his phenotype only when no dominant factor is present.

Mendel studied the seven characteristics of the pea plant and gave his three laws, which are also known as the law of inheritance. These laws are:

Law of segregation
Law of dominance
Law of independent assortment

But the work of Mendel was unrecognized for a long time.

Friedrick Miescher (1869).

In 1869, F.Miescher discovered the DNA from bandages with white blood cells.

He took some bandages which have blood cells on them. He separated the nucleus from the cell and then observed this nucleus under the microscope. He actually wanted to study the white blood cells present in the body. He extracted some white material from the nucleus of the cell which he called Nuclein. Nowadays, this white material was famous with the name of DNA.

Sutton-Boveri theory of chromosomes (1902).

These two scientists proposed the theory of chromosomes.

Walter Sutton

Walter Sutton worked on grasshoppers while Theodor Boveri worked on sea urchins. The work of both these scientists proposed a theory that we called the theory of chromosomes. Walter Sutton found that chromosomes are in the form of pairs while Theodor Boveri told that for the formation of the embryo, the chromosome numbers should be completed first.

Morgan (1915).

Morgan gave experimental proof of both Mendal’s work and Boveri-Sutton's theory of chromosomes. One day, Morgan observed a mutant fruit fly. This fruit fly had white eyes while a normal fruit fly had red eyes.

He crossed this mutant fruit fly with a normal fruit fly and observed the next generations of these fruit flies.

He observed that the mutant fruit fly or white-colored eye fruit fly is only a male fruit fly and it was a recessive trait of the male fruit fly. He also showed that white eye color is a sex-linked character of the male fruit fly.

His experiment explains Mendal’s work and theory of chromosomes very well and shows how characters transfer from one generation to the next generation.

Muller (1927).

Morgan observed the natural mutation in fruit flies but Muller was able to induce the mutations in organisms. He used X-rays to induce mutations in the sperm cells of fruit flies (Drosophila). Some of the fruit flies died (i.e. about 88) showing lethal mutations while most of them survived against these mutagenic actions. Muller's experiment highlights the both positive and negative effects of X-rays. He got a Nobel prize in 1946 for his excellent work on fruit flies.

Griffith (1928).

Griffith shows the transformation principle of characters from one generation to the next.

He infected mice with two types of bacteria of Streptococcus pneumonia I.e. smooth streptococcus pneumonia (S-type bacteria) and rough streptococcus pneumonia (R-type bacteria).

When he injected S-type bacteria in mice, the mice died
When he injected R-type bacteria in mice, the mice remains alive.
He used heat-killed S-type bacteria in mice and the mice remains alive this time also.

When he used both heat-killed S-type bacteria and R-type bacteria in mice, the mice died. This was very strange for him. He concluded that something was transformed from heat-killed S-type to R-type bacteria and made the R-type bacteria virulence but he was unable to recognize that thing.

Hammerling (1930).

Hammerling showed that inherited information is present in the nucleus of the eukaryotic cell.

He took algae named Acetabularia. This alga was about 10cm long in size. This is the largest unicellular algae. The special thing about this algae was that nucleus of this algae was present at the base of this algae.

He cuts the Acetabularia into three parts, I.e. cap, stalk, and base. He placed these three parts in three different culture plates. He observed that only the base region of Acetabularia was able to regenerate and develops its body while the caps and stalk region degenerated.

He concluded that the nucleus controls this activity and the nucleus performs a functional role in inheritance.

Barbara McLinktock (1931).

Barbara demonstrates the genetic recombination of corn cells. She also discovered the Jumping genes or mobile genes.

She was studying the chromosomes of corn for several years. Corn has a totally 10 numbers of chromosomes and she was dealing with chromosome number 9.

She observed that some genes can move within its genome and shows different colors in corn. She also observed Ds (dissociation) and Ac (activator) in corn cells. The Ds and Ac are responsible for the movement of genes within its genome.

McClintock got the Nobel Prize for her discovery in 1983.

Beadle and Tatum (1941).

Beade and Tatum proposed the one gene one enzyme hypothesis. However, their work was not accepted by scientists but it was very helpful for scientists because it shows a one-to-one relationship between gene and enzyme (protein).

They used a fungus named Neurospora and grow it in two different kinds of culture mediums. One is minimal media while the other is complete media. They placed the spores of Neurospora in both these mediums and the spores show growth.

Then they induced the mutations in Neurosporal spores with the help of X-rays and this time the spores of Neurospora show growth only in complete media. They observed this thing and concluded that X-rays cause mutation in the cells of Neurospora and due to this mutation arginine was not prepared by the spores of Neurospora.

Avery, Mcleod, and McCarty (1944).

Oswald Avery, Colin Mcleod, and Maclyn McCarty.

Avery, Mcleod, and McCarty solved the mystery of Griffith’s experiment. They identified the mysterious thing that transformed from heat-killed S-type bacteria to R-type bacteria and that thing was DNA.

They repeated Griffith’s experiment but this time they used DNAaes enzyme (DNA digesting enzyme) and Protease enzyme (protein digesting enzyme). They observed that transformation was inhibited with DNAaes enzyme. So, it was confirmed that DNA is the material that transformed from one generation to the next.

Chargaff (1950).

In 1950, Erwin Chargaff gave its famous rule which is called Chargaff’s Rules. He showed the relationship between the nucleotides of DNA. He took the DNA from many organisms and form a mixture of all of the DNA he collected. He used the paper chromatography technique in his experiment to differentiate the purines (adenine and guanine) and pyrimidines (cytosine, thymine, and uracil.

According to Chargaff’s rule, adenine binds with thymine with two hydrogen bonds and guanine binds with cytosine with three hydrogen bonds.

Hershey and Chase (1952).

Hershey and Chase used radioactive techniques to prove that DNA is a hereditary material. They used radioactive phosphorous 32 and sulfur 35 for their experiment.

They took some bacterial cells and formed some bacterial cells with radioactive phosphorous while creating some of the bacterial cells with radioactive sulfur.
They took some bacteriophages and allow them to attack these bacterial cells separately.
They had done this because they wanted to know that is DNA is the genetic material (if radioactive phosphorous is found in the next generation of bacteriophages) or protein is the genetic material (if radioactive sulfur was found in new generations of bacteriophages).
They found radioactive phosphorus in the next generation of bacteriophages which shows that DNA is the hereditary material because phosphorous is part of the composition of DNA.

Watson and Crick (1953).

In 1953 Watson and Crick showed the double helix structure of DNA. According to this model, DNA is made up of three components, I.e. nitrogenous bases, pentose sugar, and phosphate groups. DNA has four types of nitrogenous bases, I.e. adenine, guanine, cytosine, and thymine (as shown by Erwin Chargaff in 1950). All of these units form a double helical structure of DNA.

The replication in DNA is also a point to be discussed by scientists for a long time. Many models have been presented that show replication in DNA.

In 1958 two scientists performed an experiment that we called Meselson-Stahl experiment that shows the replication in DNA.

They used two different isotopes of nitrogen, one is a lighter isotope while the other is a heavier isotope of nitrogen. They inserted these isotopes in E.coli bacteria and observed that DNA is replicated in a semi-conservative manner (a half strand of DNA remains conserved).