Unit – I A
History of Genetics
- From Aristotle (384 – 322 B.C.) to 18th century it was assumed that the mother provides inert matter and father imparts the motion or a life to this inert matter.
- Leonardo da Vinci (1452 – 1519) and Regnier de Graff (1641 – 1673) proposed that the male and female parents contribute equally to the heredity of offspring.
Theory of Preformation:
- This theory was proposed by J. Swammerdam (1679).This is a theory (popular in the 18th century)
- This theory proposes that an individual develops by simple enlargement of a tiny fully formed organism (a homunculus) that exists in the germ cell.
- The homunculus can be present in the sperm or ovum.
Theory of Epigenesis:
- The theory suggested that many new organs and tissues, which were originally absent, develop de novo (totally new begining) due (totally new begining) to mysterious vital forces.
Theory of Pangenesis:
- Charles Darwin (1809 – 1882) proposed Pangenesis, a developmental theory of heredity.
- He suggested that all cells in an organism are capable of shedding minute particles called gemmules or pangenes, which are able to circulate throughout the body and finally congregate in the gonads.
- During fertilisation, gemmules from both parents are bought together for redistribution to different organs during development. Thus new characters are developed.
Contribution from Other Biologists
- Joseph Gottlieb Koldreuter (1733 – 1806) a german botonist with is experiments with hybrids obtained from tobacco species concluded that, inherited traits are particulate in nature.
- Knight (1799) and Goss (1824) performed experiment on garden pea (Pisum sativum) and observed that the hybrids were uniform in character and segregation of characters occured in second generation. These experiments were basis of Mendel’s work. But Knight and Goss failed to formulate the laws inheritance.
Theory of Blending Inheritance:
- By this theory inherited traits were determined, randomly, from a range bounded by the homologous traits found in the parents.
- Thus, the height of a person, with one short parent and one tall parent, was thought to always be of some interim value between its two parents’ heights.
- The theory failed to explain that the offspring of a black and white horse were black and not grey.
- Theory failed to explain how traits that seemingly disappeared for several generations often reasserted themselves down the line, unaltered.
Gregor Mendel (July 22, 1822 to January 6, 1884)
- Gregor Johann Mendel was born on July 22, 1822, in village Heinzendorf, Austria. He spent his early youth in that rural setting. At the age of 11 he was sent to secondary school in Troppau. Mendel, excelled in his studies, and in 1840, he graduated from the school with honors.
- In 1843 Mendel began studying to be a monk: He joined the Augustinian order at the St. Thomas Monastery in Brno, and was given the name Gregor. At that time, the monastery was a cultural center for the region, and Mendel was immediately exposed to the research and teaching of its members, and also gained access to the monastery’s extensive library and experimental facilities.At he University of Vienna, at the monastery’s expense Mendel studied mathematics and physics under Christian Doppler, after whom the Doppler effect of wave frequency is named; he studied botany under Franz Unger.
- In 1853, upon completing his studies at the University of Vienna, Mendel returned to the monastery in Brno and was given a teaching position at a secondary school. It was during this time that he began the experiments for which he is best known. He performed number of experiments with garden pea plant and proposed laws of inheritance.
- He presented his work in 1865 for the first time before Brunn Society for the Study of Natural Science His work was published in the proceedings of the society in 1866. His work was ignored at that time. The ignorance was may be due to
- His work was ahead of his time.
- He published his work in obscure journal.
- He published his work in a period, when there was controversy due to Darwin’s theory of origin of species.
- In 1900 eminent biologists, Karl Correns of Germany, Hugo de Vries of Netherlands and Erich Von Tschermak of Austria working independently discovered Mendel’s work. In 20 th century, Thomas Morgan and his team from United States worked with Drosophila to gain evidence of Mendel’s work.
Concept of Factors
- Concept of factors is given by Mendel. Mendel proposed that the characters are transmitted from one generation to the next through particle. He called these particles as factors.
- Nowadays these factors are called genes.