GENETICS : Concept of Heredity and Variation

GENETICS : Concept of Heredity and Variation

The children or offsprings closely resemble their parents and to some extent their grand parents and great grand parents. Still the offsprings of a set of parents differ from each other and from their parents in different degrees. They have certain unique characteristics by which we can understand that they belong to the same family. The Science that deals with the mechanisms responsible for inheritance of similarities and differences in a species is called Genetics. It is a branch of biology that encompasses the study of the mechanism of transmission of characters from parents to offsprings. The word "genetics" is derived from the Greek word "genesis" meaning "to grow" or "to become".

The Science of Genetics helps us to differentiate between heredity and variations and seeks to account for the resemblances and differences due to heredity, their source and development.

Heredity refers to the transmission of characters, resemblances as well as differences from one generation to the next. It explains how offsprings in a family resemble their parents.

Variation refers to the differences shown by individuals of the same species and also by offsprings (siblings) of the same parents. It explains why offsprings eventhough born to the same parents differ from each other. They are similar, but not identical (except in identical twins). These similarities and differences are not coincidental.

In brief, genetics is the study of heredity and variation.

Heredity

Heredity refers to the transmission of characters from parents to the offsprings. In the very early ages though improvement of the races of plants and animals were conducted by the Babylonians and Assyrians, it was not known what exactly caused the characters to be passed from one generation to the next.

Some early views of heredity

Many view points were put forward before Mendel to explain the transfer of characters to the subsequent generation.

1. Moist Vapour Theory

This was put forward by the Greek philosopher Pythagoras who believed that each organ of the animal body produced vapours and new organism was formed by combination of different organs.

2. Fluid Theory

This was propounded by Aristotle who was of the view that both male and female produced semen and when these mix the female semen which is not so pure provided the inert substance for the formation of the embryo and the male semen gave form and vitality to the embryo.

3. Preformation Theories

Anton Von Leeuwenhoeck observed human sperms for the first time. This theory according to Swammerdam (1679) postulates that the sex cells either the sperm or egg contained within itself the entire organism in a miniature form called "homunculus

Development was only an increase in size of the miniature. This theory was supported by Malpighi (1673), Delepatius (1694) and Roux (1800).

4. Particulate Theory

French biologist Maupertius propounded that the body of each parent gave rise to minute particles for reproduction which blend together to form the offspring.

5. Pangenesis

This theory proposed by Aristotle (384-322 B.C.) holds that the animal body produces minute bodies called gemmules or pangeneswhich were carried by blood to the reproductive organs. Here the pengenes from two parents blend to give rise to a new individual.

This theory prevailed for many centuries and was accepted by great biologists such as Charles Darwin (1809 - 1882).

Evidences against the Blending Theory

The individual, according to the views of the Pre-Mendelian era represents the mixture of characters of both parents. This was theblending theory. Under this concept the progeny of a black and white animal would uniformly be grey. The progeny from further crossing the hybrids would all remain grey as the characters once blended can never be separated again. But however in daily life it is seen that children of black and white parents may be dark, fair or of a intermediate complexion. So also their children may be dark or fair.

Pattern of inheritance shown by atavism is also against blending inheritance. In atavism, the grandchildren may exhibit a feature of an earlier generation not seen in the parents. The traits of sex (male or female) do not blend in unisexual organisms.

Basic features of Inheritance / Heredity

The Swedish taxonomist Carolus Linnaeus and two german plant breeders Kolreuter and Gaertner performed artificial cross pollination in plants and obtained hybrids. Kolreuter was able to obtain evidence to show the inherited traits remained discrete without blending. Though his results were similar to that of Mendel, he was not able to interpret them correctly.

Mendel's great contribution was that he replaced the blending theory with the particulate theory. Mendel first presented his findings in 1865, but they were not accepted then and remained unknown for many years. Their rediscovery in 1900 by de Vries of Holland, Carl Correns of Germany and Tschermak of Austria independently, led to the beginning of modern genetics.

Few important characteristics of inheritance are:

   Every trait has two alternative forms.

   One alternative form is more commonly expressed than the other.

   Any alternative form can remain unexpressed for many years.

   Hidden character may reappear in original form.

   Characters or traits are expressed due to discrete particulate matter and so do not get blended or modified.