INTRODUCTION
- The term ‘ Genetics ‘ was used for the first time by W. Bateson in 1905, although beginning of the science of genetics was made in 1900 by rediscovery of Mendel’s work.
- Genetics is often described as a science which deals with heredity and variation.
- Heredity includes those traits or characteristics which are transmitted from generation to generation and is, therefore, fixed for a particular individual .
- Variations on the other hand, are mainly of two types, namely (i) hereditary and (ii) environmental.
- Hereditary variations refer to differences in inherited traits.
- Such variations are found not only in progenies of different parents but also among progeny from the same parents.
- Traits which are present in parents can reassort and give rise to different combinations, which are responsible for hereditary variations within the same progeny.
- Identical twins, however, is an exception, where no hereditary variation exists.
- Environmental variations are those which are merely due to environment.
- Study of genetics, therefore, should enable us to differentiate between hereditary variations and environmental variations.
- While hereditary variations are transmitted from generation to generation, environmental variations are temporary and have nothing to do with the last or the next generation.
- Hereditary variations within a progeny result due to sexual reproduction.
- If there were no sexual reproduction, there could be no hereditary variation except those due to mutations among the individuals of the same progeny.
- It is due to this reason that in asexually reproducing plants, the population appears fairly uniform.
- Obviously sexual reproduction has played a very important role in evolution.
- A brief account of sexuality in animals and plants has, therefore, been included here.
Sexuality: A Source of Hereditary Variation
Sexuality in animals
- Before the development of light microscope in seventeenth century, the idea about sexuality in animals was based on speculation rather than facts.
- Harvey (1578-1657) speculated that all animals arise from eggs and that semen only plays a vitalizing role.
- de Graaf (1641-1673) observed that the progeny would have characteristics of father as well as mother and, therefore, suggested that both the parents should contribute to heredity.
- He also studied the development of embryo to some extent, although the egg was discovered by Von Baer in 1828.
- V. Leeuwenhoek observed sperms of several animals in 1677 and also suggested their association with eggs.
Sexuality in plants
- Reproductive parts of plants were reported for the first time by N.Grew in 1682.
- Subsequently R. Camerarius in 1694 described sexual reproduction in plants for the first time.
- Camerarius is also known to be the first to produce a hybrid between two different plant species.
- It was however, in 1717 that T. Fairchild produced hybrid having characteristics of both parents.
- This hybrid was called Fairchild’s Sweet William or as Fairchild’s mule.
- This provided a means of artificial hybridization in plants.
