Mendel studied the pattern of inheritance using several true-breeding varieties of garden peas. He concentrated on transmission of seven easily identified traits, and crossed plants showing alternate aspects of each of these(e.g., tallness and dwarfness; yellowness or greenness of peas; smooth; rounded versus crinkled versions of seeds; and so on). Mendel’s application of probability methods to study of the patterns of heredity may be considered to be the foundation of genetics.
Mendel proposed that a trait was but the visible manifestation of certain particulate entities in the organisms; he named them ‘factors’. This idea was not totally original with Mendel. Earlier, Charles Naudin, a botanist, had postulated the existence of ‘essence’ in gametes–the essences being responsible for the characteristics of species. However, like other biologists and breeders before Mendel, no significance was attached to the patterns observed in the data.
The ‘factors’ of Mendel, were proposed as the true determiners of hereditors. These were postulated to be discrete bodies which segregated in offspring, and which remained independent and intact even in these offspring where the trait was not evident. The factors did not mix, but maintain their discrete particulate nature in their passage through generations. The mono, di and tri – hybrid ratios were interpreted by the hypothesis of dominance and recessiveness. Each trait was though to exist in alternate states, and was, therefore represented by alternate form of the same factor (e.g. the factor for height existed as alternate factors for tallness and dwarfness). The validity of Mendel’s postulates was tested by performing test or back-crosses. A neat foundation for the mechanism of inheritance was, thus, laid down.
A sudden change in the gene, which was perpetuated in succeeding generations, was designated as a mutation Mutations occuring spontaneously – that is nit directed continuously – were too low in frequency to be amenable for genetic investigations. It was Muller who discovered, that mutations could be induced by physical agents in large enough proportions to allow experimentation. Some of the mutants he recovered from X-rayed Drosophila had traits which were identical to those found in the natural population. Even without an understanding of the molecular nature of the gene, it was soon accepted that
a) Gene could exist in alternate states
b) At least, in some of them, the changes between two states were reversible
c) If more than one alternate state occurred together in an organism there were several alternatives as to which trait would be evident to observer
These alternative states of a gene are the ‘alleles’. Presence of dissimilar alleles in an organism is distinguished from that of identical alleles by terms like heterozygous and homozygous, respectively. The early concepts of mutations were the outcome of observations of phenotypes which do not and cannot always mirror the actual events in mutation which caused them. Sophistication in our appreciation of the mutation process came only since the discovery of the Watson and Cricks model of DNA. Mutations, in today’s jargon, are sudden, heritable changes in the nature, number and sequence of nitrogen bases in the nucleotide building blocks of nucleic acids. The term heterozygous and homozygous and allele were coined by Bateson.