The hereditary units which are transmitted from one generation to the next generation are called genes. A gene is the fundamental biological unit, like the atom which is the fundamental physical unit.
Mendel while explaining the result of his monohybrid and dihybrid crosses first of all conceived of the genes as particulate units and referred them by various names such as hereditary factors or hereditary elements.
But his concept about the gene was entirely hypothetical and he remained ignorant about the physical and chemical nature of gene.
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Even before the rediscovery of Mendel’s laws in 1900 it was already established that chromosomes have a definite role in the inheritance because it was found that chromosomes were the only link between one generation and the next generation and a diploid chromosome set consists of two morphologically similar sets, one is derived from the mother and the other from the father at fertilization. Later on, a parallel behaviour among chromosomes and genes was discovered.
Earlier workers proposed various hypotheses to explain the nature of genes. For instance, De Vries postulated one gene one character hypothesis according to which a particular trait of an individual is controlled by a particular gene.
Bateson and Punnett proposed the presence or absence theory. According to them, in a cross the character which dominates the other has a determiner while, the recessive character has no such determiner.
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But all the theories were discarded by Morgan who produced the particulate gene theory in 1926. He considered genes as corpuscles which are arranged in a linear order on the chromosomes and appear like beads on a string.
Each gene was supposed to be different from all others. The particulate theory of gene was widely accepted and supported by cytological observations.
But, the discovery of DNA molecule as a sole carrier of genetic information’s has altogether discarded the Morgan’s theory. Therefore, before defining the gene it will be advisable to consider the both classical as well as modern definitions of gene.
Classical definition of gene:
The definition of gene is an operational one and had different meanings according to different viewpoints. Classically, gene as assumed to be a genetic unit by the following three criteria:
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(i) A gene is a unit of physiological function that occupies a definite locus in the chromosome and is responsible for a specific phenotypic character, e.g. vestigial or long wings and white or yellow eyes in Drosophila.
(ii) A gene is a unit of transmission or segregation, because it can be segregated and exchanged at meiosis by way of crossing over.
(iii) A gene is a unit of mutation, because by a spontaneous or induced change it can give rise to different phenotypic expressions.
These classical definitions of genes have been revised thoroughly due to recent discoveries in the field of molecular genetics.