In the 1930s, G.W. Beadle, B. Ephrussi, E.L. Tatum, J.B.S. Haldane and others provided a basis for understanding the functional properties of genes and suggested functional extensions of the classical gene concept. The gene was at first characterized as an indivisible unit of structure, unit of mutation and unit of function. A.E. Garrod, had indicated in 1902 that genes in humans function through ‘enzymes’.
Early triumphs were the identification of maromolecules carrying genetic information in a bacterium by O.T. Avery and associates and in a virus by A. Hershey and M. Chase.
The Avery et al experiments showed that a chemical, deoxyribonucleic acid (DNA) could bring about genetic change (transformation) in a Peumococcus bacterium; Hershey and Chase demonstrated that the nucleic acid component (DNA) and not the protein is the genetic material carried by the bacteriophage. H. Fraenket Conrat & B. Singer showed that ribonucleic acid (RNA) is the genetic material in tobacco mosaic virus. Thus, RNA performs the function in some viruses that DNA performs in other organisms.
J.D. Watson and F.H.C. Crick worked out the double helix structure of the chemical DNA. The central problem of genetics was thus resolved with the discovery that DNA is the genetic material. Genetic mechanisms could now be forumlated in biochemical terms. Also, it became clear how do units (genes) of DNA control specific traits in organisms and how do assemblages of DNA units carried in fertilized eggs provide ‘blueprints’ for development of entire organisms.
(A) Functional Perspective:
Genes accomplish their function (a) through replication that results in more units like themselves & (b) through transcription and translation, whereby, proteins that function as determiners in the metabolism of the cell are synthesized. Although genes are usually stable, they are susceptible to occasional change or mutation, which provides altered forms of genes (alleles).
Mendel first postulated the existence of genes from their end effects, as expressed in altered characteristics. Now genes have been defined chemically and are know for what they do in directing the formation of traits through the specificity of protein enzymes. Thus, DNA carries the specifications for growth, differentiation & functioning of cells in the organism.
Gene versus Allele Concepts (Pseudoalleles):
Gene: “Hereditary unit specifying the production of a distinct protein (e.g. an enzyme) or RNA.” In modern sense, an inherited factor that determines a biological character of an organism is called a gene. This is a functional unit of hereditary material. In the part, gene was believed to be unit of structure also, but recent knowledge has shown that it is no longer a unit of structure.
Alleles, the abbreviated form of the term “allelomorphs” (meaning one form or the other) indicates alternative forms of the same gene. i.e. “One of two genes for a given trait that occurs in a specific position on each homologous chromosome”. For instance, ‘D’ & Y are two alleles of the gene for plant height. In pure tall or pure dwarf plants, same allele is duplicated (DD or dd), while in hybrid tall both the alleles will be present (Dd).
An individual having only one allele or in other words, two identical genes, is known as homozygous (DD or dd). Similarly, an individual, having two different alleles will be called heterzygous or hybrid (Dd). In the present usage the term gene and the allele are interchangable, but while gene can be used for any factor, allele is used with reference to another allele. For instance, while ‘D’ & ‘d’ are alleles to each other, they cannot be allelic to any other gene.
Pseudogenes are partial duplicates of structural genes that have incorporated sufficient changes that they are not biologically active and are usually not transcribed. Pseudogenes are turning out to be quite common in eukoryotes. There are inactive but stable component of a genome resembling a gene, apparently derived from active genes by mutation.
The pseudogenes, which are relatively frequent in the mammalian genome, can be identified because they have undergone mutations that render them unable to produce functional m RNA e.g., a pseudogene might contain premature chain termination codons, (insertions or deletions) that cause the message to be read out of frame.
Most frequently the promoters of pseudogenes are inactive. Because there is no selective pressure to maintain a functional gene product, in due course, pseudogenes accumulate multiple mutations and eventually become random sequence DNA.
Quantitative Genetics and Multiple Factors
Soon after rediscovery of Mendel’s experiments, two main groups of geneticists emerged:
Who believed that all evolutionary important heritable differences are qualitative and discontinuous?
Who proposed that heritable variation is basically quantitative and continuous and that genes did not exist as separate units?
To prove that both “mendelian” and “biometricians” were only partly correct, Johannsen (1903) used the character of seed weight in beans and analysed its breeding behaviour. In a commercial seed lot of a variety, seed weight ranged from 1 eg (eg = centigram) to 90 eg.
In the progeny of seed of every weight group, great-variation was observed, and by successive self pollinations for several generations, it was possible to isolate 19 pure lines. Each pure line had a distinct average seed weight, which was maintained in the progeny, although variation did exist in each pure line and was attributed to environmental effect.
Thus by Johannsen’s experiment, it could be shown that continuous variation can be heritable, a belief opposed by ‘mendelians’. However since differences between 19 different pure lines isolated by Johannsen (1903) could not be ascribed to specific genes, genetic basis of quantitative traits still remained disputed and it could not be proved that like qualitative traits, quantitative traits exhibiting continuous variation are also controlled by individual genes comparable to Mendelian factors.
Subsequently, it was Yule (1906), who suggested that quantitative variation may be controlled by large number of individual genes, each having a small effect. Soon after Yule’s proposal, H. Nitsson-Ehle (1908) and East (1910), demonstrated segregation and assortment of genes controlling quantitative traits, kernel colour in wheat and corolla length in tobacco. Such genes were later called polygenic systems and inheritance due to such a system was explained in the form of “multiple factor hypothesis.”
Alleles that had small but cumulative effect with semidominance rather than complete dominance were postulated to behave in a Mendelian fashion. Several too many nonallelic genes were postulated to have cumulative effects on particular traits. An explanation for continuous variation thus emerged in the form of the ‘multiple-gene-hypothesis’.
Experimental results and interpretations substantiating this hypothesis were obtained from the classical investigations of H. Nilsson Ehle in Sweden & E.M. East in the United States during the period 1910-1913.