- • The hereditary units which are transmitted from one generation to the next generation arc called genes.
- • A gene is the fundamental biologic 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.
- • Even before the rediscovery of Menders 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 behavior 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.
- • 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 ail 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 base altogether discarded the Morgan’s theory.
- • Therefore, before defining the gene it will be advisable to consider both the classical as well as modern definitions of gene.
Changing Concept of Gene
- • The concept of gene has been the focal point of study from the beginning of twentieth century to establish the basis of heredity. The gene has been examined from two main angles, i.e.,
- (1)genetic view, and
- (2)biochemical and molecular view. These aspects are briefly described below:
(2)A Genetic View
- • The genetic view or perspective of gene is based mainly on the Mendelian inheritance, chromosomal theory of inheritance and linkage studies.
- • Mendel used the term factors for genes and reported that factors were responsible for transmission of characters from parents to their offspring.
- • Sutton and Boveri (1903) based on the study of mitosis and meiosis in higher plants established parallel behaviour of chromosomes and genes.
- • They reported that both chromosomes and genes segregate and exhibit random assortment, which clearly demonstrated that genes are located on chromosomes.
- • The Sutton- Boveri hypothesis is known as chromosome theory of inheritance.
- • Morgan based on linkage studies in Drosophila reported that genes are located on the chromosome in a linear fashion.
- • Some genes do not assort independently because of linkage between them.
- • He suggested that recombinants are the result of crossing over. The crossing over increases if the distance between two genes is more.
- • The number of linkage group is the same as the number of chromosomes.
- • The chromosome theory and linkage studies reveal that genes are located on the chromosomes.
- • This view is sometimes called as bead theory.
The important points about the bead theory are given below:
- 1.The gene is viewed as a fundamental unit of structure, indivisible by crossing over. Crossing over occurs between genes but not within a gene.
- 2.The gene is considered as a basic unit of change or mutation. It changes from one allelic form to another, but there are no smaller components within a gene that can change.
- 3.The gene is viewed as a basic unit of function. Parts of a gene, if they exist, cannot function.
- • The chromosome has been viewed merely as a vector or transporter of genes and exists simply to permit their orderly segregation and to shuffle them in recombination.
- • The bead theory is no more valid for any of the above three points.
Now evidences are available which indicate that:
- (1)a gene is divisible
- (2)part of a gene can mutate, and
- (3)part of a gene can function.
The Gene is Divisible
- • Earlier it was believed that gene is a basic unit of structure which is indivisible by crossing over.
- • In other words, crossing over occurs between genes but not within a gene.
- • Now intragenic recombination has been observed in many organisms which indicates that a gene is divisible.
The intragenic recombination has following two main features.
- 1.It occurs with rare frequency so that a very large test cross progeny is required for its detection. Benzer expected to detect a recombination frequency as low as 10-6, the lowest he actually found was 10-4(0.01 x 2
- 2.The alleles in which intragenic recombination occurs are separated by small distances within a gene and are functionally related.
- • Examples of intragenic recombination include bar eye, star asteroid eye and lozenge eye in Drosophila.
- • The bar locus is briefly described below.
- • Lozenge eye and star asteroid have been discussed under pseudoalleles.
- • Bar Eye in DrosophilaThe first case of intragenic recombination was recorded in Drosophila for bar locus which controls size of eye.
- • The bar locus contains more than one unit of function.
- • The dominant bar gene in Drosophila produces slit like eye instead of normal oval eye.
- • Bar phenotype is caused by tandem duplication of 16A region in X chromosome, which results due to unequal crossing over.
The flies with different dose of 16A region have different types of eye as follows:
- 1.Single 16A region → Wild type oval eye
- 2.Double 16A region → Bar eye small in size
- 3.Triple 16A region → Double bar or ultrabar eye very small in size
- • The homozygous bar eye (B/B) produced both wild and ultra bar types though at a low frequency which indicated intragenic recombination in the bar locus but the frequency was much higher than that expected due to spontaneous mutations.
Part of a Gene Can Function
- • It was considered earlier that gene is the basic unit of function and parts of gene, if exist, cannot function.
- • But this concept has been outdated now. Based on studies on rll locus of T4 phage, Banzer (1955) concluded that there are three sub divisions of a gene, viz., recon, muton and cistron.
These are briefly described below:
- • Recons are the regions (units) within a gene between which recombinations can occur, but the recombination cannot occur within a recon.
- • There is a minimum recombination distance within a gene which separates recons.
- • The map of a gene is completely linear sequence of recons.
- • It is the smallest element within a gene, which can give rise to a mutant phenotype or mutation.
- • This indicates that part of a gene can mutate or change.
- • This disproved the bead theory according to which the entire gene was a mutate or change.
- • It is the largest element within a gene which is the unit of function.
- • This also nocked down the bead theory according to which entire gene was the unit of function.
- • The name cistron has been derived from the test which is performed to know whether two mutants are within the same cistron on in different cistrons. It is called cis-trans test which is described below.
Cis – Trans Test
- • When two mutations in trans position produce mutant phenotype, they are in the same cistron.
- • Complementation in trans position (appearance of wild type) indicates that the mutant sites are in different cistrons.
- • There is no complementation between mutations within a ciston. It is now known that some genes consist of only one cistron; some consist of two or even more.
- • For example, the mutant miniature (m) and dusky (dy) both decrease wing size in Drosophila and map in the same part of X chromosome.
- • But when brought together in dy +/+m heterozygote, the phenotype is normal which indicates that the locus concerned with wing size is composed of at least two cistrons.
(2)A Biochemical View
- • It is now generally believed that a gene is a sequence of nucleotides in DNA which controls a single polypeptide chain.
- • The different mutations of a gene may be due to change in single nucleotide at more than one location in the gene.
- • Crossing over can take place between the altered nucleotides within a gene.
- • Since the mutant nucleotides are placed so close together, crossing over is expected within very low frequency.
- • When several different genes which affect the same trait are present so close that crossing over is rare between them, the term complex locus is applied to them.
- • Within the nucleotide sequence of DNA, which represents a gene, multiple alleles are due to mutations at different points within the gene.