Chromosomal Basis of Inheritance

Chromosomal Basis of Inheritance

The factors of Mendel were called genes by Johansen 1909, who did not know their exact nature and structure.

Gene Concept

Sutton introduced the gene concept which was elaborated by the studies of Morgan, Bridges and Miller.

The important features of the gene concept are:

   Genes are transmitted from parents to offsprings and are responsible for the physical and physiological characteristics of the organism which are present inside the nucleus of the cell.

   The genes are present on the chromosome.

   Since the number of genes far exceeds the number of chromosomes, several genes are located on each chromosome. In man about 40,000 genes are present in 23 pairs of chromosomes.

   The genes are present at a specific position on the chromosome called locus.

   Genes are arranged on the chromosomes in a linear order like beads on a string.

   A single gene may have more than one functional state or form. These functional states are refered to as alleles.

   The alleles may be dominant or recessive but sometimes co-dominance or incomplete dominance may be seen.

   Genes may undergo sudden heritable changes called mutations, induced by chemical and physical factors.

   Due to mutation a gene may come to possess more than two alternative states and these states of the gene are called multiple alleles.

   Genes undergo duplication by a phenomenon called replication.

   Genes are responsible for the production of proteins called enzymes by which they show their expression which brings about a change in the organism.

A gene is a particular DNA segment which contains the information to synthesize one polypeptide chain or one enzyme. The information is contained as a sequence of nucleotides which is called genetic code. The sequence of three nucleotides that code for an aminoacid is called Codon.

Molecular structure of a gene

A gene, is made of DNA. The gene may be subdivided into different units according to Benzer such as Recon, Muton, Cistron and Operon.

Recon

It is that smallest portion of a gene which can undergo crossing over and recombination and may be as small as a single nuclecotide pair.

Muton

It is the smallest unit of a gene that can undergo mutation and can involve a pair of nucleotides.

Cistron

It is the functional unit which can synthesize one polypeptide.

Operon

It is a group of genes having an operator a structural gene and other genes in sequence which all function as a unit.

Exons and Introns

In Prokaryotes generally, the genes are continuous segments of DNA occurring collinearly without interruption. But in Eukaryotes, the genes on the DNA strand have coding regions called exons interrupted by non-coding DNA segments which do not carry genetic information called introns. This led to the concept of interrupted genes or discontinuous genes. Such genes while producing m-RNA will first form a primary transcript which will then cut off the introns to form the functional m-RNA and this is called splicing.

Chromosomal basis of inheritance

The chromosome basis of inheritance was put forth by Sutton and Boveri independently in the year 1902. W.S. Sutton and Theodor Boveri faced and solved the problem of drawing a parallel between chromosomes and genes.

Both had concluded that the genes are contained in chromosomes. Allelic genes present in a heterozygote segregate independently because the chromosomes carrying these genes segregate when the sex cells are formed. This conclusion of Sutton and Boveri was verified extensively by further studies conducted by various geneticists and cytologists.

In order to accept this conclusion we must be able to understand the behaviour of chromosomes in the light of Mendel's assumption.

  Individuality of Chromosomes :Every organisms has a fixed number of chromosomes. The nuclei of gametes contain haploid (n) and those of zygotes have double the number or diploid (2n) number of chromosomes.

  Meiosis : At the time of meiosis, for the formation of gametes, the pairs of chromosomes of the diploid sets undergo pairing.

  The chromosomes of each pair segregate independently of every other pair during their distribution into gametes. This is similar to Mendel's law of independent assortment in the segregation of factors.

  During the fusion of haploid gametes, the homologous chromosomes from two parents are brought together to form the diploid zygote. Accordingly Mendel had maintained that maternal and paternal characters mix up in the progeny.

   The chromosomes maintain the structure and uniqueness during the life time of the individual whether observable or not. Mendel had also demonstrated that the characters are never lost though they are not expressed in a particular generation.

From these points it is evident that a clear parallelism exists between Mendel's factors and chromosomes and so there is a firm basis for Mendel's Laws of heredity in the behaviour of chromosomes during meiosis and fertilisation and therefore theChromosomal Theory of Inheritance has been proposed.

Postulates of the Chromosomal Theory of Inheritance

   The factors described by Mendel are the genes which are the actual physical units of heredity.

   The genes are present on chromosomes in a linear order.

   Each organism has a fixed number of chromosomes which occur in two sets referred to as diploid (2n). A pair of similar chromosomes constitute the homologous pair.

   Of this, one set is received from the male parent (paternal) and the other from the female parent (maternal).

   The maternal and paternal chromosomes are contributed by the egg and sperm respectively during zygote formation. But only sperm nucleus is involved proving that chromosomes are present within the nucleus.

   The chromosomes and therefore the genes segregate and assort independently at the time of gamete formation as explained in Mendel's law of segregation and Law of Independent Assortment.

Physical and Chemical Basis of Heredity

                        Physical Basis

                        Gregor Johann Mendel put forward in 1866 that particles called germinal units or factors controlled heredity. These were present in both the somatic cells as well as the germinal cells. Though he was not able to actually see these particles, he did explain the pattern of inheritance of genetic characters. It was the gamete which carried these factors to the next generation and so gametes form the physical basis of heredity.

                        Chemical Basis

                        Now it is known that genes control heredity and these are definite segments of chromosomes and so are particulate bodies. The genes travel from one generation to the next carrying the traits and since gene is composed of DNA and protein, the DNA part functions as the chemical basis of heredity.