Genes

   The classical principles of genetics were deduced by Gregor Mendel in 1865 on the basis of the results of breeding experiments with peas. Mendel studied the inheritance of a number of welldefined traits such as plant height and was able to deduce general rules for their transmission. In all cases, Mendel could correctly interpret the observed patterns of inheritance by assuming that each trait is determined by a pair of inherited “factors”, which are now called genes. The term gene (Gr genos descent) was coined by W. Johannsen in 1909 to refer to the hereditary “factors” of Mendel.
The gene can be defined as a functional unit of heredity which occupies a specific place (locus) on a chromosome, is capable of  reproducing itself exactly at each cell division and directs the
formation of an enzyme or other protein.

 Cytologic and genetic studies show that genes are the fundamental units of inheritance regarded as indivisible units of the chromosomes on which they are located like “beads on a string”. Genes as the chief functional genetic unit, determine the basic architecture of every cell, the nature and life of the cell, the specific protein synthesis, the enzyme formation and the selfreproduction of the cell. Genes are molecular patterns that can maintain their identities for many generations, can be self-duplicated in each generation and can control cell processes by allowing their specificities to be occupied. Genes can mutate, can be assorted, can be shuffled in different combinations, therefore, genes are regarded as the basis for modern interpretation of evolution. On operational or functional basis as a cistron, a muton or a recon as determined or encountered by different approaches.

 
 A Cistron:  A gene can be referred to as a unit of function called a cistron which is the smallest functional region on a chromosome. This idea replaced the unitary concept of the physical gene with the concept of an operational gene composed of one or more functional components (mutant
alleles) that embraces an array of mutant sites, that behaves in a Mendelian fashion. The term cistron was coined by Seymour Benzer and is regarded as a portion of DNA specifying a single polypeptide chain. Each cistron is responsible for coding one messenger RNA molecule, which in turn participate in the formation of a polypeptide chain. It  has been discovered that hundreds of units of mutation
(mutons) and recombination (recons) exist within each cistron. Cistrons, therefore, occupy a much greater chromosomal length than mutons or recons.

 Muton: There are many positions or sites within a cistron where mutations can occur. A muton is the smallest genetic unit or length of DNA that can mutate i.e. a change in muton could result in mutation to produce a phenotypic effect. A muton

  may consist of a single nucleotide or many nucleotides.

 
 Recon: Sometimes crossing over or recombination occur in a cistron to provide another sub-divisional concept of the cistron, the recon. The recon is the smallest genetic unit that
can undergo crossing over (exchange of genetic material), or recombination i.e. the smallest unit within the DNA capable of being independently involved in recombination.

 CHROMOSOMES

Thomas Morgan after looking at Hugo De Vries theories of mutations, bred the fruit fly Drosophila melanogaster for a year with no mutations, but a fly appeared with white eye instead of red eyes, within another year. 40 different kinds of mutations had been noticed. In trying to explain how organisms inherit characteristics Morgan discovered that the various mutations of the flies were
associated with 4 pairs of chromosome possessed by Drosophila. Thomas Morgan’s work proved the chromosomal theory of inheritance which states that chromosome are the elements that transmits inheritable characteristics, also discovered that chromosome are the carriers of genes which cause the expression of individual characteristics.

  Chromosomes are filamentous rod-like or thread-like gene bearing bodies found in the nucleus during cell division. Each nucleus contain information coded in the form of DNA and organised into groups called genes. Genes are arranged on the chromosome and each gene contain enough information for the production of one protein which can have some effects on the individual chromosome vary widely between different organisms.

  The chromosome molecule may be circular or linear, typically eukaryotic cells (cells with nuclei) have large linear chromosomes and prokaryotic cells (cells without defined nuclei) have circular
chromosomes. Chromosomes are the essential unit for cellular division and must be replicated, divided and passed successfully to their daughter cells so as to ensure the genetic diversity and
survival of their progeny.

 Chromosome Structure

Chromosome within a cell occur in matched pairs called homologous xomes, joined at the centre by a centromse. Each chromosome contains many genes, and each gene is located at a
particular site on the chromosome, known as the locus. Like chromosome, genes typically occur in pairs. A gene found on one chromosome in a pair usually has the same locus as another gene
in the other chromosome of the pair, and these two genes are called alleles. Alleles are alternate forms of the same gene. In organisms that use sexual reproduction, offspring inherit
one-half of their genes from each parent and then mix the two sets  of genes together. This produces new combinations of genes; so that each individual is unique but still possesses the same genes
as its parents.

Chromosome Number

In the cells of most organisms that produce sexually, chromosomes occur in pairs; one chromosome is inherited from the female parent; and one is inherited from the male parent. The two chromosomes of each pair contain genes that correspond to the same inherited characteristics. Each pair of chromosomes is different from every other pair of chromosome in the same cell.
The number of chromosome pairs in an organism varies depending on the species. The number of chromosome characteristic of a particular organism is known as the diploid number. Dogs, for example, have 39 pairs of chromosomes and a diploid number of 78 while tomato plants have 12 pairs of chromosomes and a diploid number of 24.

Gametes or sex cells (eggs and sperm) contain only half the number of chromosomes found in the other cells of an organism. This reduced number of chromosomes in the gametes is known as the haploid number. During fertilisation the gametes unite to form a cell known as a zygote containing the diploid number of chromosomes characteristics of the species.

 Sex Chromosomes

Most organisms have complete sets of matching chromosomal pairs, known as autosomes. In mammals, birds and some other organisms, one pair of chromosomes is not identical.
Known as the sex chromosomes, this pair plays a dominant role in determining the sex of an organism. Females have two copies of the X chromosome while males have one Y chromosome and one X chromosome. Both males and females inherit one sex chromosome from the mother (always an X chromosome) and one sex chromosome from the father (an X chromosome in female
offspring and a Y chromosome in male offspring). The presence of the Y chromosome determines that a zygote will develop into a male.

 Human Chromosomes and Genetic Disorders

  Humans have 23 pairs of chromosomes, with a diploid number of 46 numbered according to their size. The largest is chromosome 1 and the smallest is chromosome 23. Physical and
chemical meiosis (formation of gametes) can damage chromosomes or alter their number in a cell, to give rise to embryos with more of less genetic material, sometimes resulting in developmental disabilities or health problems. In a process called non disjunction, paired members of chromosomes fail to separate from one another during meiosis.

  Non disjunction can lead to a condition known as Down Syndrome, in which a person inherits three copies of chromosome 21. Another condition that may result from non disjunction is Turner Syndrome, a disorder in which a female inherits only a single X chromosome. Breakage of a chromosome can lead to four types of changes in chromosome structure. A deletion occurs when a chromosome fragment lacking a centromere is lost during cell division. In some cases the fragment may join to the homologous chromosome to produce a duplication. It may reattach to the original chromosome but in a reverse orientation, producing an inversion; the fragment can join a non homologous chromosome a rearrangement called translocation.