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 Genetics is based structure, function, and behavior of genes.Gene is the molecular unit of heredity in organisms. Gene is the name given to some stretches of Deoxyribonucleic acid (DNA) and ribonucleic acids (RNA). DNA and RNA are the molecules that encode the instructions used in the development and functioning of all known living organisms and many viruses. Both DNA and RNA are found in the nucleus of cells and store all the information related to that particular cell in which they reside. DNA has a double helix structure in which each of the strands is identical. The two strands are joined to each other by hydrogen bonds.

Following are the basic concepts on which the Genetics is based on.

Genotype refers to the genetic component of an individual or group of individuals usually with respect to a specific characteristic under consideration.
Phenotype refers to the physical appearance of the individual such as morphology, development, biochemical and physiological properties, behavior and product of behavior such as nests of birds. Phenotype results from expression of the organism’s genes as well as the influence of environmental factors and interaction between the two.
A chromosome is the structure of DNA, protein, and RNA found in cells. Humans have 23 pairs of chromosomes that make 46 chromosomes in total. Out of the 46 chromosomes, 2 chromosomes, named X and Y, are called sex chromosomes whereas the remaining 44 chromosomes, 22 pairs are Autosomes. Man produces two types of gametes (sperm) – one with X chromosome and another with a Y chromosome. A woman produces only one type of gamete (eggs) – with X chromosome. Y chromosome triggers the formation of the testis and hence is called the sex determining chromosome. Therefore, the sperm determines the sex of the offspring. The offspring with XY chromosome is a male whereas one with XX chromosome is a female.
Sex Chromosomes are the pair of chromosomes that control inheritance of sex (called as X and Y chromosomes) whereas the other chromosomes referred to as Autosomes, control the inheritance of other features. In humans, 2 out of 46 chromosomes are sex chromosomes and the remaining 44 chromosomes are Autosomes.
Sex-linked Inheritance: The inheritance of characteristics controlled by genes in the sex chromosomes varies the sex and is therefore called sex-linked inheritance.

Gene is a segment of DNA (Deoxyribo Nucleic Acid). DNA is the genetic material present in animals. RNA (Ribo Nucleic Acid) is the genetic material in some viruses and therefore in these cases, genes are the segments of RNA. Genes, through their control of enzymes, control cellular metabolism and thereby control heredity. Genes are the basic unit of heredity and the science of genetics mainly revolves around the study of genes that make up chromosomes.

Linkage, Crossing Over, and Recombination of Genes

A number of chromosomes in each species are fixed (23 pairs in humans, 46 in total). Each chromosome comprises many genes; sometimes even hundreds or thousands. Genes on the same chromosome are inherited together and the phenomenon of inheritance is called Linkage of genes. If all genes are inherited together, it is called Complete Linkage. Sometimes, genes get segregated during inheritance and partial inheritance takes place.This phenomenon of incomplete linkage is called Crossing over.

As a result of crossing over of genes, Recombination of genes takes place which is an importance source of variation. This process is essential for natural selection and separation. The frequency of recombination is proportional to the distance between genes. The distance between genes is the number of changes required to be done in one gene to convert it into another gene.

Mutation is a sudden, discreet and abrupt change in genotype. Mutation can be induced by radiations like X-rays, UV-rays etc and also by certain chemicals like mustard gas, nitrous acid, and other such harmful compounds. Mutation is an important process in genetic engineering and genetically modified organisms. It is used for improving crop plants and pest control.

Biology of Genes

Gene (DNA) is the ultimate controlling factor in heredity. The process of replication of gene is described below.

DNA molecules are double helix structure (except for a few single stranded DNA viruses). These helices (poly-nuclide chains) are held together by hydrogen bonds between nitrogen bases (Adenine (A), Guanine (G), Cytosine (C) and Thymine (T)) of nucleotides. Pairing between nucleotide bases is fixed and the two helices are complementary to each other. Under normal circumstances, the two strands are identical to each other.
During replication, the two helices separate and synthesize a complimentary copy. One-half of each of the DNA molecule is old (original) and the other half is newly synthesized. This mode of replication is called Semiconservative Replication. The enzymes involved in this process are helicase, topoisomerase, primase, RNA polymerase, and DNA polymerase.
In animals, most DNA molecules are very long and replication starts simultaneously at a number of points. Each of these units that replicate themselves is known as Replicon. Some human chromosomes may contain thousands of replicons.
The nucleotide sequence in a DNA molecule determines the sequence of amino acids in a protein. This is known as co-linearity hypothesis meaning that certain length of DNA corresponds to (is collinear with) a sequence of amino acids in a polypeptide.
Some of the structural genes encode essential metabolic enzymes or cellular components are expressed all the time (constitutionally). Such genes are called Housekeeping Genes.
Certain movable genetic elements could detach from one site and move to a new site in either the same of a different chromosome. These elements called transposable controlling elements affect the expression of genes on either side of it producing a new phenotype. These movable genes are known as transposons, insertion elements, or Jumping genes.
Genetic Engineering: It is possible to isolate specific DNA segments, even specific genes and transfer them to the altogether unrelated organism, e.g. transfer of genes from bacteria to men and vice versa. This results in an entirely new combination of genes or recombinant DNA. This technology is called Recombinant DNA technology or Genetic Engineering. The transfer is brought about through a bacteriophage or plasmid (small circular DNA) in bacterial cytoplasm –called vehicle or vector. Genetic Engineering is applied to increase the yield of products, develop varieties that are resistant to certain pests and diseases and so on.
The living organisms developed by the process of genetic engineering, i.e. developed by altering some of the genes or inducing mutation in some of the genes are called living modified organisms or genetically modified organisms. Such organisms are developed to produce organisms with certain desired characteristics like crops are developed with increased resistance to herbicides, with increased productivity and so on. As contrasted with hybrid organisms which are developed by crossbreeding of two different species, living modified organisms have greater potential to have desired traits.
Living modified organisms have great potential use in agriculture, industry, and medicine.