Basic conceptHeredity, process of transmitting biological traits from parent to offspring through genes, the basic units of heredity. Heredity also refers to the inherited characteristics of an individual, including traits such as height, eye color, and blood type. Variations can be defined as the occurrence of differences among the individuals. Genetics is that branch of biology with leads with Heredity and variation of organisms. Mendel’s Theory: Mendel proposed the presence of two copies of the ‘factors’ controlling every character which was based on his experiment. `When pea plant with two different contrasting characters are crossed, only one appears among the offspring’s of the next generation. But in the successive generation both the characters appear. Mendel considered each and every character as a unit, which is controlled by a ‘factor’ Chromosome: A chromosome is a large macromolecule into which DNA is normally packaged in a cell. Gene: A gene is the fundamental physical and functional unit of heredity in living organisms. Genes are encoded in an organism’s genome, composed of DNA or RNA, and direct the physical development and behavior of the organism. Deoxyribonucleic acid (DNA): Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions for the biological development of a cellular form of life. All known cellular life have DNA. DNA is a long polymer of nucleotides (a polynucleotide) that encodes the sequence of amino acid residues in proteins, using the genetic code. Θ The term ‘gene’ was coined by Danish botanist Johanssen in 1909. Θ T. Boveri and W.S. Sutton in 1902 in their experiment provided the evidence that gene is part of the chromosome. Θ In humans there are 23 pairs of chromosomes. The total number of genes in humans is estimated at 30,000 to 40,000. Θ Genes are located on chromosomes at fixed positions. Θ Griffith, Avery, McLeod and McCarty established that DNA is the carrier of the genetic information. Θ Every chromosome contains one molecule of DNA and genes are segments of DNA.