Heredity and Variation
All members of a species have many characteristics in common, but no two individuals are exactly alike. The common characteristics are due to common heredity. These are transferred from parents to progeny via gametes. The differences are called variations and are caused by heritable mutations, i.e. sudden changed in the genetic material, change in the number of arrangement of genes on the chromosomes, and rearrangement of chromosomes of maternal and paternal origin in the zygote. Certain variations are established for a number of generations, it leads to a new characteristic, sometimes leading to the formation of a new species. In organisms reproducing asexually, a part of the parent body (bud, rhizome etc) separates and develops into new individual resembling parent without any variation.
In 1866, Gregor Johann Mendel reported the results of eight years of experiment and thought. These results were rediscovered after 34 years. Mendel formulated laws of inheritance based upon his experimental results. Later work by Sutton and Morgan suggested that the behaviour of chromosomes at mitosis and meiosis and their transmission to next generation follows a pattern similar to that postulated by Mendel for his characteristics controlling factors. They therefore hypothesized that chromosomes bar these factors (now called genes) or that the chromosomes are the physical basis of heredity. Each chromosome contains thousand of genes.
Genes reproduce themselves in exactly the same form, with only an occasional change – the mutation. Gene is made up of a chemical called DNA (Deoxyribo Nucleic Acid), which is the hereditary material, or the chemical basis of heredity.