The principles or laws of “Heredity” were discovered by a monk named Gregor Mendel in the 19th century. He conducted his experiments involving hybridization experiments in garden peas. (Pisum Sativum) He cultivated and tested some 29000 pea plants. He measured only binary characteristics such as color, shape, and position of offspring, rather than quantitative characteristics. His method of data analysis and incorporation of huge sample size gave credibility to his data, which was then published in 1865. Mendel’s finding further provided a chance to other scientists to predict the expression of traits on the basis of mathematical probabilities.
The law of segregation can be defined in a simple way as when the egg and sperm unite during fertilization, each contributes its allele, finally making the paired chromosome in offspring. It also means that allele pair present in egg and sperm separates or segregates during gamete formation, and randomly unite at fertilization. This phenomenon was later on proved when the meiotic cell division process was found. In meiosis, the paternal and maternal chromosomes get separated (according to the law of segregation) and the alleles with the hereditary character are segregated into two different gametes.
Mendel carried out his experiments on garden pea plants. He discovered that by crossing a white flower plant and a purple flower plant, the result was not a hybrid offspring. The offspring was purple-flowered and not the mix of two. He then derived the idea of heredity units, which he called ‘factors’, later on, known as genes. He also planted the theory that the factors are generally present as a pair in ordinary body cells, yet segregate during formation of sex cells. After segregation, the factor which is dominant gets expressed and the other has a recessive characteristic. Thus, during his crossing experiment, as stated above, he obtained purple flowers which hide the recessive genes of white flower. After that Mendel self-fertilized the first generation and obtained 3:1 ratio, this is how he proved that genes can be paired in 3 possible ways: AA, aa, Aa. [A] represents the dominant factor, and [a] represents the recessive factor.
In 1900, the Mendelian theories were rediscovered by three European scientists, Hugo de Vries, Carl Correns, and Erich von Tschermak. The rediscovery made Mendel’s theory important and also more controversial that it can be applied to a certain category of species or traits. A scientist, also a promoter in Europe, William Bateson, coined the terms term "genetics", "gene", and "allele" to describe many of the findings of the Mendelian theory. The heredity model was criticized by many scientists, as it inferred that heredity was discontinuous, in contrast to the fact that obvious and apparent continuous variations in heredity were observed in most of the traits. Many biologists dismissed the Mendelian theory because of the uncertainty that it would be applicable on all species, and there seemed to be a very few true Mendelian characters in nature. However, later on, works carried out by biologists and statisticians such as R.A. Fisher showed that if multiple Mendelian factors were involved in the expression of an individual trait, they could produce diverse result observed. Furthermore, a scientist called Thomas Hunt Morgan and his team later integrated the theoretical model of Mendel with the chromosome theory of inheritance, in which the chromosome of cells were thought to hold the actual hereditary information created a scientific field, now known as classical genetics, which was extremely successful and cemented Mendel’s place in history.
To ensure the applicability and robustness of law of dominance many scientists undertook cross-breeding experiments. The experiments were conducted by Correns on peas and maize, by Bateson on a variety of organisms. The general observations of the scientists were that a large number of characters in organisms are related as dominant and recessive. It is only because of the law of dominance that the harmful recessive character gets suppressed and is not expressed by the normal dominant character in the hybrid. In human beings a form of idiocy, diabetes, hemophilia etc. are recessive characters. A person appears healthy if all these characters are suppressed in the hybrid. For generations, these recessive hybrids are present but are not expressed and are passed on silently to various generations.