Who could think that experiments of the ordinary monk Gregor Mendel will lay the foundation for such difficult science as genetics? He opened three fundamental laws which serve as the base of classical genetics. These principles were explained in terms of molecular interaction subsequently.
The first law of Mendel
Mendel made all the experiments with two grades of peas possessing yellow and green seeds respectively. When crossing these two grades all their posterity appeared with yellow seeds, and this result did not depend on to what grade maternal and fatherly plants belonged. Experience showed that both parents are equally capable to transfer the hereditary signs to children.
It was confirmed also in other experience. Mendel crossed the peas with wrinkled seeds with other grade possessing smooth seeds. As a result the posterity appeared with smooth seeds. In each similar experiment one sign is prevailing over another. It was called prepotent. It is shown at posterity in the first generation. The sign which is extinguished by prepotent was called recessive. In modern literature other names are used: "prepotent alleles" and "recessive alleles". Inclinations of signs are called genes. Mendel suggested to designate them by letters of the Latin alphabet.
The second law of Mendel or law of splitting
In the second generation of posterity interesting regularities of distribution of hereditary signs were observed. For experiments seeds from the first generation (heterozygotic individuals) undertook. In case of seeds of peas it turned out that 75% from all plants appeared with yellow or smooth seeds and 25% with green and wrinkled respectively. Mendel conducted many experiments and made sure that this ratio is precisely carried out. Recessive alleles are shown only in the second generation of posterity. Splitting happens in the ratio 3 to 1.
The third law of Mendel or law of independent inheritance of signs
Mendel opened the third law, investigating two signs inherent in peas seeds (their rugosity and color) in the second generation. Crossing homozygous plants to yellow smooth and green wrinkled, he found the surprising phenomenon. In posterity of such parents there were individuals having signs which were never observed at last generations. It were plants with yellow wrinkled seeds and green smooth. It turned out that at homozygous crossing the independent combination and heredity of signs is observed. The combination occurs in a random way. Gena defining these signs have to settle down in different chromosomes.