Mendel's Law

In 1865, the Austrian monk Gregor Joham Mendel, abbot of the monastery of Brünn (Czech Republic), formulated the hereditary laws that take their name, fruit of his studies after a discovery which take place in his garden with certain vegetal species.
Mendel worked on the transmission of the characters of the plants through successive generations, in which today is the foundation of the modern genetics. The interest to know those principles starts from its experimentation with seven different characteristics from pure varieties of peas. Mendel observed that hybrids were obtained if a variety of short stem and another one of long stem were crossed; these descendants conserved the similarity with the ascending ones of long stem.
The studies of Mendel were based on four aspects: a) to study the transmission of isolated characters; b) to count the number of descendants of each type; c) to cross pure stocks or races; and d) to choose a plant in which the origin of the gametes could be controlled.
In the first place it crossed two pure individuals that differed in one from the characters. The descendants of the first crossover were hybrid. Next he crossed these hybrids to each other. The first generation was called the paternal P, or F0; the second, first filial generation or F1; the third, second filial generation or F2.
His main experiments were summarized in laws, those of the dominancy and the segregation of characters. In 1865 he presented the results to the Society of Natural Sciences of Brünn, which were published the following year. His studies were not valued until 1900, when Hugo de Vries, Karl Frich Correns and Erich Tschermack, rediscover them.
The three laws of Mendel enunciate as follows:

(P - pure races fathers, F1 - equal hybrids, F2 - Pure races)

First law, or Law of Uniformity of the first filial generation.
If two homozygous individuals (p) for a single pair of alleles cross themselves, but with different expression, all the descendants from the first generation, which will be denominated hybrids F1, are identical. Expressed with other words: when the crossover between individuals pertaining to pure races of the same species is made, all the hybrids of the first filial generation are equal.
These hybrids shoe entirely the character of one of the ancestors (dominant character), whereas the character of the other ancestor is not, as if it was hidden or missing (recessive character), or the hybrids both show an intermediate character between parents (co-dominancy).
Mendel called "factors" to the responsibles of the biological inheritance. Today these "factors" are called genes, which are located in specific places of the chromosomes called locus.
The homologous chromosomes have the same genes, of such form, that they correspond point by point exactly; therefore, each cell does not have one, but two genes to rule a certain character.
Therefore, any hereditary character will be determined by two genes, one coming from the father and another one of the mother. These genes that rule a character are called alleles. If these alleles are equal, the individual is called homozygous or pure, if they are different, heterozygous or hybrid.
The set of the genes of an individual is called genotype, and the set of characteristics of this individual is called phenotype.

Second law, or Law of the segregation (or disjunction) of the antagonistic genes.
When crossing each other the hybrids of generation F1, they are obtained in the F2 different types of descendants, part of which are like the individuals of P. The genes that have constituted even in the individuals of the F1, separate when forming the reproductive cells of these. In the F2, the 3/4 parts of the obtained individuals presented smooth seeds, and the 1/4 rest, rough.

Third law, or Law of the Recombination of the genes (independent transmission of the genes)
Mendel also carried out crossings with plants that differed in two characteristics (dihybridism): for example, peas of smooth and yellow seed with others of green and rough seed. This is how the third law was obtained, which states:
If races that differ in one or but alleles are crossed, the alleles are bound or independent and follow the two first laws of Mendel. That is to say, each one of the hereditary characters is transmitted to the lineage with total independence of the rest.
The proportion obtained by Mendel was of 9 plants of yellow and smooth seed; 3 plants of yellow and rough seed; 3 plants of green and smooth seed; and 1 plant of green and rough seed. Therefore, 9:3:3:1. The possible combinations between the masculine and feminine gametes are describe by the called boards of Punnett.