Laws of Inheritance Formulated by Mendel

The basic structure of a species is transmitted from generation to generation due to a set of chemical instructions encoded in the DNA that individuals receive through their sex cells parents; thus, children resemble their parents and this is known as inheritance. However, between the individuals of a species there are sometimes barely perceptible differences or manifestations markedly different from each other, these differences is what is known as variation, that is, individuals are not identical.
The physical characteristics of individuals, that is, what can be seen or measured, are called phenotype, and it is the physical expression or the way in which a hereditary character is manifested. Eg: skin color, eyes, height, physiology in general.
For a phenotype to manifest itself, the genetic instructions inherited from its parents must be present in cells, that is, the genetic makeup of an individual or genotype.
Gametes (the result of meiosis) have a haploid number of chromosomes and during fertilization they mate and form the zygote, the which has a diploid number of chromosomes or normal number of the species, so that in the somatic cells of individuals (cells normal non-gametic), the genotype is represented by pairs of homologous chromosomes which share an identical site for the same character.

Each member of a gene pair is called an allele. If the alleles are identical, the individual who possesses them is said to be homozygous in that character, and can be homozygous dominant or homozygous recessive. In contrast, an individual who in a genotype carries opposite or contrasting alleles for a trait is said to be heterozygous or hybrid.
Austrian father Gregorio Mendel tried to decipher the complicated proficiency of heredity by experimenting with a plant.
And to avoid the risk of obtaining doubtful results, he selected a plant that had the following characteristics: Possess constant differential characters; Hybrids, during the flowering period, must be protected from the influence of foreign pollen. Hybrids and their offspring should not present fertility problems in successive generations. For all this he decided on the genus Pisum, since it possesses the qualities that he considered necessary.
Finally, for his experiments, Mendel selected seven characters that, by having contrasting alleles and being easily observable, helped him understand the process of inheritance.
To start his experiments, he allowed self-fertilization for several generations, which favored the constancy of the characters through the formation of pure lines (homozygous).
He also proceeded to carry out reciprocal crosses between contrasting plants, taking care that the plant that originally provided the ovules for fertilization provided later the pollen grains, which allowed him to realize that the results were always the same, regardless of whether the plants provided ovules or grains of pollen.
When analyzing the results of the first crosses of him, he realized that the characteristics selected regarding the size of the stems, some plants had long stems and others were short, but there were no in-between, allowing him to hypothesize that heritable traits existed as individual factors and could be manipulated in the progeny. Currently these heritable factors are known as genes.
His observations have been embodied in two laws: the law of segregation and the law of independent distribution.