Gregor Mendel examined resulted of crossings of peas, arriving the conclusion that the factors that today are called
genes, are considered as messenger chemistries that lead information of the parents the descent. He was possible to prove that the genes are not abstraction, but that it has material localization in the cellular corpúsculos, if presenting in filamentosas structures of the nucleus, called
chromosomes. The gene that controls the
color of the eyes for example occupies definitive point of a chromosome. The cells of the body possess a double collection of chromosomes, therefore it has two genes for the color of the eyes. If the two are equal or produce the same color, the individual is called homozigoto, or pure with respect to color of the eyes. If each gene will be of a type that is, one for blue and the other for the chestnut, the individual is heterozigoto, or impure for the color of the eyes. The pairs of chromosomes if separate and each one goes for its gameta. The color of the hair is controlled in the man for a gene for the black hair and another one for the blond hair, is observed that the first one masks the effect of as, and the result (fenótipo) is the black hair. We say then that the gene for the black hair is dominant and of the blond hair he is recessivo. The hair alone will be blond if the 2 genes will be for the blond color. If the parents will be impure for the color of the hair will approximately have ¼ of the children with the blond hair. If one of the parents is homozigoto for the black hair, all will have the hair of this color, therefore the blond gene would be dominated. The color of the blue hen appears as consequence of the crossing of the blacks with the whites the controlling genes of the coloration act together, in order to produce the coloration bluish. When blue birds are crossed between itself the results are: ¼ black, ¼ white and ½ blue. Calls of incomplete dominância. Many animals and the majority of vegetables are hermaphrodites, that is, they possess agencies masculine and feminine. Although we had affirmed that the man possesss 23 pairs of chromosomes, we now want to say that the male possesss only 22 pairs, increased of 2 others that do not form pair, called sexual chromosomes. The woman has 2 chromosomes X, completing the 23. The half of the masculine spermatozoa possesss a chromosome X and to another half Y. If half X to fecundar óvulo, the result will be a female; in case that contrary, it will be Y, the resultant individual will be a male. As both are gifts in equal n.º, the numerical equality in the probabilities of births between boys and girls occurs.