All human genes are divided into dominant and recessive, and dominant (strong) genes always and in any combination will suppress the recessive. Weak recessive genes can manifest in the human genotype only when both father and mother, the child will get two genes. Otherwise, when connecting the dominant and recessive gene, the trait will not occur, but the person will be a carrier of such a "weak" gene. In addition, gender is determined by the combination of sex chromosomes, and female is a combination of two XX chromosomes, whereas male XY genotype obtained if the mother will be obtained in the X chromosome, but the father's y-chromosome. Sex chromosomes in addition to information about the gender of the unborn child, also contain data about some human diseases. Recessive genes, sex-linked X chromosome will appear only if the pathological gene is contained in the maternal and paternal X-chromosomes girls, or when the boy receives y chromosome from the father and the diseased X chromosome from the mother. Its properties simply will not be blocked the second chromosome (y chromosome these genes just yet), so all sons of hidden media will be cheering this disease, a recessive sex-linked X chromosome.This disease is hemophilia, the genes of which are located in the X chromosome. Therefore, in 90% of cases the disease affects boys in the blood which no specific proteins that determine the processes of blood coagulation. The disease is manifested by profuse and prolonged bleeding, and hemorrhage, which occur at the slightest injury, and in some cases even without any external influences. Bleeding may continue for hours and even days, with massive blood loss leads to impaired activity of all organs and body systems of the patient, and is constantly increasing in the volume of the hematoma leads to mechanical compression of the tissues.The only way to treat hemophilia becomes a constant introduction into the organism of special preparations obtained from the blood of healthy people, antihemophilic plasma and cryoprecipitate. Such therapy patients should receive throughout their life, otherwise, once bleeding can be fatal.