Each of us carries about half a dozen defective genes. We remain unaware of this fact unless we, or one of our close relatives, are amongst the many millions who suffer from a genetic disease. About one in ten people has, or will develop at some later stage, an inherited genetic disorder, and approximately 2,800 specific conditions are known to be caused by defects in just one of the patient's genes. Some single gene disorders are quite common-cystic fibrosis (纤维症) is found in one out of every 2,500 babies born in the Western world- and in total, diseases that can be traced to single gene defects account for about five percent of all admissions to children's hospitals. Most of us do not suffer any harmful effects from our defective genes because we carry two copies of nearly all genes, one derived from our mother and the other from our father. The only exceptions to this rule are the genes found on the male sex chromosomes(染色体). Males have one X and one Y chromosome, the former from the mother and the latter from the father, so each cell has only one copy of the genes on these chromosomes. in the majority of cases, the normal gene is sufficient to avoid all the symptoms of disease. If the potentially harmful gene is recessive(隐性的), then the normal gene will carry out all the tasks assigned to both. Only if we inherit from our parents two copies of the same recessive gene will a disease develop. On the other hand, if the gene is dominant, it alone can produce the disease, even if the other one is normal. Clearly only the children of a parent with the disease can be affected, and then on average only half the children will be affected. Huntington's chorea(舞蹈症) a severe disease of the nervous system, which becomes apparent only in adulthood, is an example of a dominant genetic disease.1. What does the author state about the genetic disease?A. It's very likely that the defects in just one gene can result a many as 2800genetic diseases.B. Most genetic diseases are developed in childhood.C. Everyone Knows clearly that he carries some defective genes which can cause genetic diseases.D. The defective genes can cause genetic diseases inevitably.2. What do you know about the male sex ceil?A. Each has both one X chromosome and one Y chromosomeB. Each has only one X chromosomeC. Each has either one X x chromosome or one Y chromosomeD. Each has only one Y chromosome.3. In most cases, on what occasion can a person avoid suffering from genetic?A. only if he inherits from his parents two copies of the same recessive gone.B. If he has at least one normal gene.C. Only when he has two normal genes.D. When he has two potentially harmful recessive genes.