Defective Genes and Human Health Each of us carries about half a dozen defective(有缺点的) genes. We remain blissfully(快乐地) 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(遗传的)genetic disorder, and approximately 2,800 specific conditions are known to be caused by defects (mutations) in just one of the patient’’s genes. Some single gene disorders are quite common-—cystic(胞状的) fibrosis (纤维化) 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 5% 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 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, one normal gene is sufficient to avoid all the symptoms of disease. If the potentially harmful gene is recessive(后退的), then its normal counterpart(配对的) 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(显性的), it alone can produce the disease, even if its counterpart is normal. Clearly only the children of a parent with the disease can be affected, and then average only half the children will be affected. Huntington’’s chorea (舞蹈病) , a severe disease of the nervous system, which becomes apparent only in hood, is an example of a dominant genetic disease. Finally, there are the X chromosome-linked genetic diseases. As males have only one copy of the genes from this chromosome, there are no others available to fulfill the defective gene’’s function. Examples of such diseases are Duchenne muscular dystrophy(营养不良) and, perhaps most well known of all, hemophilia(血友病). Queen Victoria was a carrier of the defective gene responsible for hemophilia, and through her it was transmitted to the royal families of Russia, Spain, and Prussia. Minor cuts and bruises, which would do little harm to most people, can prove fatal to hemophiliacs, who lack the proteins(Factors VIII and IV) (凝血因子VIII和IV)involved in the clotting(血凝结)of blood, which are coded for by the defective genes. Sadly, before these proteins were made available through genetic engineering, hemophiliacs were treated with proteins isolated from human blood. Some of this blood was contaminated(污损) with the virus, and has resulted in tragic(悲惨的) consequences for many hemophiliacs. Use of genetically engineered proteins in the utic applications, rather than blood products, will avoid these problems in the future. Not all defective genes necessarily produce detrimental(有害的)effects, since the environment in which the gene operates is also of importance. A classic example of a genetic disease having a beneficial effect on survival is illustrated by the relationship between sickle-cell,(镶形血球)anemia (贫血症) and malaria(疟病). Only individuals having two copies of the sickle-cell gene and one normal gene are unaffected and, more importantly, are able to resist infection(传染) by malarial parasites (寄生虫). The clear advantage, in this case, of having one defective gene explains why this gene is common in populations in those areas of the world where malaria is endemic(特有的). Which of the following statement is "TRUE"