根据材料请回答 36～40 题
The genes that help determine a person's individual characteristics -- from the color of his eyes to the score he makes on an IQ test-- are located on chromosomes(染色体)within the cells of his body.Half of a person's chromosomes come from his father, hail from his mother.Many diseases are the result of a .single defective(有缺陷的)gene on one of the chromosomes.Achondroplastic dwarfism (软骨营养障碍性矮小), for example, is caused by a dominant gene ,and any child who inherits it will have the disease.A genetic counselor confronted by a parent with such a disease could warn that half of his children risk the disease.More often, genetic diseases are caused by recessive(隐性的)genes.The most common is cystic fibrosis(囊肿性纤维化), a disorder that affects at least .one in every 1,600 babies and causes their lungs and other body organs to become congested(充赛)with mucus (黏液).Some diseases, such as the blood-clotting(血栓) disorder, hemophilia (血友病), are, sex-linked recessive defects carried On the female X chromosome.
Many genetic defects, such as the hemophilia of European royalty, can be traced back through the family tree.And a genetic counselor can calculate probable risks for couples e-ven before they have defective children.But faulty genes may .also occur without warning by mutation(突变)in any generation.
In recent years, researchers have detected a number of disorders caused by an extra chromosome, or lack of part of a chromosome.Mongolism(先天愚型), a form. of retarda-tion(迟钝) accompanied by ,short stature(身材矮小), a flattened nose and broad hands and feet, is caused by an extra chromosome.The parents of such a child have little risk of hav-ing another Mongoloid(先天愚型患者).
There are blood, urine and other tests which show promise in detecting more than 100genetic diseases, including cystic fibrosis, hemophilia and some forms of muscular dystro-phy(先天愚型患者 ).
Researchers are also detecting genetic defects even before a child is born."Intrauter-ine detection (子宫内检测)," notes Dr.Henry L.Nadler of Northwestern University Medical School, "brings a new dimension to genetic counseling.The physician may now inform. the parents that they will hare either an affected or a normal child."
第 36 题 The original title of the passage can most probably be
A．Genes and Diseases
B．Genes and One's Characteristics
C．Genetic Defects and the Family Tree
D．Dominant and Recessive Genes
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