问题 填空题 案例分析题

下面左图表示人类镰刀型细胞贫血症的病因,右图是一个家族中该病的遗传系谱图(控制基因为B与b),请据图回答(已知谷氨酸的密码子是GAA、GAG)。

若图中正常基因片段中CTT突变为CTC,由此控制的生物性状不变,原因是密码子具有()现象。

答案

参考答案:简并

解析:

(1)由图可知图中①是发生基因突变,基因突变发生在细胞分裂的间期。由正常的可知应是以a链为模板进行转录的,根据碱基互补配对原则,b链应是GUA。

(2)由图可知,I3和I4没有病,而其女儿有病说明镰刀型细胞贫血症的致病基因是位于常染色体上隐性遗传病。

(3)II8没有该病,但其有个姐妹是患者,双亲应是携带者,故II8基因型应是BB或Bb。II6和II7没有病,生出孩子有病,说明都是携带者,再生一个患病男孩的概率是1/4*1/2=1/8。II9是患者要保证后代不患病,应与BB的人结婚。

(4)基因上碱基对发生替换后,翻译出的蛋白质不变是因为密码子具有简并性,氨基酸可以有一种或多种密码子编码。

考点:本题考查基因突变相关知识,意在考察考生对知识点的理解掌握程度。

阅读理解

第三部分:阅读理解(共20,满分40分)

第一节(共17小题;每小题2分,满分34分)

阅读下列短文,从每题所给的四个选项(A、B、C和D)中,选出最佳选项,并将答案写在答题卡上。

Mr. Smith worked in an office. He had neither a wife nor children. And he lived in an old house alone. He liked nothing but drinking. He almost spent all of his money on drinks. Sometimes he was hungry; he had to borrow some money from his work-mates to buy a little food.       

One evening he met a friend of his in the street. The man asked him to have dinner in a restaurant. He was happy and drank a lot. When they left there at midnight, he could hardly stand. The man had to stop a taxi and asked the driver to take him home. Soon they arrived at the door of his house. With the help of the driver, he got out. “Thank you, sir.” said Mr. Smith. “Now I can open the door myself.” The taxi drove away, but he couldn’t put the key into the keyhole. He was trying to do it while a policeman came. “Can I help put the key into the keyhole, sir?” asked the policeman.

“Thank you, sir.” said Mr. Smith. “The house is circling now. If you can stop it moving, I can open the door myself.”

56. He spent all his money          

A. on his clothes                      B. on drinks

C. on food                           D. on his house

57. One evening Mr. Smith’s friend asked him          

A. to visit his old friend                    B. to drive a taxi for him

C. to have a meal with him                 D. to watch a football match

58. One night Mr. Smith could hardly stand because he          

A. drank too much                       B. was too happy

C. felt too tired                          D. was too hungry

59. Which of the following is TRUE?          

A. Mr. Smith opened the door himself.

B. Mr. Smith was very angry with the policeman.

C. Mr. Smith wouldn’t like the policeman to help him.

D. Mr. Smith couldn’t open the door without the help of the policeman.

单项选择题 A1/A2型题