单项选择题 Passage Two：Questions 26 to 30 are based on the following passage.Normally a student must attend a certain number of courses in order to graduate, and each course which he attends gives him a credit which he may count towards a degree. In many American universities the total work for a degree consists of thirty-six courses each lasting for one semester. A typical course consists of three classes per week for fifteen weeks; while attending a university a student will probably attend four or five courses during each semester. Normally a student would expect to take four years attending two semesters each year. It is possible to spread the period of work for the degree over a longer period. It is also possible for a student to move between one university and another during his degree course, though this is not in fact done as a regular practice.For every course that he follows a student is given a grade, which is recorded, and the record is available for the student to show to prospective employers. All this imposes a constant pressure and strain of work, but in spite of this some students still find time for great activity in student affairs. Elections to positions in student organizations arouse much enthusiasm. The effective word of maintaining discipline is usually performed by students who advise the academic authorities. Any student who is thought to have broken the rules, for example, by cheating has to appear before a student court. With the enormous numbers of students, the operation of the system does involve a certain amount of activity. A student who has held one of these positions of authority is much respected and it will be of benefit to him later in his career.第26题：Normally a student would at least attend ________ classes each week.A) 36B) 20C) 12D) 15

未知题型 下图是患某罕见遗传病的家系图： (1)该病的遗传基础最可能是什么? (2)家系中有没有携带突变基因而不表现出病态的个体?如果有，请列出。

问答题 果蝇中P因子的转座酶由4个外显子（0，1，2，3）编码。此基因在生殖细胞中的转录产物长度为2．1kb，包含所有这4个外显子，并编码具有转座酶活性的蛋白质。体细胞中P因子的转录产物长度为2．5kb，除了含有全部的4个外显子之外，位于外显子2和3之间的内含子也被保留下来。（1）体细胞中产生的较长的mRNA编码的蛋白质的大小只相当于在生殖细胞中产生的较短的转录产物编码的蛋白质的75％。较长的转录产物为什么会编码较小的蛋白质产物？（2）反转录酶能够用来从mRNA合成得到相应的双链DNA，这个DNA拷贝称为cDNA。如何利用cDNA和转基因技术得到在体细胞中也有转座酶活性的果蝇？（3）假设用限制酶SaZ工消化P因子后得到8个片段。用来自果蝇生殖细胞的转座酶的cDNA作为探针对它们进行Southern杂交，结果发现所有的8个限制酶切片段都显示阳性条带。上述结果说明了什么？