①在野生型和突变型这对相对性状中，显性性状是____，隐性性状是____．

②根据上述杂交实验的结果能否确定突变基因在X染色体上还是在常染色体上？请简要说明推断过程．____．

③若要通过一次杂交实验鉴别突变基因在X染色体上还是在常染色体上，最好选择F₁中雌性为____型，雄性为____型作为杂交的亲本．请简要说明推断方法．____．

草履虫在水中能旋转前进和自由运动是借助于(　　)

    Big Ben is not the name of a man. It's the name of a big clock in London. This clock has four faces. So, no matter(无论)where you stand, you can read the time on the face of Big Ben. Each face is the size of a bus. The hands are about four metres long. If you go to London, you may want to visit the Houses of Parliament(英国国会大厦). In that place you will find Big Ben sits at the top of the clock tower in the Houses of Parliament. You will probably hear it and see it. The big clock makes such a loud noise. It goes every quarter of an hour. Big Ben has been the most famous symbol(标志)of  London