若检测发现Ⅱ₄携带致病基因，致病基因是由正常基因序列中一个碱基对的替换而形成的．图2显示的是B和b基因区域中某限制酶的酶切位点．分别提取家系中Ⅱ₃，Ⅱ₄和Ⅲ₉的DNA，经过酶切、电泳等步骤，再用特异性单链核酸片段做分子杂交，结果见图3．另已知一个处于平衡状态的群体中b基因的频率为q．

    Most Chinese eat three meals a day. In the past,they had breakfast in the morning, a big lunch at noon, and a light supper at night.

    But now both men and women in the big cities have to work on the weekdays. So they often eat bread, eggs, milk and porridge forbreakfast at home. For lunch, Chinese take-away food is very popular. They often have a quick lunch in their workplaces.

    After work in the afternoon, they have more time for supper. They usually eat a big meal at home. Wives often cook meat, fish, chicken, vegetables and rice for their family.You can also find dumplings and noodles on the table in most families of northern(北方的)China.