A long time ago, there ________ a king.

某植物的花色受不连锁的两对基因A与a、B与b控制，这两对基因与花色的关系如图所示，此外，a基因对于B基因的表达有抑制作用。现将基因型为AABB的个体与基因型为aabb的个体杂交得到F_l，则F₁的自交后代中花色的表现型及比例是

    A person has been badly injured in a car accident. It is necessary to give him a transfusion (输血) because he has lost much blood in the accident. However, special care must be taken in selecting new blood for him. If the blood is too different from his own, the transfusion can kill him.
    There are four basic blood groups: A, B, AB and O. A single test can tell us a person's blood group. Everybody is born with one of these four blood groups. People's blood group, like hair colour, is inherited (继承) from parents.
    Because the substances contained in each group are different, transfusion must be given carefully. Basically, A and B cannot be mixed. A and B cannot receive AB, but AB may receive A or B. O can be given to any other group, so it is often called the universal donor. On the contrary, AB is sometimes called the universal recipient (接受者). However, before being given the transfusion, patients usually receive only salt or plasma (血浆) until their blood can be matched as exactly as possible. In this way, it is possible to avoid any bad results of the transfusion.
    There is a relationship between our blood groups and our nationalities. Among the Europeans, about 42 percent are blood group A while 45 percent are blood group O. The rarest is blood group AB. However, some American Indian groups are nearly 100 percent blood group O.