STUDY HINTS

The ability to determine the probability of an event or series of events is fundamental to many applications of genetic principles. Thinking in terms of probability is not easy at first, but with a set of guidelines and some practice, you should find that you soon have no difficulty. In the following section we will discuss some important terms and then summarize the general types of probability problems you might expect to encounter in genetics.

First, let us contrast two important phrases. Independent events are events that have no causal interrelationship. The conception of the first child in a family, for example, cannot biologically influence the fusion of sperm and egg at fertilization for a second child. Each fertilization is an independent event, and each probability for segregation or sex determination must be assessed independently. Mutually exclusive events, on the other hand, are related in that the occurrence of one eliminates the possibility that the other will occur. A normal child cannot be both a boy child and a girl child. Sex determination yields either of two mutually exclusive events.

Both of these ideas play a role in solving a probability problem in genetics. Depending upon the genotype of the parents, the probabilities of mutually exclusive events, for example, the birth of “normal” progeny as opposed to the birth of “affected” progeny, may be different.