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*RISK* *risk*, we mean the probability that a particular event will happen. Probability is conventionally measured with a number ranging from 0 to 1. A probability (*p*) of zero means never, and a probability of 1 means always. For two or more mutually exclusive possible outcomes, the individual probabilities sum to 1.0 (or 100%). Thus, someone who is a heterozygote for a particular rearrangement might, in any given pregnancy, have a probability of 0.10 (10%) of having an abnormal child and a probability of 0.90 (90%) of having a normal child. We may speak in terms of risks of *re*currence or of *oc*currence: the probability that an event will happen again, or that it will happen for the first time. Risk can also be presented as *odds*: the ratio of two mutually exclusive probabilities. The odds for the preceding hypothetical heterozygote would be 9:1 in favor of a normal child.

The work *risk* has two important meanings in the English language. First, there is the scientific sense of probability that we already discussed. Second, as most people use the word, it conveys a sense of exposure to danger. Our hypothetical heterozygote runs the risk that an unfortunate outcome may occur (an abnormal child, or an abnormal result at prenatal diagnosis). In the genetic counseling clinic these meanings of risk coalesce in some ways, to which the counselor needs to be sensitive. We might instead use such everyday words as *chance* or *likelihood*, which have no negative connotation, to refer to the fortunate outcome of normality. The words *fortunate* and *unfortunate* are also chosen deliberately: the wanted or the unwanted event will occur entirely by chance, analogous to tossing a coin, throwing a dice, or being dealt a card.

*Chapter.*
*3696 words.*
*Illustrated.*

*Subjects: *
Clinical Genetics

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