FIGURE 5.1 Autosomal dominant transmission of deafness in a hypothetical four-generation family would show that each person with the trait passes the trait along to about half of their children, both sexes can have the trait, and both sexes can pass the trait along. Individuals who do not have the trait do not pass deafness along to their descendants. Stop and consider this: every time someone deaf in this family has a child, there is a 50% chance that their child will be deaf. Does that mean that a deaf individual will have exactly 50% of their children turn out to be deaf? No. On average, about 50% of those who are at risk (have a deaf parent in this family) will end up deaf. This is like flipping coins. If you flip a coin many times, you will find that about half of the coin flips come out heads, but they will not exactly alternate heads, tails, heads, tails, etc. In fact, in some of the families we work with, we sometimes see a case in which someone with a trait will have as many as five children in a row who all have the trait, but when we look at many families, we find that they are balanced out by some other family who had five at-risk children who did not end up with the trait. In between, most of the families we work with have about 50% of the at-risk children turn up affected if it is a dominant trait. Before Nick was born, his parents might have thought that the next child would probably not be deaf since they already had two deaf children. Was this valid? No. This is a key concept: with each new genetic flip of the coin, the chance is once again 50%, and that chance is not affected by whether some, none, or all of the previous children have the trait.

individuals have the recessive allele and the accompanying recessive trait. Thus a recessive trait may suddenly pop up in a family that has never heard of that trait before, but a dominant trait is usually evident across a series of successive generations of a family.

Consider what a family would be like with deafness in all four generations for which information is available (Figure 5.1). Individuals who inherit the family's hearing loss trait are deaf from birth. If you look at descendants of Jacob, who is deaf, and his wife Adelaide, who is not, you find that about half of their children are deaf and half are not. Like the heterozygous pea plant with both green and yellow alleles, a deaf member of this family has one "deaf" allele and one "hearing" allele and has an equal chance of passing either along to their child. The difference is that individuals with one "deaf" copy and one "hearing" copy of the gene have the affected phenotype, so they do not have to marry someone else who is a carrier to have a deaf child. Thus, even though the trait is dominant rather than recessive, the transmission of information between generations is actually consistent with Mendel's model that half the progeny get one allele and half the progeny get the other allele. There are no carriers in this family because anyone who gets the information manifests the trait. Thus we also see consistency with one of Mendel's other predications: that some traits are dominant over others.

In fact, there are dozens of different genes that can cause hearing loss that is transmitted like this, in a dominant manner. However, there are dozens more that can cause recessive hearing loss. In theory, it is easy to make the kinds of predications we make here, that the mode of inheritance is dominant and that the risk to children of affected individuals is 50%. However, in real-life situations, things are often more complicated. Small families, adoption, divorce, early death of some family members, geographic distances between family branches, and other complications can sometimes limit the information that is available to help sort out the level of risk to a new child.

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