HEREDITARY DEFECTS DUE TO MULTIPLE GENES
Several well-known hereditary defects are missing from Table 16.1 because they involve more than one gene. These may be subdivided into two types. Some multigene defects are due to the interacting effects of several individual genes. Some examples are cleft palate, spina bifida, certain cancers, and diabetes.
Other multigene defects are due to the presence of an extra copy of an entire chromosome. Although most errors involving whole chromosomes are lethal, a few are viable. The best known of these is Down syndrome, which causes mental retardation and is due to an extra copy of chromosome 21. The overall frequency of Down syndrome is about one in 800, but like most instances of getting a whole extra chromosome, it is due to an accident during cell division in the newly fertilized egg and it is not normally inherited. The relative chances of such a mishap increase with maternal age, but even so, most Down syndrome children are born to young women, simply because more younger women have babies. Extra sex chromosomes are found in about one of every 1000 people; there are three relatively common possibilities: XXY, XYY, and XXX. The XYY individuals are best known for their supposed tendencies toward violent crime, but the others show various abnormalities also.