Male-to-male transmission in extended pedigrees with multiple cases of autism.

Despite strong genetic influences in autism, the true mode of inheritance remains unknown. Sex differences in autism have been described in both singleton and multiplex families [Lord et al., 1982; Volkmar et al., 1993; McLennan et al., 1993; Lord, 1992]: Boys outnumber girls by 3 or 4 to 1, and so a sex-linked mode of transmission must also be considered. The key characteristic of X-linkage is that all sons of affected men are unaffected (no male-to-male transmission). In the present study, which is part of an ongoing linkage project in autism, we describe 77 multiplex autism families, 11 of who are affected cousin or half-sibling families. By using these families, it is possible to trace the path of genetic transmission and observe whether the hypothesis of X-linkage is tenable. Of 11 extended pedigrees from 77 multiplex families, six show male-to-male transmission; in these families, X-linkage can be excluded as the genetic basis for their autism. The data from the other five families are compatible with either an autosomal or an X-linked mode of transmission. The key point to emerge, then, is that autism cannot be exclusively an X-linked disorder; there must be an autosomal mode of transmission at least in some families. Thus we must consider the alternative hypotheses that autism is either entirely autosomal, or it is genetically heterogeneous, involving at least one autosomal locus with genderspecific expression, as well as a possible locus on the X-chromosome.