Genetics in disorders of language.

As is typical in science, early work in an area is bound to have weaknesses. Therefore, it is not unexpected that the extent to which conclusions can be drawn from the current research on the genetics of DLD is limited. These limitations stem from the use of heterogeneous samples, the use of overly broad phenotypes, survey rather than objective test data, and only a few studies looking beyond simple familiarity. Future research is in progress that attempts to correct for these weaknesses. Still, some practically useful information can be gleaned from the work done thus far. First, we can now hypothesize something about the pathway from gene to DLD phenotype. Much of DLD, like RD, may be a manifestation of early genetic effects on the structural development of the brain (e.g., Plante, Swisher, and Vance, 1991; Plante, 1991; Molfese and Betz, 1988). However, consequences of these genetic effects for language development can still be modified by environmental events (e.g., treatment). Furthermore, genetic effects do not act in isolation and they are not necessarily static. A number of genetic and non-genetic events in the course of development may positively or negatively modify the disorder (e.g., Tomblin, 1989, Tallal et al., 1991; Molfese and Holcomb, 1989). A related point is that there is evidence for genetic heterogeneity in DLD. That different modes of transmission have been put forth by different authors, suggests that a variety of genetic forms of DLD may exist. This idea is further supported by noting that DLD is a common outcome of a number of clinical syndromes having very different genetic bases (e.g., Williams syndrome, Down syndrome, fragile X, etc.; Siegel-Sadewitz and Shprintzen, 1982; Bellugi et al., 1991). This does not rule out the possibility that a majority of nonsyndromic DLD is due to one or a few major genes, however. Finally, there is also evidence for behavioral heterogeneity in DLD. The data suggest that the genes for DLD are variably expressed. For example, in families selected through a DLD proband, a number of different language-related problems have been noted. Furthermore, when a globally defined DLD individual is examined closely, it is common to find a collection of symptoms, sometimes spanning all the major domains of language (Weiss and Lillywhite, 1981). Overall, the data and concepts presented in this article suggest that the clinician should take careful family histories of clients and, when doing so, attempt to ascertain the specific symptoms that family members other than the client exhibit.(ABSTRACT TRUNCATED AT 400 WORDS)