2. Theories of genetic causation in human disease.

The central problem of the mid-20th century genetic counselor has been to 
determine the place of the genes in the origin of complex 
human characteristics - such as neural tube defects, cleft palate, diabetes, 
and congenital heart disease - in short, the major malformation and 
polygenic syndromes which make up the bulk of what we call 
birth defects. Of comparable importance in terms of genetic counseling is 
understanding the origin and incidence of the newly recognized genetic heterogeneity 
lying behind what had previously been regarded as simple, single-gene determined 
diseases. Topics include dealing with genetic heterogeneity, phenotypes and genotypes, where 
genetic causation defies simple analysis, and sources of variation. Behavior disorders 
with presumptive single-gene causation (Lesch-Nyhan disease), presumptive sex-linked causation (aggressivity), and 
presumptive polygenic causation (alcoholism) are also discussed. 