II. SOURCE 2 : JOURNAL
Journal of Immunotoxicology, 2011; 8(1): 68–79
Theoretical aspects of autism: Causes— A review
Helen V. Ratajczak
There is indisputable evidence for a genetic component in autism (Rodier, 2000). With identical (monozygotic) twins, if one is autistic, the likelihood that the other twin will have some form of autism is 90%. In great contrast, for fraternal (dizygotic) twins, the likelihood that the other twin will have a form of autism is only 2–3% (De Francesco, 2001). The results fit best with models in which variants of several genes contribute to the outcome. Relatives of people with autism may have some of its symptoms but fail to meet all the criteria for the disorder. Muhle et al. (2004) confirm De Francesco’s findings but with different percentages: twin studies reported 60% concordance for classic autism in monozygotic twins vs. 0% in dizygotic twins, the higher monozygotic concordance attesting to genetic inheritance as the predominant causative agent. Reevaluation for a broader autistic phenotype that included communication and social disorders increased concordance remarkably from 60–92% in monozygotic twins and 0–10% in dizygotic pair. This suggests that interactions between multiple genes cause “idiopathic” autism, but epigenetic factors and exposure to environmental modifiers may contribute to variable expression of autism-related traits. Data from whole genome screens in multiplex families suggest interactions of at least 10 genes in causation of autism
Age of parents
There are well-documented effects of aging on human genetic traits, especially those that have their effects in early embryonic life (Strickberger, 1968). There are known changes in the rates of chromosome abnormalities at different maternal ages. For example, rates for Down, Edwards, and Klinefelter syndromes associated with XXY genotype increase exponentially from about age 30 to 49, with less dramatic change below age of 30 (Hook, 1981)....