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Recent studies have suggested that ASD in families where multiple members are affected by autistic traits may be caused more by genetic predisposition. In contrast, ASD in families where only one member displays autistic traits are more likely to be caused by rare gene mutations which have a large effect or by copy number variations. One way of studying this difference is by comparing autistic traits in unaffected family members from multiplex autism families to simplex autism families.5
Linkage and Association Studies
Successful linkage studies in the past have been mostly based on affected sibling-pair designs in multiplex families. However, there were no genome wide significant results probably because of small effect sizes that were a result of any single gene. Even large scale studies showed only minor overlap, likely because of variety of phenotypes in ASD. Recently though, use of endophenotypes and QTL mapping have increased the power of linkage and association studies.
Endophenotypes can help genetic studies by defining more etiologically homogenous subgroups. Furthermore, endophenotypes are measurable in both affected and control groups, thus allowing for larger sample sizes. Language phenotypes such as the age at which the child speaks their first word, are very promising endophenotypes because they show significant linkage in many samples and the support has been lent at implicating the 7q region to this language development, thereby raising hypothesis that the 7q region is home to other loci that are associated with the autism language phenotype3.
Some areas of investigation include