Quest for QTLs for intelligence
Identification of complex traits like intelligence using QTLs has shown a very slow progress. The primary reason being a high degree of polymorphism shown in genes affecting intelligence. Monogenic disorders, occurring at low frequency (0.0001), causing mental retardation are usually highly pleiotropic and exert their effect primarily on brain regions. Whereas common disorders such as mild mental retardation and learning disabilities, which influence intelligence, are usually epistatic and their detection using QTLs have met with limited success.
Candidate genes underlying those QTLs have been identified and organized into 3 categories of gene function: synaptic transmission, immune function and development. Synaptic transmission defects have been reported to show some association with intelligence development. Influence of immune system on intelligence has shown to affect older individuals where an impaired immune response causing brain dysfunction is seen as the reason for lowering intelligence. Genes influencing brain development affects brain function which eventually effects intelligence. Despite identifying the candidate genes affecting intelligence, no significant association could be deduced that consistently replicate when tested on different sample population size.
QTLs associated with intelligence are also possibly known to spread out into the non-coding regions such as the promoter regions and introns which regulate gene expression for intelligence. Humans have the highest ratio of non-coding DNA to coding DNA and the QTLs, associated with intelligence, are possibly interspersed in to those non-coding regions making it even more difficult to detect the genetic basis for intelligence. Monogenic disorders occur due to mutation in the coding regions and tend to be very severe.
This study highlights the potential importance of non-coding DNA. In our quest for QTLs for intelligence, a consideration of the whole genome is required rather than limiting the search to exons(protein-coding regions).
References:
Robert Plomin, Joanna K.J. Kennedy and Ian W. Craig “The quest for quantitative trait loci associated with intelligence” Intelligence 34(6) November-December 2006, Pages 513-526
Written by: 41104899
Topic: Quest for QTLs associated with intelligence (Complex traits in mice)
