Multifactorialtraits are those traits that are influenced by both the interactionof environment and genes. One important characteristic ofmultifactorial traits is that they are polygenic. What this means isthat the traits are controlled by more than one gene. These genesnormally act additively, in which each individual gene contribute asmall fraction to the outward expression (Cummings, 2009). Individualgenes separately obey Mendelian laws, yet the trait does not.Multifactorial genes are not inherited as either recessive ordominant trait. It is very difficult to measure interactions betweenthe environment and genotype (Cummings, 2009). This is becausegenotype is unique to every individual.
However,familial and twin studies are used to measure multifactorial traitsquantitatively. Researchers often compare monozygotic twins withfraternal twins to find evidence for environmental and geneticinfluences (Lewis, 2015). In family studies, researchers look for thenumber of affected children in families. Children of autistic parentsare at higher risk of autism compared to the general population.Furthermore, twin studies show that autism is an inheritablecondition.
spectrum disorder (ASD) is a group of neurodevelopment abnormalitiescharacterized by social communication impairment, languagedifficulties, and behavioral problems. ASD consist of classicalautism, Rett syndrome, pervasive developmental disorder andAsperger`s syndrome (Lewis, 2015). Furthermore, other chromosomaldisorders like Fragile-X syndrome and Down syndrome are implicated inautism (Lewis, 2015). Numerous genes have been associated with thepathogenesis of autism, some of which are implicated insynaptogenesis. Several environmental factors have also beenimplicated in the autistic pathophysiology. ASD starts early inchildhood under age 3. Therefore, it affects the development of brainrelatively late, more so during the formation and maturation of thesynapse. ASD is a neurodevelopment disorder of brain function. ASDsare mostly influenced by genes rather than the environment.
Thediagnosis of ASD is based on three characteristics: imaginativethought, social relationships, and social communication. A personwith autism must display at least one of these characteristics. Thesefeatures exist in a range of combinations in that two individualsdiagnosed with similar label may have differing deficits and skills.
Thescientific community is not sure about the underlying cause ofautism, but they generally agree it is due to a product ofenvironmental factors on a genetic tendency. They observe that mostcases of autism are of idiopathic (unknown cause), only a few aresecondary. About 15% of autistic individuals are diagnosed with thissecondary autism, meaning the remaining 85% cases fall underidiopathic autism (Lewis, 2015). Secondary autism is believed to becaused by childhood vaccinations like rubella and valproic acid(Lewis, 2015). Researchers have attributed the cause of idiopathicautism to polygenic mutations, abnormal brain regions and abnormalserotonin levels in the brain.
Recentstudies attribute the cause of autism to an interaction of multiplegenes that produce and lead to autistic conditions. Family studiesseem to be consistent with the polygenic mode of inheritance. Thesestudies estimate that 5-20 genes play a role in autism (Cummings,2009). However, twin studies and advanced genetic analysis have foundthat many of the recognized genes control only one out of the threeimpairments caused by autism (Cummings, 2009). Some theoriespostulate that brain chemicals, activities and brain structurecontribute to autistic development. Neuro-imaging studies havereported that autistic brain tends to grow rapidly between 1-3 yearsand then slows down. Such brains fail to process or refineinformation as normal brains do (Robertson). Another school ofthought from the UK believes autism is caused by vaccines like DPTand MMR. These contradictory studies make it difficult to pin downthe exact cause of autism.
Thebrain is responsible for processing and refining information bysending out signals at synapses. The synapses, in turn, link neuronsinto networks of communicating cells. Neuroligins and neurorexins,which are synaptic cell-adhesion molecules, connect pre-synaptic andpost-synaptic neurons at synapses and facilitate trans-synapticsignaling (Sudhof, 2008). Changes in neuroligins or neuroxin arelinked to autism in humans (Sudhof, 2008). Therefore, neuroligins andneuroxins are major ingredients of the molecular mechanismcontrolling synaptic transmission. Deletions in neuroxin-neuroligincomplex lead to higher risks of autism and suggest dysfunctionalsynapse as the originating region of autism (Pizzarelli &Cherubini, 2011).
Genemutations have been linked to familial ASDs recent studies which havediscovered mutations in genes encoding Nlgn4, Nlgn3, and Nlgn1. Thesestudies detected 2 unique translocation events, 7 point mutations and4 varied deletions in Nrxl genes in parents with autism (Marshall,2008). Different mutations have also been detected in Nlgn4 includingthree internal deletions, two frame-shifts and five missensemutations (Yan, et al., 2005).
Thoughautism is a heritable condition, its mechanism of inheritance islargely unknown. No single cause has been identified so far. Thegenetic cause has been implicated in cases of co-occurring withautism such as Down syndrome. Owing to its complex genetics, it isnot clear whether autism is explained by rare gene mutations or bypolygenic interactions.
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PizzarelliR, & Cherubini, E. (2011). Alterations of GABAergic signaling inautism spectrum disorders. NeuralPlasticity, 2011(2011),ID 297153.
Sudhof,T. (2008). Neuroligins and Neurexins Link Synaptic Function toCognitive Disease. Nature,16455(7215): 903–911.
YanJ, et al. (2005). Analysis of the neuroligin 3 and 4 genes in autismand other neuropsychiatric patients. MolecularPsychiatry, 10:329–332.