Introduction
Disease and disorder both basically mean the disruption of structure or function in a mechanism. This straightforward definition concludes that genetic disorders are caused by a dysfunction of DNA. Some diseases show up by a single gene, also called Mendelian inheritance. These mutations could be X-linked, Y-linked, or autosomal. For instance, a single gene called dystrophin (Xq21), located on the X chromosome and inherited recessively, causes Duchenne muscular dystrophy. Environmental factors can affect these mutations, but they are highly genetic. However, not all disorders occur because of one single gene. The world and the human body are more complex, which leads to disorders that stem from multiple genes and environment collaboration. I want to give an example of this: attention deficit hyperactive disorder – ADHD.
Genetic Background of ADHD
The genetic background of ADHD is still a mystery, but the evidence shows that this disorder is highly affiliated with genetics. A research with questionnaire measures showed that ADHD is highly heritable; the heritability gauge is 74%. The research further concluded that MZ (monozygotic) twins showed a higher correlation than DZ (dizygotic) twins, indicating the genetic factor. The complexity of this disease is due to the multiple gene inheritance.
Keeping in mind that humans with ADHD indicate changes in the brain where dopaminergic systems are more active, let’s take a look at a meta-analysis. Some genes showed significant association with ADHD across several studies: the serotonin transporter gene (5HTT), the dopamine transporter gene (DAT1), the serotonin 1B receptor gene (HTR1B), D4 and D5 dopamine receptor genes (DRD4/DRD5), and gene SNAP25 that regulates synaptic transmission. Furthermore, a study indicated a significant relationship between adult ADHD and the BAIAP2 gene, which helps the growth and operation of neurons within the central nervous system.
These examples are only sand particles in a desert, but all studies share one common key point: the association odds ratios between these genes and ADHD are generally 1.5-3. This value is weak but also indicates that ADHD has a genetic background. We can further interpret from this research value that genes don’t directly cause ADHD but play an essential role in increasing the risks. All these values and findings may not be enough to present details but prove the big picture: the tendency for ADHD is related to genetics and inherited by several genes.
Interplay of Environment with Genes in ADHD
On the other hand, ADHD is an excellent example of gene-environment interaction (GXE). This phrase indicates that the risk of genetic factors depends on environmental conditions. A child with ADHD parents may not have the disorder but most likely to have a high risk of getting it; people can develop this disease later. This frankly proves the influence of external factors. Genes and environment are collaborating; a human with a higher genetic tendency for ADHD would develop the disease easier from environmental factors than a human with a lower tendency.
For instance, recent research found that when a mother drank alcohol during pregnancy, the DAT1 haplotype and ADHD association of the child was much more robust. We can observe that an environmental factor, ethanol, interplayed with genetics at an embryonic level. In an analysis, researchers found that people carrying the COMT gene variant, which increases risk, were more likely to experience the effects of lower birth weight—a factor highly related to ADHD—with an odds ratio of 2.64. Aside from these, there is a whole psychosocial perspective to look from for a better understanding of GxE.
Summary
To sum up, there are multiple reasons for some disorders. Both genetics and environment have been cooperating since time immemorial, and complex disorders are most likely to be a consequence of this. The genetic and environmental backgrounds of ADHD have a lot to discover, but the big picture is obvious: genes and environment are in a vicious circle. While genes determine the tendency to have ADHD, environmental factors reinforce the association between genes and the disorder, which is an underrated psychiatric example of gene-environment interplay. There are numerous examples of GxE, such as diabetes, obesity, etc. However, we should remember that psychology and biology connect with each other profoundly and affect each other continuously by the interaction of genes and the environment.
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