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Genetic Research of Identical Twins
Studying identical twins has provided greater insight into whether disorders are caused by “nature or nurture." According to the Child Development Reference, monozygotic, or identical twins, are of particular interest because both individuals have the same genes: one egg is fertilized by one sperm, but the embryo it is split into two identical halves. If a disorder is fully genetic, then both twins will have it; if the environment is a factor, then the second twin will have a decreased chance of developing it.
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Genetics of Asthma
One area of research where identical twin studies have been used is asthma. The National Institutes of Health (NIH) defines asthma as “an inflammatory disorder of the airways, which causes attacks of wheezing, shortness of breath, chest tightness, and coughing." Brooklyn College references a 1995 study by Sarafino and Goldfedder, where 325 pairs of twins were studied; 84 pairs had at least one asthmatic individual. When the pairs were divided up, 39 were identical twins, and the other 55 were non-identical. Sarafino and Goldfedder found that in 59 percent of the identical twin pairs, both twins had asthma. This lead to the conclusion that while asthma has genetic links, it also has an environmental factor. Since the non-identical twins had a lower chance of both twins developing asthma (24 percent), genetics still plays a large factor in asthma.
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Genetics of Schizophrenia
Schizophrenia research has also used identical twins studies to determine the presence of a genetic link. The NIH states that schizophrenia is a psychological disorder where the patient has problems with telling the difference between reality and fiction. Schizophrenia.com notes that when the prevalence of schizophrenia is traced through familial relations, a second identical twin has a 48 percent of having schizophrenia as well; this is compared to a 17 percent risk with a fraternal twin.
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Genetics of Drug Abuse
The National Institute on Drug Abuse (NIDA) states that genetics could also be a factor in a person's penchant for drug use. One mentioned study focused on marijuana usage and the person's reaction: 352 pairs of identical twins and 255 pairs of fraternal twins were interviewed, all who have smoked marijuana at least five times. The researchers found that identical twins responded to the questions about their reaction to marijuana more similarly than fraternal twins, leading them to believe there are genetic factors involved. Further testing can pinpoint genes, or malfunctioning genes, that predispose someone to become addicted to drugs, or be drawn to narcotics. The NIDA notes that the genes in question could be dopamine-producing cells, as dopamine is involved in pleasure.
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Child Development Reference: Twin Studies (http://social.jrank.org/pages/666/Twin-Studies.html)
MedlinePlus Medical Encyclopedia: Asthma (http://www.nlm.nih.gov/medlineplus/ency/article/000141.htm)
Brooklyn College: Asthma and Genetics (http://acc6.its.brooklyn.cuny.edu/~scintech/asthma/Genetics2.htm)
MedlinePlus Medical Encyclopedia: Schizophrenia (http://www.nlm.nih.gov/medlineplus/ency/article/000928.htm)
Schizophrenia.com: Heredity and the Genetics of Schizophrenia (http://www.schizophrenia.com/research/hereditygen.htm)
National Institute on Drug Abuse: Promising Advances Toward Understanding the Genetic Roots of Addiction (http://www.drugabuse.gov/NIDA_Notes/NNVol12N4/Promising.html)