Just like snowflakes, no two people are alike, even if they're identical twins according to new genetic research from The University of Western Ontario. Molecular geneticist Shiva Singh has been working with psychiatrist Dr. Richard O'Reilly to determine the genetic sequencing of schizophrenia using identical or monozygotic twins.
The study is published in this month's PLoS ONE. Singh looked at about one million markers of identical twins (and their two parents) where only one twin had schizophrenia. "The most informative feature of schizophrenia is that it sometimes runs in the family. So, for example, the risk of developing schizophrenia is much higher if your brother, sister, mother or father have the disease," says Singh, noting in the general population about one percent have schizophrenia. "We started with the belief that monozygotic twins are genetically identical, so if one member of identical twins has schizophrenia, then the risk for the other twin should be 100 percent, if it's all due to genes. However, studies over the years have shown that the risk of the disease in both twins is only 50 percent." That means either the twins are genetically not identical or the familial disease involves non-genetic (random) effects. Singh and his team have now demonstrated that the monozygotic twins are not genetically identical. "So if schizophrenia is in the genes, then the difference in the genetic makeup of monozygotic twins, with only one disease twin, must have something to do with the disease." Singh found about 12 per cent of DNA can vary across individuals, "Cells are dividing as we develop and differentiate. More importantly, these cells may lose or acquire additional DNA. The genome is not static." Dr. O'Reilly hopes this research will lead to better understanding and improved treatments for schizophrenia. "If we had a genetic test for schizophrenia, it could be applied early in the disease when it's hard to make that diagnosis," says Dr. O'Reilly. The research was funded through the Canadian Institutes of Health Research, the Ontario Mental Health Foundation and the Schizophrenia Society of Ontario.
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Story Source: Materials provided by University of Western Ontario. Note: Content may be edited for style and length. Journal Reference:
The following glossary of common terms has been culled from the Common Terms page of the DNA Initiative. Comprehensive glossaries are available at: the FBI and The National Human Genome Research Institute. Allele - The characteristics of a single copy of a specific gene, or of a single copy of a specific location on a chromosome. Autosomal DNA - DNA found in chromosomes which are not sex chromosomes. Chromosome - The biological structure by which hereditary information is physically transmitted from one generation to the next; located in the cell nucleus, it consists of a tightly coiled thread of DNA with associated proteins and RNA; the genes are arranged in linear order along the DNA. Combined DNA Index System (CODIS) - The generic term used to describe the FBI’s program of support for criminal justice DNA databases as well as the software used to run National DNA Index System (NDIS) databases; CODIS is made up of the National DNA Index System (NDIS), the State DNA Index System (SDIS) and Local DNA Index Systems (LDIS). DNA (Deoxyribonucleic acid) - Often referred to as the “blueprint of life;” genetic material present in the nucleus of cells which is inherited from each biological parent that determines each person’s individual characteristics. An individual’s DNA is unique except in cases of identical twins. DNA Profiling - The result of determining the relative positions of DNA sequences at several locations on the molecule; each person (except identical twins) has a unique DNA profile when used in the context of the CODIS database, which evaluates 13 specific DNA locations. DNA Fingerprinting - Analyses of the lengths of the fragments reveal that when looking at multiple VNTRs (variable number of tandem repeats) within and between individuals, no two people have the same assortment of lengths, except identical twins; this technique became known to the public as “DNA fingerprinting” because of its powerful ability to discriminate between unrelated individuals. Epithelial cells - Cells that cover the inner and outer linings of body cavities. Forensic DNA Analysis - The process of identifying and evaluating biological evidence in criminal matters using DNA technologies. Genotype - The genetic constitution of an organism, as distinguished from its physical appearance (its phenotype); the designation of two alleles at a particular locus is a genotype. Locus - The specific physical location of a gene on a chromosome; the plural form is loci. Low Copy Number Analysis - The analysis of samples containing a small amount of DNA (approximately 30 cells or less); analysis of samples falling into this category often requires enhanced analysis methods to increase the sensitivity of detection. Mitochondrial DNA (mtDNA) - DNA located in the mitochondria found in each cell of a body; sequencing of mitochondrial DNA can link individuals descended from a common female ancestor. National DNA Index System (NDIS) - Authorized by the DNA Identification Act of 1994, the FBI administers this national index. NDIS enables comparison of DNA profiles associated with a crime scene to DNA profiles collected from known convicted offenders, as well as to other crime scene profiles. DNA profiles uploaded to NDIS are searched against the other DNA profiles submitted by other participating states. Nuclear DNA - DNA located in the nucleus of a cell. Partial DNA Profile - DNA evidence that does not yield identifiable results in all 13 core loci. Quality Assurance Standards (QAS) - Quality assurance methods developed by the Scientific Working Group of DNA Analysis and Methods (SWGDAM). QAS provides guidelines to ensure the quality and integrity of data generated by the laboratory and uploaded into the CODIS database(s); published by the FBI. Reference Samples - Material of a verifiable/documented source which, when compared with evidence of an unknown source, shows an association or linkage between an offender, crime scene, and/or victim. Short tandem repeat (STR) - Multiple copies of a short identical DNA sequence arranged in direct succession in particular regions of chromosomes. Y-STR - STR located on the Y chromosome; often examined when investigating sexual assaults involving male suspects. Back to top of page ▲
Asked by: Griffin Windheuser, by email Your DNA is arranged into chromosomes, which are grouped into 23 pairs. When a sperm cell is manufactured, the father’s genome is split in two, so that each sperm receives one chromosome from each of the 23 pairs, and the same thing happens with the egg cells in the mother. When the egg and the sperm combine to form an embryo, the resulting genome contains half the mother’s chromosomes and half the father’s, chosen essentially at random. Theoretically, same-sex siblings could be created with the same selection of chromosomes, but the odds of this happening would be one in 246 or about 70 trillion. In fact, it’s even less likely than that. Before the chromosome pairs fully separate, they often swap individual genes from one chromosome to another in the pair. This means that even if successive sperm were manufactured with exactly the same chromosome selection, they wouldn’t contain the same genes. Of course, a lot of the gene pairs in your genome are actually the same, so it wouldn’t matter which copy you used, but the odds of constructing an exact duplicate genome by chance are still vanishingly small. And even identical twins don’t necessarily have identical DNA. Stray cosmic rays and chemicals in our diet, such as caffeine and nicotine, can cause point mutations in random genes, here and there. Read more: Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts. |
Except for identical twins, no two people on earth have the same:
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