What is DNA (Deoxyribonucleic Acid)?
DNA (Deoxyribonucleic Acid) is the molecule responsible for all inherited genetic traits. There are two types of DNA found in each cell (with the exception of red blood cells), nuclear DNA and mitochondrial DNA. Nuclear DNA is found in the nucleus portion of the cell and mitochondrial DNA (mtDNA) is found in the cellular organelle called the mitochondria and is inherited only from the mother. Each individual inherits one half of their DNA from their father and the other half from their mother. Each person's DNA is unique, except for identical twins who have the same DNA. When a sufficient nuclear DNA profile from the victim's remains matches the nuclear DNA profile from a family member's reference sample, we can be very sure of the identity of the victim. The majority of forensic DNA tests are performed on nuclear DNA, however, simultaneous analysis of mtDNA from a maternal relative may be necessary in order to improve the identification process. Cases with limited fragments of materials such as hair samples or deteriorated samples, the more specialized mitochondrial DNA testing may be the only option.
Short Tandem Repeats
DNA in the human genome can have a variety of functions: Some codes for specific genes that become enzymes and other proteins, other sections are inactive and do not code for any function, often referred to as non-coding regions. These non-coding regions are of particular interest and importance to DNA Identity testing. Sections of DNA that do not code for critical enzymes and other functions have the freedom to mutate and change without causing any physical or physiological consequences. As a result, non-coding regions provide us with dependable markers that can be used to differentiate between individuals, maternal lineages and paternal lineages. Short Tandem Repeats or STRs consist of small blocks of DNA two to six nucleotides in length repeated back-to-back. The STRs are distributed throughout the human genome. The DNA testing community, particularly in the areas of parentage testing and forensic testing, has evaluated a series of these markers containing blocks of 4 or 5 nucleotides that can be reliably used for human identification. Similar STR systems have been developed for dogs and horses and are used by the American Kennel Club registry, the Jockey Club and other livestock groups.
Human Identity testing regularly employs a suite of 13 well-characterized STR markers. Population statistics for these genetic loci have been determined and are used in the calculation of genetic profile frequencies and parentage statistics. STR loci are characterized by the number of repeated blocks are present in a sample. As each individual has two copies of each of their chromosomes, each STR locus will give two types, such as 13, 14 for the D8S1179 locus shown below. One type or allele was donated to the individual by their mother and the other allele came from their father. If an individual receives two copies of the same allele, they would show only one allele at that locus. In addition to the 13 standard loci that are in routine use, our laboratory can test an additional eight STR loci as well as seven RFLP (Restriction Fragment Length Polymorphism) loci for complex cases involving genetic reconstruction or mutations.
Nuclear DNA (hnDNA) vs mitochondrial DNA (mtDNA)
Almost every cell in the human body contains 46 chromosomes that inhabit the nucleus. These chromosomes hold the vast bulk of genetic information that has been inherited from your parents. Outside the nucleus, but still within the cell, lie mitochondria. Mitochondria are tiny structures that help cells in a number of ways, including producing the energy that cells need. Each mitochondrion (there are hundreds to thousands in every human cell) includes an identical loop of DNA about 16,000 base pairs long containing 37 genes. In contrast, nuclear DNA consists of approximately 3.2 billion base pairs and an estimated 30,000 to 40,000 genes.
Whenever an egg cell is fertilized, nuclear chromosomes from a sperm cell enter the egg and combine with the egg's nuclear DNA, producing a mixture of both parents' genetic code. The mtDNA from the sperm cell, however, is left behind, outside of the egg cell. Therefore the fertilized egg contains a mixture of the father and mother's nuclear DNA and an exact copy of the mother's mtDNA, but none of the father's mtDNA. The result is that mtDNA is passed on only along the maternal line. This means all of the mtDNA in the cells of a person's body are copies of his or her mother's mtDNA and all of the mother's mtDNA is a copy of her mother's, and so on. No matter how far back you go, mtDNA is always inherited from the mother. If you went back six generations in your own family tree, you'd see that your nuclear DNA is inherited from 32 men and 32 women. Your mtDNA, on the other hand, would have come from only one of those 32 women.
The fact that the tiny mitochondrial genome, unlike the much larger nuclear genome, is directly transmitted through the maternal line, makes it an ideal piece DNA with which to trace family lineages. Mitochondrial DNA has also become a powerful tool for analyzing relationships in humans and other animals. The last few years have seen an extraordinary number of studies on the relationships of human population groups throughout the world. These studies have approached important questions, not the least of which is how long ago the great native populations of Africa, Europe, and Asia diverged from each other.