Biology 201
DNA is a helical structure, similar to a ladder. The outside of the ladder is a sugar-phosphate backbone and the rungs are made of paired nitrogenous bases. These bases come in four varieties, Adenine (A), Thiamine (T), Cytosine (C), and Guanine (G). A always pairs with T and C always pairs with G along the rungs of the ladder.
The portion of the DNA looked at in genealogical studies is found traditionally in the “junk DNA” sections, or the non-coding regions of the DNA. Typically there are three main kinds of analysis performed for genealogical purposes: DNA sequencing, Single Nucleotide Polymorphism testing (SNPs), and Short Tandem Repeat testing (STRs).
DNA Sequencing is the process of determining the exact base pair sequence in a predetermined section of DNA. This technique is most often used in mitochondrial DNA (mtDNA) testing where the sequence of hypervariable region I (HVR I) and hypervariable region II (HVR II) is determined. This kind of testing is useful for direct maternal lineage analysis or for maternal ancestry typing. (See the DNA for the Genealogist section for more details.)
Single Nucleotide Polymorphisms (SNPs, pronounced snips) are single base pair changes that occur throughout the genome. These changes occur relatively infrequently, therefore this kind of testing places an individual in a large deep-ancestral group also known as a haplogroup. SNP testing is available for mtDNA, Y chromosome, and autosomal DNA. This testing in its current form may not be directly applicable to the genealogist as it can only give you information about your ancestry 1,000’s of years ago. (For more information, please see the DNA Testing for the Genealogist section.)
Short Tandem Repeats (STRs) are junk segments of DNA that repeat themselves, for example a GATA sequence repeated several times (GATA GATA GATA). Every individual has STR segments at specific locations on the chromosome. The specific locations are referred to as Loci or Markers and are generally noted as DYS###, for example DYS459. DYS indicates the location is on the Y chromosome at position 459.
While everyone has these repeated regions, the distinction between one individual and another is the number of times the segment repeats. For example, at a given locus or marker on the chromosome, one individual may have 12 GATA's, and another individual may have only 10 GATA's. These numbers, 12 and 10, are called alleles. It is the collection of alleles for an individual at specific locations (loci) that make up his or her unique DNA profile or haplotype. These profiles or haplotypes are used for identification purposes, to determine paternity, or to establish relatedness. (More information on the use of STRs in genealogical studies can be found in the section titled DNA for the Genealogist).
