Introduction to DNA

Your DNA is your genetic blueprint - the code that defines what and, to a certain extent, who you are.Twisted into the endless corkscrew spirals of your DNA are the coded instructions for building every single part of your body.

What is DNA?

DNA is short for deoxyribonucleic acid, a molecule with an extraordinary structure. DNA is a polymer, a long molecule created by the joining together of many smaller repeating units in a seemingly endless series.

The simplest way to visualize a molecule of DNA is to imagine a tall ladder. The uprights of the ladder are formed from molecules containing sugar, oxygen, and phosphorus. Each rung of the ladder contains a separate pair of molecules known as bases. Now imagine twisting the ladder along its length. Twisting causes the uprights to spiral round each other into a characteristic shape known as the double helix.

The bases that form the rungs of the ladder come in four different types known as cytosine, adenine, thymine, and guanine. Scientists habitually describe them by their initials C-A-T-G.

Four letters might not seem much, yet they have the potential to form a limitless variety of coded messages rom random strings of letters. In reality the order of those letters is far from random; it defines you precisely. That string of Cs, As, Ts, and Gs contains all the information needed to create and maintain you.

Different Types of DNA

Autosomal STR: STRs are short tandem repeats found in the genetic material (or DNA) that makes up the human genome. Several different STRs, regions that are highly variable in length from individual to individual, have been located and examined. The size of the STR is what scientists look at to help answer questions regarding relatedness. The different sizes of DNA found at these STR regions correspond to, what geneticist call, alleles. These alleles are passed on from parents to offspring. By examining random individuals within a particular population, each allele is found to have a frequency associated with it within that group. It is these frequencies that researchers use to calculate the probabilities of relatedness. DNA Worldwide uses STRs to measure probabilities of paternity, and most other DNA relationship test, including the Y-chromosome Haplotyping.

Y-chromosome Haplotype: Y-chromosome haplotypes are used for testing descendants with a common paternal link in their genealogical lineage. For example, if a group of males have strictly a male descent line (may have the same last name, such as Nicholson), and they are thought to all be related to a common male ancestor, examining the Y-chromosomes of the individuals in common makes it fairly easy to support or disprove this hypothesis. By looking at Y-chromosome markers from all of the terminal males with the same paternal descent (all the males with the Nicholson Surname) we would suspect that they should all share the same Y-chromosome markers (with allowances made for calculated mutation rates, which should be small given less than 8 or 9 generations). This test could also be used to distinguish non-paternity in the line, a question that one might not want answered.

Mitochondrial DNA (mtDNA) Analysis: You must note that in using this method, you are tracing ancestors through the maternal, not paternal, line. This is because mitochondrial DNA is passed from the mother to her children. As long as there is a female lineage connecting some of these people, we would expect that they would share the same mtDNA, from generation to generation to generation, etc. (with allowances made for calculated mutation rates, which are higher than what is observed from the Y-chromosome).

How are the probabilities calculated?

If we were to focus on testing only one locus, an area on the Y-chromosome, we can provide an easy illustration to help understand this process. At the y-chromosome locus there can be 7 - 20 different possible identification markers (or alleles). On the Y-chromosome each male will only have one of these identification markers (representing one Y-chromosome).

Each of the possible 7 - 20 identification markers (found at one locus) have different frequencies in a given population. This is the key to understanding how these probabilities are calculated. One identification marker may have a frequency of 50% in the Caucasian population; another may have a frequency of 2% in the Caucasian population, and etc. Now 2 Caucasian people with a common identification marker of 50% frequency, are not calculated to have a high degree of relatedness. Therefore, the probability of relatedness is less than what would be calculated between 2 people that share the less common identification marker (i.e. the one with the 2% probability in the Caucasian population).

Furthermore, the more loci that you examine between two individuals the more revealing your results will be. Therefore, the identification markers that scientists find at different loci when examining your DNA may be either a high frequency identification marker, which provides less to work with, or a low frequency identification marker, which provides more information when common between individuals.