Genetic markers used to determine ancestry occur in different forms. Some genetic markers can establish maternal mitochondrial DNA) or paternity (Y-Chromosome) lineage where there is an unbroken chain in the pedigree. Such markers are valuable in resolving ancestry issues, but any break in the pedigree can cause difficulties. For example, the Y Chromosome is inherited from father to son. Any generation where there are no male offspring, the Y Chromosome is not passed on. Likewise, the mitochondria is inherited from mother to offspring. Any generation where there are no female offspring, the mitochondria are not passed on.
Another type of genetic marker is known as a SNP (single nucleotide polymorphism). SNPs are utilized for many applications in genetics including disease detection and relationship testing. SNPs are single base (letter) differences in the DNA. DNA is composed of four bases (the nucleotides adenine (A), thymine (T), cytosine (C), and guanine (G)) abbreviated A, T, C, and G. If at a location the sequence of letters is A-C-C-G-A-T-T in one person and at the same location A-C-A-G-A-T-T in another person, the C to A difference is a SNP. Once a SNP occurs, it tends to remain. The rate of change at a single nucleotide site is about 1 in 100,000,000.
SNPs have been identified that are helpful in the determination of ancestry and are termed Ancestry Informative Markers (AIMs). Such markers occur in high frequency in some population(s) and at a very low frequency in a different population. Such ancient markers often allow a view of a person’s genome from a deep, primordial level.
STR (Short Tandem Repeat) genetic markers form alleles that differ in size by the number of repeats present in the DNA. For example at the D2S1338 locus, the typical repeat unit is four letters; TTCC. Thus a 20 allele at the D2S1338 locus has 20 repeats of the four base motif, while the 21 allele at the D2S1338 locus has 21 repeats of the four base motif, and is one repeat (4 bases) longer than the 20 allele.
These STR loci are highly polymorphic in that there are many possible forms or sizes of DNA. Since each STR locus used in ConnectMyDNA™ is assigned to a specific location on a chromosome and there are two chromosomes (one from your mother and one from you father), the number of possible genotypes is a function of the number of alleles detected. In the example above, at D2S1338, allele frequency for alleles 15 through 25 is show. This means there are 11 possible. The number of genotypes is calculated as:
(n x n+1)/2 (n = number of alleles) or (11 x 12)/2 = 66 possible genotypes. Thus you could take the world population and place them in 1 of 66 bins reflecting the possible genotypes. For example:
Bin 1 – 11, 11
Bin 2 – 11, 12
Bin 3 – 11, 13
Bin 66 – 25, 25
When a number of loci are combined, the number of possible bins exceeds the world population many fold. This is the reason why these loci are used for individual identification.
The STR loci change more rapidly than the SNP loci discussed above. The frequency of allele change at a STR locus is about 1 in 1,000. Such changes occur during the formation of gametes (egg or sperm). One theory for the greater frequency of change at the STR loci involves errors that occur when DNA is copied. Most allele changes at the STR loci involve the addition or subtraction of a single repeat unit (4 bases). It is hypothesized that the enzyme that copies DNA (DNA polymerase) can more easily make errors in replicating the number of repeats than in making single letter changes which are corrected. The end result is that alleles change frequency at STR loci many times faster than at SNP loci.
Because of the ability of STR alleles to change more rapidly than SNP alleles, STR loci are not useful for ancestry testing. The alleles detected in a population are a snap shot of the frequency which is always changing due to mutational and migratory events. The country matches determined by ConnectMyDNA™ are a snap shot of how your DNA profile fits in with the databases used for comparison.
In many cases there will be connections that seem surprising. There may be different connections between people that are related. The reason for this is due to the individual alleles that make up your Gene Ring and their corresponding frequency in the databases used to make the connections.