Single nucleotide polymorphisms, frequently called SNPs (pronounced “snips”), are the most common type of genetic variation among people. Each SNP represents a difference in a single DNA building block, called a nucleotide. For example, a SNP may replace the nucleotide cytosine (C) with the nucleotide thymine (T) in a certain stretch of DNA.

A short tandem repeat (STR) occurs when a pattern of two or more nucleotides are repeated and the repeated sequences are directly adjacent to each other. The pattern can range in length from 2 to 16 base pairs (bp). By identifying repeats of a specific sequence at specific locations in the genome, it is possible to create a genetic profile of an individual. Autosomal STR analysis has become the prevalent analysis method for determining genetic profiles in forensic cases. Y-chromosome STRs (Y-STRs) on the Y chromosome) are used in genealogical DNA testing in surname DNA projects.

Generally, STR loci are named D + the chromosome the locus is on + S + the order in which the location on that chromosome was described. For example, D3S1358 is on the third chromosome and is the 1358th location described.

STRs are useful genealogically, to determine to whom you match within a recent timeframe, of say, the past 500 years or so, and SNPs define haplogroups which reach much further back in time. 10-15 years ago, Y-DNA matching and haplogroup definitions were done primarily with STRs. But in the last few years, sequencing technology has improved such that SNPs can be useful in genealogical years.

The FBI uses STR loci to identify/match persons. Their system is called CODIS (Combined DNA Index System). The original core group had 13 STRs, 7 additional were added in 2017.

The CODIS loci list can be found here.

The loci used in CODIS were chosen because they are in regions of noncoding DNA, sections that do not code for proteins. These sections should not be able to tell investigators any additional information about the person such as their hair or eye color, or their race. However, new advancements in the understanding of genetic markers and ancestry have indicated that the CODIS loci may contain phenotypic information

None of these are found in the Big-Y test at FTDNA.

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