At its most basic level the human genetic sequence is a three-billion letter string of A's, T's, G's and C's (the code of DNA). Most of that sequence is the same in everyone. But there are an estimated 10 million places in the genome where a single letter of the sequence sometimes differs from one person to the next.
Those one-letter spelling variations are known as SNPs, or single nucleotide polymorphisms, and they form the basis for the genotyping used by 23andMe. A person who has a "C" at a particular spot, for example, might be more sensitive to bitter tastes than a person with a "G" in that spot.
In all, 23andMe analyzes hundreds of thousands of SNPs scattered across 23 pairs of chromosomes that make up the human genetic sequence.
The genotyping technology we've chosen is referred to as a BeadChip and although the process relies heavily on massive computer power, the chip itself is not a microprocessor but a miniaturized genetics lab. The BeadChip is a small glass slide with millions of tiny beads on its surface. Attached to each bead are probes - bits of DNA complementary to sites in your genome where SNPs of interest are located. Your DNA will stick to the probe that matches whichever SNP you happen to have.
In the genotyping process, a person's DNA is chopped up into pieces and washed over the chip, where each fragment sticks to any probes that are complementary to it. Then fragments of DNA that have been specially tagged are introduced to the chip in such a way that they stick to any probes that are themselves paired perfectly with sample DNA. At that point, the tagged DNA fragments are triggered to glow indicating which version of each SNP is present in the sample. It's a scientifically complex process, but a simple and reliable method in practice! The technology that we use, the Illumina HumanOmniExpress-24 format chip, analyzes hundreds of thousands of SNPs that cover the entire genome. Although this is still only a fraction of the more than 10 million SNPs that are estimated to be in the human genome, these were specially selected because they provide a lot of information about other nearby SNPs. This maximizes the information we can get from every SNP we analyze, while keeping the cost low.
In addition, we have hand-picked more than 30,000 additional SNPs of particular interest from the scientific literature. As a result, we can provide you with unique, genetic information available through no other service. If you would like to view more details on Illumina's genotyping platform, please visit the Illumina website.