23andMe

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What is the difference between genotyping and sequencing?

Though you may hear both terms in reference to obtaining information about DNA, genotyping and sequencing refer to slightly different things.

Genotyping is the process of determining which genetic variants an individual possesses.    Genotyping can be performed through a variety of different methods, depending on the variants of interest and resources available.    At 23andMe, we look at SNPs, and a good way of looking at many SNPs in a single individual is a recently developed technology called a “DNA chip.”

Sequencing is a method used to determine the exact sequence of a certain length of DNA.    Depending on the location, a given stretch may include some DNA that varies between individuals, like SNPs, in addition to regions that are constant.    So sequencing is one way to genotype someone, but not the only way.

You might wonder, then, why we don't just sequence everyone's entire genome, and find every single genetic variant they possess.    Unfortunately, sequencing technology has not yet progressed to the point where it is feasible to sequence an entire genome quickly and cheaply.    It took the Human Genome Project over 10 years' work by multiple labs to sequence the three billion base pair genomes of just a few individuals.    For now, genotyping technologies such as those used by 23andMe provide an efficient and cost-effective way of obtaining more than enough genetic information for scientists — and you — to study.
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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.

BeadChip

The Illumina HumanHap 550+ BeadChip, analyzes more than 550,000 SNPs that cover the entire genome.    Although this is still only a fraction of the 10 million SNPs that are estimated to be in the human genome, these 550,000 are 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 and added them to the chip.

23andMe looks at nearly 600,000 SNPs scattered across the 23 pairs of chromosomes that constitute the human genetic sequence.    We also look at a few thousand places on the mitochondrial DNA, an odd loop of genetic material outside the nucleus that is involved in producing energy for the cell.

Ron Scott has compiled an interesting list of phenotypes tested by 23andMe which may be seen by clicking here.