[Keeblo]

Think of a genome as 3 billion character sequence. "Sequencing" a genome gives you the actual content of the sequence. Think: "....GAAAAGATACCCACAGAGATTACAAAAC...." (I'll call this string FOO) Until recently, sequencing an entire genome was really, really expensive. I'm not sure what the price is now, but I know that it has dropped considerably.

When you sample a genome, you're checking for one specific string. Traditionally this is done by mixing a sample of the DNA with an enzyme that breaks apart a specific DNA sequence and then measuring whether the size of the DNA molecules have changed. Take the string that I mentioned above (FOO). Suppose that I mix it with an enzyme that cleaves along "GATTACA" such that "GATTACA" becomes "GAT| |TACA". If I mix a DNA sample containing the string FOO with the enzyme I just mentioned, then the molecule would be split into: "....GAAAAGATACCCACAGAGAT| |TACAAAAC...." If I mix a DNA sample that does _not_ contain FOO with the enzyme, no change occurs.

You know when you watch CSI shows* and the DNA evidence is presented as a bunch of light and dark bands? Those are produced by mixing the suspect's DNA sample with a standard cocktail of enzymes, treating them so that the DNA molecules are electrically charged (DNA might already have a charge, not sure), placing them at one end of a container of a standard gelatin with known pore size, and applying an electric field for a standard amount of time. The cocktail of enzymes cleaves the DNA at multiple different substrings and breaks down the DNA strands into multiple smaller substrings. Different size molecules will progress through the gelatin at different rates, so you can effectively compare two people's DNA to see whether the substrings match. You _don't_ know the person's genetic sequence, just whether or not his or her's DNA contains the same substrings as the DNA of the person that you're comparing it to.

* I'm assuming those TV shows present the DNA evidence this way. I don't watch them.

Disclaimer: I'm not a biochemist by any stretch. This is based on my memory from school, so don't take my explanation as absolute truth :-)

[samatman]

There are three techniques in play here: Sequencing, fingerprinting, and SNP sampling.

You have described sequencing and fingerprinting. These services use SNP sampling.

Sampling has been described several places in this thread so I won’t repeat that here;