how is"DNA fingerprinting" is done?

Answer 1

DNA fingerprinting relies on a specific region of DNA that doesn't do anything. Actually, about 90% of your DNA doesn't do much of anything, but the section that's used for DNA fingerprinting has done nothing for so long that random mutations have made the section... well... random. A typical 'fingerprint' includes 13 units, each of which has from 5 to 20 variations in the population.

So basically scientists can analyze this one area of DNA and come up with a pattern that is ALMOST unique to you. Given the above variation in the units they use, the chance that a total stranger happens to have the exact same sequence can be as small as one in a quintillion. Since there's less then 10 billion people on earth, it is so fantastically unlikely that any two people have the same strands by chance that most would agree that it probably just isn't going to happen.

And that's where the 'almost' comes in. Any child will have two copies of this random sequence, one from each parent... and since copy errors are fairly common, it's possible even that it's very slightly different from each. Still, a slight difference is quite different from total randomness. Thus DNA fingerprinting can usually determine if someone is a parent or not with better than 99% accuracy, and tell if any trace DNA is yours with perhaps more accuracy than that, depending on how much time the particular piece of DNA has had to degrade and the like.

The actual procedure involves adding chemicals which cut the DNA into fragments and then using electrophoresis to separate those fragments based on size. You end up with a series of bands whose placement and intensite are (more or less) unique to you.

Of course, you have to have some DNA to test, but most people are constantly shedding skin and stuff that still contains living cells. An actual swab of cells is probably best, followed by saliva and other bodily fluids. Hairs usually don't have many cells on them unless they are pulled out, so hair from a brush is preferred to hair that is just shed naturally.