How many of my physical characteristics can be determined from a sample of my DNA? For example, I leave a nice-sized splatter of blood behind at a crime scene . . . can they analyze it and say “Okay, we’re looking for a 5’4, brunette woman with green eyes”?
I don’t think they’re quite that sophisticated yet. However, they can take the DNA samples at a crime scene and, once they find you, they can take DNA samples (subject to various legalities) and match the two. Sort of like fingerprints, though it’s much more likely that your fingerprints are on file somewhere than your DNA.
Well, genes for height and eye color are not known. Most likely that are pleiomorphic characteristics, that is many many genes combine to give you green eyes (which by the way are a sign of pure genius). But even if the genes were known, I’d imagine it would be impractical (time and expense) to analyze DNA from a crime scene that way.
I don’t know precise forenzic techniques for purifying DNA from blood, but I suppose one could look for a Y chromosome in the white blood cells to determine if the suspect is male. But when it comes down to it, standard detective work is quicker: good old motive weapon and opportunity. The DNA stuff works better when you have a limited group of suspects.
The way they identify someone’s DNA is by using enzymes that break DNA strands in specific places. When you run the fragments out according to size you get a specific pattern. DNA from a different person has a different sequence and will give a different pattern. I think. I work with the stuff every day but I don’t know forenzic protocols.
I think one country was considering the establishment a DNA databank but only for known felons. Drawing blood from every citizen for a complete DNA library might not be too popular.
Alphagene
Brunette, huh? How old are you? What are your measurements? Are you cute? 
It would be cool if they could do that. In theory that could be done if we were really advanced (like in that movie “Gattaca”- they had handheld analyzers that you could stick something like a stray eyelash into, and see what the person would look like on the screen). It’s not that simple though. Even if they were able to sequence your entire genome (the way the Human Genome Project has been sequencing the complete genome of two unnamed individuals, one male, one female, for the past couple years) it wouldn’t be easy to figure out what the phenotype would be. Genes are dominant, recessive, mixed dominant,etc. They interact with each other and suppress, enhance, and modify each other’s gene expression. They are also subject to varying levels of gene expression during your lifetime. The most straightforward way to tell your hair color, eye color, etc. would be to clone your DNA sample (assuming it’s in excellent condition) and see how the baby comes out. And that would create an ethical racket.
What the cops actually do in their labs is take your DNA and expose it to restriction nucleases that are “restricted” because they are specific to a particular target nucleotide sequence, like “GATTACA” (guanine-adenine-thymine-thymine-cytosine-adenine, as in the name of that movie). So once in a while your DNA will purely by chance contain a GATTACA base sequence. The restriction nuclease finds it, and snip snip. The actual site of the cleavage (huh huh, I said “cleavage”, huh huh) may be within the target sequence itself or it may be a few bases upstream or downstream relative to the target. It depends on the particular nuclease that the cops are using. For more accuracy, and to further impress a jury, the sample may be divided up and treated with several different restriction nucleases (which are each going to look for a different target sequence).
The result is a bunch of fragments of DNA, with varying lengths. Each fragment has been cut wherever a “GATTACA” was present. The whole idea is that if the crime scene sample is your DNA, the lengths of the fragments produced from it will be the same as the lengths of the fragments produced from your blood sample.
Gel electrophoresis is the method they use to determine the fragment lengths. In gel electrophoresis the DNA segments are injected into a gel in a tray that is combed (with a special “gel comb”) into “tracks”- one for each sample. Sometimes molecular weight markers with known weights are also injected into a track, to provide a standard for comparison. The tray is electrified on each end, producing an electric field that is applied for a couple hours. DNA is negatively charged because of the phosphate in its sugar-phosphate “backbone”. So the DNA fragments slowly push their way through the gel toward the positive electrode on the other side of the tray. Since the smaller fragments are not as impeded by the “holes” in the gel matrix, they manage to move the farthest along their tracks before the electricity is shut off. Then they take a picture by staining the gel with ethidium bromide, which binds to DNA, and which fluoresces under UV light.
Mr. Fung then shows the picture to the jury, and then Barry Sheck comes in and convinces everyone that it’s all police hocus-pocus that doesn’t prove anything. And the bloody glove doesn’t fit so the defendant is found not guilty and vows to find the “real killers”, then immediately gets his clubs together and goes to the golf course. The manhunt continues. 
Nowhere does this process concern itself with what the DNA actually does in the body, such as making your eyes green or coloring your hair. The whole idea is just to compare two samples of DNA and see if they “match up” when analyzed using a certain laboratory technique. It’s much like fingerprinting- in that a fingerprint by itself is worthless. It doesn’t give you any information about the perpetrator. It’s only useful because you’re able to compare it to other fingerprints.
While we can’t tell physical characteristics from a DNA sample, we can check for the presence of certain genetic markers, which have been found to exist in limited segments of the population. This is mainly useful because it doesn’t require as large a sample as a DNA “fingerprint” does.
For a simple example, say that the DNA at the crime scene contains the gene for hemophilia. Previous studies have shown us what percentage of people carry that gene. So if your next-door neighbor was suspected of the crime, and she does not carry that gene, then the police can rule her out as a possibility.
This sort of thing was being done with blood types-- O, A, B, AB-- long before we had the ability to do it with DNA.
The other concern with identifying physical characteristics via DNA is that many parts of a persons physical appearance are decided by an undeterminable mixture of genetics and envirnoment. DNA has only a single role in the body: it codes for proteins. All DNA can do is tell your body what kind of proteins to make.
Now, while this sounds simple, most of your bodies interesting chemistry comes from proteins. Without getting too heavy into biochem here, basically things which are coded for by simple proteins (hair color, hair texture, eye color) could in theory be easily matched up with the genetic code that caused them. Other things further down the line, (i.e. they are products of products of products of reactions of proteins, and ad infinitum) are infinitely more difficult to assign to a specific genetic code. Plus there’s the problem that things like height and weight are heavily (though not entirely) influenced by environment; healthier people tend to be taller, given similar genetic make-up. Which is to say that genes (and DNA, related but not in anyway the same concept) have a say in what you look like, but so does the environement in which you grew up in or live in currently.
And all of it comes to naught when put in front of Judge Ito and a jury of twelve boneheads. :>
Certain races carry more genes for some diseases.You could probably guess at the race of a person that way. You may carry genes for all kinds of hair color, but it follows what gene is dominant.
I thought they had genes pretty well figured out for cat fur choices, for instance.
Lipochrome sed: It would be cool if they could do that. In theory that could be done if we were really advanced (like in that movie “Gattaca”- they had handheld analyzers that you could stick something like a stray eyelash into, and see what the person would look like on the screen).
I thought in Gattaca that the analyzers were determining the genetic pattern, then looking up the person’s picture in a database keyed off of that pattern.
No, that would make more sense, but they could show a persons picture without even knowing the name.