I was browsing some past mailbags and happened on:
http://www.straightdope.com/mailbag/mvasectdna.html
It got me to thinking about the value of sperm cells in DNA fingerprinting. If I remember correctly, a sperm cell has only HALF of a normal cell’s DNA. So, if ALL you had from a rape victem was the perp’s sperm cells, could you reliably ID the perp?
Well, my understanding of DNA is hazy at best, but I know it’s true that sperm cells and egg cells each provide half the genetic material of the offspring. It would make sense, then, that sperm and eggs each have half the normal DNA.
But here’s the tricky part: how do we divide that half? Normal humans have 23 chromosomes. I believe the way reproduction works is that sperm and eggs provide not just half in number (11 and 1/2), but half of each of these chromosomes (23 halves).
That is, if I have 23 apples, and I want to give you half, I could give you 11 whole apples and one half-apple, or I could give you 23 half-apples, half of each apple. If I do it the first way, you can’t tell where the apples came from. If I do it the second way, you’d be able to tell where each of your apple-halves came from, because I still have the matching half.
In very, very oversimplified terms, I think this is the idea. But if I’m wrong or even a bit off-base, please do correct me.
And besides, as you read in the above-mentioned post, semen is more than just sperm cells, and therefore, complete DNA could be identified from the nucleus of those cells.
It’s not that simple, since the two chromosones in each pair aren’t matched. I have two copies of chromosone 1, for instance: One came from my father and one came from my mother. Since my father and mother weren’t related particularly any more closely than anyone else on the planet, they’re completely different chromosones. When I have, let’s say, a daughter, she’ll have one chromosone 1 from me, which will either be my mom’s or my dad’s, and one from her mother, which will again be from one of the grandparents. Repeat 22 more times.
On the other hand, there’s a lot of sperm in any given semen sample, easily enough to guarantee that every segment of the source’s DNA is represented in the sample.
Yeah, I did see that. Bit it did set me to thinking.
And Chronos gave the answer that I couldn’t remember. Although I don’t remember the vocabulary, there are pre-sperm cells that each have full DNA. A pre-sperm cell becomes two sperm cells by dividing differently than normal cell division, resulting in half the DNA in each sperm cell. Theoretically, you could find a matched pair and get one full sample of the original DNA. In practice, Chronos is right - you would examine thousands and re-build the original DNA from the samples.
Thanks Cronos!
I think they use y-chromosome identification. I was reading about it being used for paternity. I’m not too clear on the science either, but it has to do with the fact that a man can pass on only one set of y-chromosomes.