Saturday, March 03, 2012

Saturday morning reading material

Here are a few odds and ends that didn't make it into full blog posts in the last few days:


Don Dickson said...

Also don't miss the op-ed piece in yesterday's DMN by Public Safety Commissioner ADA Brown in which she opines that we give too much credence to DNA testing. It's behind the paywall but you can read it on the message board at

Don Dickson said...

I'm not the person who posted Ada's article on our website, but upon reflection I decided to take it down. Ada doesn't like our website very much, and the last thing I need is for Ada (who is an intellectual property litigator) egging on the DMN lawyers to sue TSTA for copyright infringement.

In a nutshell, Ada says that we give too much credence to DNA-based exonerations because of the existence of "chimeras." You can look up what a chimera is. I would respectfully assert that Ada's grasping at very thin straws to save bad convictions here...whatever the frequency of chimeras in the human population, if the DNA doesn't match the convict, that certainly sounds like "reasonable doubt" to me.

Anonymous said...

Speaking as a geneticist who on occasion has attempted to educate lawyers, Ada Brown's op-ed is correct - as far as it goes.

Human chimeras certainly do exist. The two examples she gives (of women whose ovaries contained oocytes that were genetically different from their blood - so they had children who appeared to not be theirs biologically) are good examples of how, when chimerism happens, it can complicate things.

What Brown's piece fails to mention, however, (probably because as a lawyer she spends her life arguing about whether things are true rather than testing to see if they are true or not) is that in any case where chimerism is proposed as a possible explanation, there are a number of experimental approaches that can be taken to either demonstrating that it is true, or is not true.

Most obviously, there can be additional genetic testing of the Y-chromosome and/or the mitochondrial chromosome. Chimeras occur when two fraternal twin embryos fuse. So the chimeric tissues are from biological siblings. Two male siblings will share the same Y-chromosome from their father. If the Y-chromosomes are different, then it isn't a chimera. Same for the mitochondrial DNA profile, which comes from the mother and is shared by all her children.

There are also some simple and straight-forward statistical tests that can be performed to ask whether it is likely or unlikely that two DNA profiles came from biological siblings. Siblings are much more likely to share portions of their genetic profile than unrelated people, and the appropriate statistical test will tell you whether two profiles are more often associated with unrelated people or siblings.

The bottom line is, don't count on getting a full story about science from lawyers. They are generally arguing either one side of a question or another.

Anonymous said...

I'm sure the anonymous geneticist knows that mitochondrial DNA can establish whether two people share a common female ancestory, but it does not indicate the degree of kinship. The shared common ancestor could have been twenty generations back.

Same for Y-chromosome testing. It can establish whether two men (men only, because women have only X-chromosomes) share a common male ancestor, but that's it.

I don't understand the comment from anonymous geneticist that, "If the Y-chromosomes are different, then it isn't a chimera." The only way fraternal twins could have two different sets of Y-chromosomes is if the ova were fertilized by the sperm from two different men. Certainly such a thing has happened, but that situation is probably rare, since it would mean that the mother had sex with two different men at the precise moment she had two eggs available to be fertilized. Or that someone at the fertility clinic screwed up. If that were to happen, I would think that the resulting twins would be as likely as any other fraternal twins to fuse into one chimera.