A rare occurrence in the earliest days of a pregnancy produces an unusual and mystifying outcome: Identical twin fetuses are conceived of the same meeting of egg and sperm. And despite their shared DNA, one of the twins has Down syndrome (the most common genetic cause of intellectual impairment), but the other does not.

For those who labor to understand how 3 billion base pairs of DNA result in the complexity of a single human, it’s difficult to discern what effect an extra chromosome has on gene expression across the genome: from individual to individual, there’s just too much natural variation for comparisons between two people to reveal truths that apply to all.

But these aborted identical twins -- one with an extra copy of chromosome 21 and the other without -- offered scientists a remarkable opportunity: given the twin fetuses’ otherwise exact DNA match, how would this one difference translate across the genome?

That natural experiment allowed a group of geneticists from Switzerland, Spain, the Netherlands and France to distill some fundamental insights into how chromosomes -- and the genetic blueprints they contain -- dictate the behavior of cells across the body. They found that when gene expression is altered by, say, an added chromosome, it is altered in consistent patterns in every chromosome, not just the one with the irregularity.


A few things follow from that: First, it lends credence to scientists’ long-running suspicion that chromosomes -- between 50 and 100 base pairs of DNA -- may be organized along functional lines, such that certain stretches of a chromosome may hold the genetic blueprint for proteins that work together in some predictable way. If they are organized functionally, they’re not random. And if they’re not random, they can (someday) be understood.

What also follows from that is that, if you want to identify a treatment for a genetic mutation such as Down syndrome, you may not have to go to its source, that extra chromosome 21: You could seek to alter gene expression almost anywhere else in the genome, since that is how far-flung the effects of that mutation are.

The study, along with an enlightening commentary by a pair of University of Florida geneticists, was published Wednesday in Nature.