Approximately one baby out of every 800 has Down syndrome, also known as trisomy 21. A rare opportunity led to the discovery that the chromosome abnormality that causes trisomy 21 seems small but it upsets the entire genome. The lead researcher of the study uses climate change to illustrate the effect.

Identical twins are two same-sex children who grew from one single egg. They share more than gender, though. Their entire genomes (all the genetic material that makes most people unique) are the same.

Down syndrome has been exhaustively studied at the genetic level. It is well understood that the genetic defect occurs when each cell in the body contains three copies (instead of two) of chromosome 21, hence the name trisomy (three chromosomes) 21.

Most scientific studies rely on a control group of study participants who do not test positive for whatever trait is being studied and a study group of participants who do have the trait under investigation. Identical twins are ideal control-study matches for most things but identical twins either both have trisomy 21 or they don't. At least, that was the theory.

Dr. Stylianos Antonarakis and his international team of researchers had the rare opportunity to study the genomes of identical twins where one twin had Down syndrome and the other did not. Antonarakis is affiliated with the Department of Genetic Medicine and Development at the University of Geneva Faculty of Medicine. His colleagues were in Amsterdam, Barcelona, Seattle, and Strasbourg.

Their research revealed that although chromosome 21 makes up just 1% of the entire genome, a flaw in it affects all the others. Genes within a chromosome are activated to produce an effect (gene expression). Study of the twins' genomes revealed that the third copy of chromosome 21 influenced the gene expression throughout all of the chromosomes of the twin with Down syndrome.

In some cases, the third chromosome influenced genes to turn off (become inactive) when they were turned on in the healthy twin. Others were turned on in the Down syndrome twin but turned off in the healthy one. These and other variations in gene expression were not limited to chromosome 21 only.

Antonarakis uses "an analogy with climate change" to describe the effect: "Even if the temperature rises by only one or two degrees, it will rain a lot less in the tropics and a lot more in temperate zones. Global climate equilibrium can thus be disrupted by a tiny element."

Source: Antonarakis, Stylianos, et al. "Domains of genome-wide gene expression dysregulation in Down's syndrome." Nature. Nature Publishing Group / Macmillan Publishers Limited. Apr 16, 2014. Web. Apr 30, 2014.