Differences in chromosome number may underlie variation among genetically identical individuals.
Aneuploidy is a condition in which cells contain an abnormal number of chromosomes, and is known to be the cause of many types of cancer and genetic disorders, including Down Syndrome. The condition is also the leading cause of miscarriage.

Disorders caused by aneuploidy are unusual in that the severity of their effects often varies widely from one individual to another.

For example, nearly 90 percent of fetuses with three copies of chromosome 21, the cause of Down Syndrome, will miscarry before birth. In other cases, people with the condition will live until they are over 60 years old.

Researchers have previously believed that this variation is the result of differences in the genetic makeup of those individuals with the condition.

But in a paper published in the journal Cell, researchers at the Koch Institute for Integrative Cancer Research at MIT reveal that aneuploidy alone can cause this significant variability in traits, in otherwise genetically identical cells.

The finding could have significant implications for cancer treatment, since it could explain why genetically identical cancer cells may respond differently to the same therapy.