Down syndrome is the most common genetic disorder, impacting about 1 in 700 newborns around the world. At some point during their first hours and days of embryonic development, their dividing cells fail to properly wriggle a chromosome pair away from each other, leaving an extra copy where it shouldn’t be. Although scientists have known for more than six decades that this extra copy of chromosome 21 causes the cognitive impairment people with Down syndrome experience, exactly how it happens remains a matter of debate.
The dominant hypothesis is that people with this syndrome make too much of the proteins encoded in the genes that reside on chromosome 21, and that this overexpression alters the timing, pattern, or extent of neurodevelopment. This theory has a name: the “gene dosage effect.”