when a cell has extra chromosomes or missing chromosomes A worldwide gene research project, called the Human Genome Project, is creating a map of all human genes and their location on chromosomes.

Theodorescu’s ultimate goal is to piece together how the Y chromosome is lost on a basic level so that it can be prevented. “I want to get to a therapeutic in the end,” he said.

In each cell, one X chromosome will randomly be silenced in a process known as X chromosome inactivation (XCI). In girls with Rett syndrome, the silenced chromosome carries a healthy copy of MECP2.