Girls with Duchenne Duchenne affects approximately 1 in every 3,500 boys that are born but only around 1 in 50 million girls. This difference is because Duchenne is caused by a fault in the DMD gene which is only found on the X chromosome (see section on how Duchenne is inherited for more information). Boys have one X chromosome and one Y chromosome while girls have two X chromosomes. A girl can have Duchenne if both of her X chromosomes have faults in the DMD gene. This is extremely unlikely although it could happen in a number of different ways. A girl could inherit a faulty DMD gene on one of her X chromosomes from her mother and also develop a spontaneous mutation in her other X chromosome. She could also develop spontaneous mutations in the DMD gene on both of her X chromosomes. It is also thought that the healthy DMD gene on one X chromosome can become ‘switched off’ by a process called X-linked inactivation in some girls that have inherited a faulty DMD gene on their other X chromosome. Alternatively, if a man with DMD and a woman who carried a defective DMD gene ‘a carrier’ had a child, it is possible that a daughter would have Duchenne. Female carriers of a faulty DMD gene can also have very mild symptoms of Duchenne that progress slowly. These women are called ‘manifesting carriers’ because they produce reduced amounts of dystrophin and may have symptoms of fatigue and mild muscle weakness and cramping.