In humans the sex of an individual is determined by inherited sex chromosomes. All individuals carry two sex chromosomes, but males carry a so-called X and a Y chromosome (and are thus called XY) whereas females carry two X chromosomes (and are thus called XX). When a male and female produce an offspring, each gives the offspring one of its sex chromosomes, chosen with equal probability. Thus a child always inherits an X chromosome from its mother since, regardless of which of her sex chromosomes is chosen, it will always be an X. But the child inherits the X chromosome from its father with probability 1/2 (in which case it will be female) or the Y chromosome from its father with probability 1/2 (in which case it will be male).
X-linked mutations are mutations that are found on the X chromosome. Because females carry two X chromosomes and males carry only one X chromosome, the inheritance pattern of conditions caused by X-linked mutations can differ between males and females.
Muscular dystrophy is an example of genetic condition caused by an X-linked mutation. Some cases of muscular dystrophy are caused by inheritance of the mutation from a parent. In such cases there is typically a family history of the disease. Other cases of muscular dystrophy are caused by the spontaneous appearance of the mutation during reproduction, and such cases can affect anyone. Example 12.3.5 of the textbook focuses on the case of spontaneous mutations. Some estimates suggest that up to 35% of the cases of muscular dystrophy are caused by spontaneous mutation.