ABSTRACT
Occasionally, girls may be affected by an Xlinked recessive disorder because of lyonisation. This is the process by which female embryos randomly inactivate one X-chromosome in each cell at around 12-16 days post conception. If the embryo happens to inactivate mostly the X-chromosome carrying the normal gene, then that female may manifest features of the disease. In addition to these forms of inheritance, some disorders arise because of mutations in genes encoded on the mitochondrial genome. These disorders are often very variable and affect many different systems of the body, and they are transmitted exclusively by the mother.
