Barth syndrome (BTHS) is a rare, genetic disorder of lipid metabolism that primarily affects males. It is caused by a mutation in the tafazzin gene (TAZ, also called G4.5) which leads to decreased production of an enzyme required to produce cardiolipin. Cardiolipin is an essential lipid that is important in energy metabolism. BTHS, which affects multiple body systems, is considered serious. Its main characteristics often include combinations in varying degrees of cardiomyopathy (a disorder of the heart muscle leading to a poorly functioning heart), neutropenia (a reduction in the number of white blood cells which may lead to an increased risk of bacterial infections), hypotonia (reduced muscle tone), muscle weakness, undeveloped skeletal muscles, delayed growth, lack of stamina, varying degrees of physical disability, and methylglutaconic aciduria (an increase in an organic acid that results in abnormal mitochondria function). Although some with BTHS may have all these characteristics, others have only one or two, and are thus often misdiagnosed. BTHS is an X-linked genetic condition passed from mother to son through the X chromosome. A mother who is a carrier of BTHS shows no sign or symptom of the disorder herself. On an average, 50 percent of children born to a carrier mother inherit the defective gene, but only boys develop symptoms. All daughters born to an affected male are carriers.
This answer is based on source information from the National Institute of Neurological Disorders and Stroke.