Review Question - QID 100094

The pedigree illustrates maternal inheritance of a mitochondrial gene. The mitochondrial genome is comprised of circular DNA that encodes genes important for energy and metabolism.

Because mitochondria are present in the maternal egg, but are lost in the sperm, mitochondrial genes are inherited almost exclusively from the maternal side. As mitochondria are involved with energy production, mitochondrial disease often leads to problems with metabolism. One well-known mitochondrial disease is Leber's Hereditary Optic Neuropathy, which causes progressive loss of retinal ganglion cells.

Chinnery presents an overview of mitochondrial diseases and described many of the individual syndromes, including MELAS, myoclonic epilepsy with ragged red fibers, and many others. He states that these diseases may occur at any age and may be focal or systemic and syndromic. He explains that while some patients may be diagnosed by their clinical presentation and then the diagnosis confirmed with genetic testing, many patients often do not fit neatly into one of the syndromes and must undergo a systematic and in-depth evaluation for the diagnosis to be made. Chinnery also provides an overview on the applications of genetic counseling on families affected with mitochondrial diseases.

Le and Bhushan describe mitochondrial inheritance in their review book First Aid for the USMLE Step 1. This section describes how mitochondrial inheritance patterns is often seen in diseases associated with defective oxidative phosphorylation. Furthermore, they describe the principle of heteroplasmy, which is characterized by variable expression of disease due to the presence of both normal and mutated mitochondrial DNA.

Figure A shows a pedigree of a condition with maternal inheritance. Illustration A shows common symptoms of mitochondrial diseases. Illustration B shows how the load of mutant mitochondria can change from generation to generation, depending on random variation in the mitochondria that are passed on.

Incorrect Answers:
Answers 1, 3, 5: This would not show a maternal inheritance pattern.
Answer 4: Most multicellular organisms have circular mtDNA. Linear mtDNA is seen in many unicellular organisms and in very few multicellular organisms.