(a) In humans the mitochondrial genome encodes a low number of proteins, rRNAs, and tRNAs but imports approximately 1100 proteins encoded by the nuclear genome. Yet, with such a small proportion from the mitochondrial genome encoding proteins and RNAs, a disproportionately high number of genetic disorders due to mtDNA mutations have been identified [Bigger, B. et al. (1999)]. What inheritance pattern would you expect in a three-generation pedigree in which the grandfather expresses the initial mtDNA defect? What inheritance pattern would you expect in a three-generation pedigree in which the grandmother expresses the initial mtDNA defect? (b) Considering the description in part (a) above, how would your pedigrees change if you knew that the mutation that caused the mitochondrial defect was recessive and located in the nuclear genome, was successfully transported into mitochondria, and negated a physiologically important mitochondrial function?

Key Concepts

Mitochondrial Inheritance

Mitochondrial inheritance refers to the transmission of genetic material found in mitochondria, which is exclusively inherited from the mother. This pattern results in all offspring of an affected mother inheriting the mitochondrial traits, while offspring of an affected father do not. This unique inheritance pattern is crucial for understanding how mitochondrial disorders manifest in families.

Nuclear vs. Mitochondrial DNA

Nuclear DNA is the genetic material located in the cell nucleus, encoding the majority of an organism's genes, while mitochondrial DNA (mtDNA) is found in mitochondria and encodes a small number of essential proteins for energy production. Understanding the distinction between these two types of DNA is vital for analyzing genetic disorders, especially those involving mitochondrial dysfunction.

Recessive Inheritance

Recessive inheritance occurs when a trait or disorder manifests only when an individual has two copies of a recessive allele, one inherited from each parent. In the context of the question, if a mitochondrial defect is caused by a recessive mutation in nuclear DNA, it would require both parents to carry the mutation for the offspring to express the disorder, altering the expected inheritance patterns in the pedigree.