page_title: Human mitochondrial genetics
section_title: Inheritance patterns
hierarchical_section_title: Human mitochondrial genetics / Inheritance patterns
caption_attribution_description: English: Fertilisation phases of a human egg. The middle part of the sperm with mitochondria of the father remains outside the zygote. The mitochondria of the mother are already in the oocyte.
context_page_description: Human mitochondrial genetics is the study of the genetics of human mitochondrial DNA. The human mitochondrial genome is the entirety of hereditary information contained in human mitochondria. Mitochondria are small structures in cells that generate energy for the cell to use, and are hence referred to as the "powerhouses" of the cell.
  Mitochondrial DNA is not transmitted through nuclear DNA. In humans, as in most multicellular organisms, mitochondrial DNA is inherited only from the mother's ovum. There are theories, however, that paternal mtDNA transmission in humans can occur under certain circumstances.
  Mitochondrial inheritance is therefore non-Mendelian, as Mendelian inheritance presumes that half the genetic material of a fertilized egg derives from each parent.
  Eighty percent of mitochondrial DNA codes for mitochondrial RNA, and therefore most mitochondrial DNA mutations lead to functional problems, which may be manifested as muscle disorders.
  Because they provide 30 molecules of ATP per glucose molecule in contrast to the 2 ATP molecules produced by glycolysis, mitochondria are essential to all higher organisms for sustaining life.
context_section_description: Because mitochondrial diseases (diseases due to malfunction of mitochondria) can be inherited both maternally and through chromosomal inheritance, the way in which they are passed on from generation to generation can vary greatly depending on the disease. Mitochondrial genetic mutations that occur in the nuclear DNA can occur in any of the chromosomes (depending on the species). Mutations inherited through the chromosomes can be autosomal dominant or recessive and can also be sex-linked dominant or recessive. Chromosomal inheritance follows normal Mendelian laws, despite the fact that the phenotype of the disease may be masked.
  Because of the complex ways in which mitochondrial and nuclear DNA "communicate" and interact, even seemingly simple inheritance is hard to diagnose. A mutation in chromosomal DNA may change a protein that regulates (increases or decreases) the production of another certain protein in the mitochondria or the cytoplasm; this may lead to slight, if any, noticeable symptoms. On the other hand, some devastating mtDNA mutations are easy to diagnose because of their widespread damage to muscular, neural, and/or hepatic tissues (among other high-energy and metabolism-dependent tissues) and because they are present in the mother and all the offspring.
  The number of affected mtDNA molecules inherited by a specific offspring can vary greatly because
  the mitochondria within the fertilized oocyte is what the new life will have to begin with (in terms of mtDNA),
  the number of affected mitochondria varies from cell (in this case, the fertilized oocyte) to cell depending both on the number it inherited from its mother cell and environmental factors which may favor mutant or wildtype mitochondrial DNA,
  the number of mtDNA molecules in the mitochondria varies from around two to ten.
  It is possible, even in twin births, for one baby to receive more than half mutant mtDNA molecules while the other twin may receive only a tiny fraction of mutant mtDNA molecules with respect to wildtype (depending on how the twins divide from each other and how many mutant mitochondria happen to be on each side of the division). In a few cases, some mitochondria or a mitochondrion from the sperm cell enters the oocyte but paternal mitochondria are actively decomposed.
caption: The reason for maternal inheritance in mitochondrial DNA is that when the sperm enters the egg cell, it discards its middle part, which contains its mitochondria, so that only its head with the nucleus penetrates the egg cell.
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