Maternal Inheritance Of mtDNA

For forensic and human identification purposes, human mitochondrial DNA is considered to be inherited strictly from one's mother. At conception only the sperm's nucleus enters the egg and joins directly with the egg's nucleus.

The fertilizing sperm does not contribute other cellular components. When the zygote cell divides and a blastocyst develops, the cytoplasm and other cell parts save the nucleus are consistent with the mother's original egg cell. Thus, mitochondria with their mtDNA molecules are passed directly to all offspring independent of any male influence.

Eggs have been reported to have as many as 100 000 mtDNA molecules creating an extreme dilution for any paternal mtDNA molecules that may pass into the zygote (Chen et al. 1995). Furthermore, any sperm mitochondria that may enter a fertilized egg are selectively destroyed due to the presence of an ubiq-uitin tag added during spermatogenesis that appears to earmark sperm mitochondria for degradation by the newly formed embryo's cellular machinery (Sutovsky et al. 1999). Thus, barring mutation, a mother passes along her mtDNA type to her children, and therefore siblings and maternal relatives have an identical mtDNA sequence. Hence, an individual's mtDNA type is not unique to them.

An example family pedigree is shown in Figure 10.2 to demonstrate the inheritance pattern of mtDNA. In this example, unique mtDNA types exist solely for individuals 1, 5, 7, and 12. Note that individual 16 will possess the same mtDNA type as seven of the other represented individuals (e.g., 2, 3, 6, 8, 11, 13, and 15).

Figure 10.2

Illustration of maternal mitochondrial DNA inheritance for 18 individuals in a hypothetical pedigree. Squares represent males and circles females. Each unique mtDNA type is represented by a different letter.

This fact can be helpful in solving missing persons or mass disaster investigations but can reduce the significance of a match in forensic cases. Since even distantly related maternal relatives should possess the same mtDNA type, this extends the number of useful reference samples that may be used to confirm the identity of a missing person. Evidence from mtDNA has been helpful in linking families and solving historical puzzles such as identifying the Vietnam Unknown Soldier (D.N.A. Box 10.1) and the Romanov family (D.N.A. Box 10.2).

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