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How the transfer of mtDNA and Y-DNA work generation to generation.

For me, sometimes the best way to understand something is to graph it. Many people have asked if DNA testing could solve a genealogy problem for them if they can't find either a living male and/or living female descendent. The answer is, it depends.

For instance, if you're trying to track down a direct male ancestor, it's very easy. If you're trying to track down a direct female line of descent, it's very easy. BUT if you're trying to track down a non-direct line, chances are it won't work. Recently, we had a request to test a female in a situation similar to the Y-DNA 3 at the bottom right of the chart below, the one in the bottom-right pink circle. However, it was more complicated than that. She was not a Sinclair, but came from a distant grandfather who was. This kind of testing requires work from all parties to find a living relative of a line. For a different project, we're currently searching for specific Sinclairs whom we believe may now live in Barbados and others in Australia.

Whenever an egg cell is fertilized, nuclear chromosomes from a sperm cell enter the egg and combine with the egg's nuclear DNA, producing a mixture of both parents' genetic code. The mtDNA from the sperm cell, however, is left behind, outside of the egg cell.

So the fertilized egg contains a mixture of the father and mother's nuclear DNA and an exact copy of the mother's mtDNA, but none of the father's mtDNA. The result is that mtDNA is passed on only along the maternal line. This means that all of the mtDNA in the cells of a person's body are copies of his or her mother's mtDNA, and all of the mother's mtDNA is a copy of her mother's, and so on. No matter how far back you go, mtDNA is always inherited only from the mother.

If you went back six generations in your own family tree, you'd see that your nuclear DNA is inherited from 32 men and 32 women. Your mtDNA, on the other hand, would have come from only one of those 32 women.

While it is commonly known that the Y - Chromosome is one of the 46 chromosmomes found in the nucleus of most cells in the body, mitochondrial DNA is entirely different. It is found as a cell inclusion throughout the cell - since its purpose is to act as a mini energy packet that helps to power the sundry functions of the cell. There are thousands or these bacteria - like organelles in each cell (again, each one coming from the mother).

Sources

"Human Evolution." Svante Pääbo. Trends in Genetics. 15(12): M13-M16, 1999.

"Neanderthal DNA Sequences and the Origin of Modern Humans." Matthias Krings, et al. Cell, July 11, 1997.

"Mitochondrial DNA and Human Evolution." Rebecca L. Cann, Mark Stoneking, Allan C. Wilson. Nature, January 1, 1987.

"The Case of Mitochondrial Eve." Frank R. Zindler. American Atheist, February 1988.

Shreeve, James. The Neanderthal Enigma: Solving the Mystery of Modern Human Origins. New York: Avon Books, 1995.

Stringer, Christopher; Clive Gamble. In Search of the Neanderthals: Solving the Puzzle of Human Origins. New York: Thames and Hudson, Inc., 1993.

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