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Inside a Cell

The nucleus of the cell contains all of the chromosomes, including the X and Y chromosomes.  The nucleus of the cell contains one X and one Y chromosome, if it's a cell from a male and two X chromosomes, if it's a cell from a female.  If it's an egg cell, it just contains one X chromosome.  If it's a sperm cell, it contains either a Y chromosome or an X chromosome.  When the egg and sperm get together, the sex of the resulting embryo depends on whether the lucky sperm cell was carrying an X or Y chromosome.  If the embryo winds up with two X chromosomes the child will be a female child and  if the embryo winds up with an X and a Y chromosome, the embryo will develop to be a male child.

The X chromosome's are autosomal chromosomes just like the rest of the 22 chromosomes, so they are of little value for genealogy beyond a generation or two.  The Y-chromosome does not recombine every generation with any other chromosome, so it is unique among chromosomes and that's why it is so valuable to genealogists. 

Outside the nucleus of the cell, there are other parts of the cell, including the mitochondrial DNA (mtDNA); which has no relationship to the nuclear DNA (the mtDNA is NOT a chromosome).  The mitochondria are the power plants for the cells.  The mitochondria produce the energy the cell needs to perform its functions.  A muscle cell would have many more mitochondria than say a lazy old skin cell.  The human body contains by volume, a lot more mtDNA than Y-DNA or X-DNA, and that's why the extraction of DNA from ancient bones or teeth is usually mtDNA.

The sperm cell has a very small amount of mtDNA; just enough to propel it upstream via its tail.  If the lucky sperm cell
has any mtDNA left when it penetrates the egg, the egg kills off the remainder.  Thus, the only mtDNA passed on to the embryo (male or female) is the mtDNA from the mother; and why the mtDNA is also a value tool for genealogists like the Y-chromosome. 

People are easily confused between the mtDNA (because we call it the female DNA) and the X-chromosome which is the 'female' chromosome.  However the mtDNA and the X chromosome are very different and they perform very different functions.

Let me know if this doesn't help you better understand these two important DNA functions that we as genealogists are now using to support our research.

Rob Noles

Sperm in comparison to the egg is a lot smaller then shown here.
 

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Last modified: September 02, 2008