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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.
