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Learn more about the power of genetics and the scientific work going on at the Genographic Project. Our Computational Biology Center (CBC) team, part of IBM’s global Research organization, will provide regular updates here at CompBio Central.
A genetics refresher
For our introductory CompBio Central article, we thought it would be helpful to describe some of the key concepts behind the Genographic Project and the use of genetics to study our migratory history.
DNA-matter of heredity
The most significant biological material we inherit from our parents is the DNA that we carry in our cells. The DNA, which carries our genetic instructions, is wrapped as chromosomes. In humans, there are 23 pairs of chromosomes-one set from the mother, the other from the father. Of these, 22 are like pairs, but the 23rd pair is different. In females this pair has two like chromosomes called X. Males have one X and one Y. The Y chromosome is transmitted from fathers only to their sons.
In addition to the DNA in our chromosomes, we also inherit the DNA carried in mitochondria, a structure within each cell, which we inherit exclusively from our mother.
It takes two
As cells divide to produce new cells, they replicate the DNA within to give each new cell a genetically identical copy of the parent cell. The cell division that produces eggs (female) or sperm (male) in our reproductive system is different in two ways.
- The resulting new cell (egg or sperm) has only one set of chromosomes, not a pair.
- The composition of each chromosome in the new cell is a recombination of segments from maternal and paternal chromosome pairs in the parent cell.
Each resulting egg or sperm is genetically distinct. But a major fraction of the Y chromosome does not undergo such recombination, since it is passed only from father to sons and is unaffected by the X chromosome from the mother. It is this unique feature that makes the Y chromosome useful to genealogists.
DNA sequence variations
Occasionally, the DNA copying process introduces mutations or variations in the DNA sequence, making the egg or sperm differ in sequence from the parental source. These mutations are carried into future generations and serve as markers of descent.
Markers of descent
Looking at non-recombinant portions of the human genome, it is possible to associate specific genetic variations to a population group and infer that they must share a common ancestor. We can also conclude that this common ancestor was the first individual with that mutation and has passed it on to all his or her descendents. Such markers of descent have been identified on the Y chromosome and mitochondrial DNA.
- The markers on Y chromosome describe the paternal line of ancestry-the father's father.
- The markers on mitochondrial DNA describe the maternal line of ancestry-the mother's mother.
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