To test if a Grandfather is related to a grandson is very accurate using a Y Chromosome DNA test.

All men have one Y chromosome which is passed down, generation to generation through their son's. If a Grandfather and a Grandson have the exact same Y chromsome, it supports them having the same male ancestory (being related).

Because the Y chromosome DNA needs to be exactly the same to support a positive relationship result, the test is highly accurate, above 99.9%.

If you are testing a Granddaughter (females do not have a Y chromsome) then other test types are required. (see below).

Y Chromosome DNA test - Details and cost

For Grandfather to Grandson testing a Y-chromosome test the best option to achieve a high accuracy with relatively low cost. Because all men have one Y chromosome (and females have no Y's). A father will pass his Y chromosome to all his sons. So a Grandfather and Grandson will have the exact same Y chromsome if they are biologically related.

Grandmother to Granddaughter, the flow of X Chromosome DNA

To test if a Grandmother is related to a granddaughter is very accurate using an  X Chromosome DNA test.

All males have one X-chromosome that they have recieved from their Mother. Subsequently, this single maternal X chromosome is passed from father's to daughters. (Son's receive a Y chromosome from their fathers).

So, by testing the two X chromosomes in a grandmother, and the two X chromosomes in the granddaughter, we can determine if they are biologically related by finding if they both have one X chromsome exactly the same.

From the illustration on right, you can see the flow of X chromosome from Grandmother to Granddaughter, via the father.

X Chromosome DNA test - Details and cost

For Grandmother to Granddaughter tests, X-chromosome testing is your least expensive option while obtaining very high accuracy (99.9%+). Men have one X chromosome that they receive from their mother. So, a grandmother will pass one of her X chromosomes to her son (the father), and then the father will pass that X chromosome to his Daugther. Therefore a biologically related Grandmother and Granddaughter will have the exact same X chromosome patterns.

Mitochondrial DNA

Cells that make up a human being contain two forms of DNA.

1. Nuclear DNA.
2. Mitochondrial DNA.

Nuclear DNA is found in the nucleus and contains all the genes that make up who we are. Nuclear DNA is the type of DNA that police use to profile a person, and what DNA labs use to perform a paternity test.

Mitochondrial DNA is a tiny circle of DNA that is found inside small cell packets called Mitochondria. Mitochondria known as the "power house of the cell" are used to create energy for the cells.

Sperm cells have mitochondria in the tail of the sperm, and not in the head of the sperm. When the sperm fertilizes a egg only the sperm head enters the egg, and the tail is dropped off. Thus, children will only recieve mitochondrial DNA from their mother. Testing the sequence of mitochondrial DNA in siblings will show if they are biologically related via the maternal line.

Mitochondrial Inheritance

Since in a fertilized egg only the mother's mitochondrial DNA is present we can find the exact same mitochondrial DNA in all children to the same mother.

Daughters will continue to pass the same mitochondrial DNA down to their children, while men will never pass on their mitochondrial DNA.

Thus mitochondrial DNA testing is a very powerful and accurate way to test for biological siblings. Mitochondrial testing is currently used by military DNA laboratories to identify skeletons discovered in old war zones, by tracing the mitochondrial DNA back to living relatives.

The US military is still using mitochondria DNA testing to identify bone fragments from world war II, Vietnam and Korean wars, and many other conflicts.