For Grandfather to Grandson tests,
*About Y-Chromosome tests:
Y-chromosome testing is your least expensive option while obtaining very high accuracy (99.9%+). Think about the father of the child (the son of the Grandfather), if this man is NOT the biological father of his son, then you need know if there is any possibility that the true biological father was not a close relative. (ie, could other possible father be a brother, cousin or other type of close relative to the alledged father?)

If the answer is no, then a Y-chromosome test is a great option. All men have one Y chromosome, it is passed down from Father to Son, woman have no Y-chromosome DNA. Therefore a Grandfather, his son and his 3 grandsons will all have the SAME Y-chromosome DNA

For Grandmother to Granddaughter tests,
*About X-chromosome tests:

X-chromosome testing is your least expensive option while obtaining very high accuracy (99.9%+). Think about the father of the child (the son of the Grandfather), if this man is NOT the biological father of his son, then you need know if there is any possibility that the true biological father was not a close relative. (ie, could other possible father be a brother, cousin or other type of close relative to the alledged father?)

If the answer is no, then a X-chromosome test is a great option. All men have one X chromosome, it is passed down from Father to Daughter, woman have two X-chromosomes. Therefore a Grandmother, her son, and her 3 Granddaughters will all have at least one of their X-chromosomes exactly the same.