Relationship DNA testing services are able to determine paternity by testing siblings, grandparents, and aunts/uncles.

Siblings test: full and half siblings

There are two types of relationships that siblings can share: Full siblingship and Half siblingship. Full Siblings will share two common parents and Half Siblings will share just one common parent. A sibling test provides the likelihood of genetic relationship between two possible siblings, specifically when no other relatives are available for study. Please note that a grandparentage or reconstruction test is more definitive than a siblingship test, especially if the mother is not involved.

Generally speaking, biological-relationship testing such as genetic reconstruction and sibling testing is not as definitive as DNA parentage testing. It is not possible to determine with absolute certainty whether two people are siblings or biologically related in some way other than the parent-child relationship. We can only test whether they are more or less likely to be related.

Grandparent test: with two grandparents

A grandparent test is performed to determine a child’s relationship to the alleged father’s family when the alleged father is unavailable or unwilling and no post-mortem sample is available. By comparing the DNA patterns of both paternal grandparents with the child, we can determine a “probability of grandparentage.”

Grandparent test: with one grandparent

A single grandparent test is performed to determine a child’s relationship to the alleged father’s family when the alleged father is unavailable or unwilling and no post-mortem sample is available. By comparing the DNA patterns of one of the paternal grandparents with the child's DNA, we can determine a “probability of grandparentage.” 

When testing one grandparent, always consider testing the grandmother in lieu of the grandfather if she is available. As with all paternity testing, the father (and the grandfather in this scenario) may be in question. If the laboratory determines the results are inconclusive, they may suggest testing additional relatives. 

Aunt/Uncle testing:

This relationship test can determine if two individuals are related to each other as aunt—nephew (or niece) or uncle—nephew.

The avuncular testing option should be discussed if the alleged father is deceased or unavailable, and post-mortem, grandparentage, or reconstruction testing cannot be performed. Generally speaking, biological relationship testing is not as definitive as DNA parentage testing. It is not possible to determine with absolute certainty whether two people are siblings or biologically related in some way other than the parent-child relationship. We can only test for whether they are more or less likely to be related.