Sibling DNA testing is used to determine whether the two people in question share the same biological parents, especially when those parents are not available for testing. Sibling testing can determine if the test subjects are full siblings or half siblings.
Full siblings are individuals who are born of both biological parents. Half siblings share only one biological parent, either the mother or the father.
Sibling analysis is not as conclusive as paternity testing. The results will provide a statistical likelihood of whether two individuals are true biological relatives, but sibling analysis cannot provide a conclusive result. The statistical likelihood in this case is expressed as a likelihood ratio. The likelihood ratio summarizes the information generated from the DNA test. A likelihood ratio of LESS than one means that the test subjects are unlikely to be related. A likelihood ratio GREATER than one means it is likely that the test subjects are related. (The range is from one to infinity)
If any of the parents of the siblings who are being assessed are available for testing, this will increase the certainty of the result. As in a paternity test a DNA profile is produced for each person made up of a number of DNA markers. The DNA profiles of the two people are then compared.
On average full siblings will share more DNA markers than half siblings, and half siblings share more DNA markers than unrelated people. The statistical analysis performed will determine the most likely relationship given the DNA results.
There are different variations of sibling testing.
BROTHERS Y-TEST
This is premised on the fact that all men have one Y chromosome which they pass to all their sons. By testing this Y chromsome in two or more brothers, we can see if they have the same father. The results of sibling testing for males is 99.9%.
SISTERS X-TEST
The basis of this test stems from the fact that all men have one X chromosome which they pass to all their daughters. By testing the X chromosome in two sisters, it is possible to detect if the test subjects have the same father. This test is best carried out for sisters with different mothers.
MITOCHONDRIAL TEST
Mitochondrial DNA is only inherited from the mother, so it is the ideal test to see if siblings have the same biological mother. The accuracy of the test is highly dependent on the type of mitochondrial DNA you have. If two matching persons have a rare type of Mitochondrial DNA then they are more likely to be siblings, than two persons who match with a common type of mitochondrial DNA.
The full sibling test is required to test a brother and sister for the same father.