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 DNA Testing for Relationship > Kinship > Siblingship

 DNA Siblingship Test

Full siblings
Half siblings

- same biological father and biological mother
- same biological father but different biological mother
  same biological mother but different father
- identical twins and fraternal twins

Tests to find out the paternity and/or maternity of 2 persons, full or half siblings
  • Only test two alleged siblings
  • Two alleged siblings + father or mother or both

Interpretation of siblingship test results, full sib, half sib or unrelated?
Combined Siblingship Index Probability, W% Likelihood Description, for Full & Half Siblingship
≥ 20 ≥ 95% Most likely are siblings
2 - 19 50 - 90% Likely, but inconclusive
Testing at least one of the parents and/or other relatives are highly encouraged; additional genetic analyses are also desirable
≤ 1 ≤50% Exclusion

Sibling analysis is not as conclusive as paternity testing. The results will provide a probability that two individuals are biologically related, but it cannot provide a yes/no answer. On average full siblings will share more genetic markers than half siblings, and half siblings share more genetic markers than unrelated persons.

Siblingship Test Extended Info
Analyses of our cases involving siblings show that the calculated Combined Siblingship Indices (CSI)(Probabilities) of full siblings, half siblings and unrelated individuals sometimes overlap, resulting in uncertainty of the relationship. (JWL 2016, GenePro)
  Range, CSI Range, Probability Median CSI Probability
Full siblings 0.01 – 1,000,000,000 0.01% to ≥99.9999% 10,000 99%
Half Siblings 0.001 – 60,000 0.001% to 99.99% 12 92%
Unrelated 0.0005 - 15 0.0005% to 93% 0.15 13%
If based on the CSI alone, full siblings could be interpreted as half siblings or even unrelated, and vice versa.
Similar observation was reported in the AABB Annual Report, 2010.

To make the sibling-ship test more definitive:
In cases where a standard marker panel does not yield an unequivocal result, the current trend is extended testing -
  a. Two alleged siblings + 1 or both grandparents
  b. Two alleged siblings + other siblings and/or blood relatives
  c. A mother or both mothers if are available should be tested;
  d. Use more genetic markers on different chromosomes;
  e. If the siblings are male, the Y-chromosome can be tested to determine if they have the same paternal lineage.
Otherwise, other evidence (such as documents, eye witnesses) must be used to support a relationship whenever a low sibling index is obtained.