Sibling test
With a sibling test , analogous to a paternity test , by examining certain genomic DNA areas, the degree of relationship between the siblings can be calculated. This special type of parentage test is used when the alleged father (the so-called putative father) cannot be tested. In specialist circles, such an ancestry constellation is somewhat unfortunately referred to as a deficiency case.
To compensate for the DNA of the alleged father, the genetic material of a close relative (brother, sister or the father or the mother of the possible father) can be examined on behalf of the latter. The aim of a sibling test (with at least two siblings) is to find out whether both siblings have the same father. The mathematical formulation of the problem uses Bayes' theorem on conditional probabilities, which was established in the 18th century , and reads here:
- Hypothesis H0 assumes that both siblings are full siblings, and the
- Alternative hypothesis H1 assumes that both siblings have different fathers.
The basis for calculating the probabilities of the two hypotheses are the DNA profiles (genetic fingerprints) of the people tested, determined in the molecular biological laboratory. The degree of relationship can be calculated using genetic similarities between the siblings. The calculation also includes whether a common hereditary trait occurs frequently or rarely in the population group under consideration . It is clear that the same genetic trait among the siblings, which is rarely observed, increases the likelihood of descent.
If you test more than two siblings, you can see with the naked eye whether alleles of more than two people can be recognized on a gene location. If so, it is a strong argument that one of the siblings has a different father than the others. For the quantitative consideration of the entire analysis, however, it is essential to derive the formulas for the probability quotients of the hypotheses made for the individual ancestry scenarios.