If the father's or mother's DNA is not available for testing (see forensic paternity section for alternative sources of DNA) we can determine a genetic relation by testing the siblings or other close genetic relative.  A sibling test is a more complicated statistical analysis than a paternity test and may or may not give you the answer you need to answer questions about paternity or maternity. 

Full Siblings
A full sibling test will indicated that you are likely or unlikely to share a common mother and father. 

Half Siblings
A half sibling test will indicate that you are likely or unlikely to share a common parent (mother or father).

A sibling test will produce a likelihood ratio. The likelihood ratio is sometimes called a kinship index, sibling index or combined sibling index.  A likelihood ratio summarizes the information generated from the DNA test.  A likelihood ratio LESS than one=unlikely to be related. A likelihood ratio GREATER than one=likely to be related. (The range is from one to infinity). 

We understand sibling and relative genetics are complicated, therefore, we encourage you to call the office if you have any questions or concerns.  Toll free:  866-344-8378.

Likelihood Ratio:  the probability that some event will occur under a set of conditions or assumptions divided by the probability that the same event will occur under a set of different mutually exclusive conditions or assumptions.  In sibling testing it would be the chance that an allele (s) would occur if two children were siblings divided by the chance the allele occurring if the two were unrelated.

Identical

First

Se Second

Third

% genes in common

100

50

25

12.5

Relationship

Identical twins

Parent-child

Grandparent-grandchild

Great grandparent-Great grandchild

Full siblings

Half siblings

Great uncle/aunts-Great nephew/niece

Aunt-niece

Half uncle/aunts-Half nephew/niece

Uncle-nephew

First cousins

Limitations of Sibling Testing:  In a paternity or maternity test, there are certain obligatory paternal and maternal genes which must be observed in both the child and his/her biological parents.  Obligatory genes are genes which must be observed in order for a positive relationship to be established.  This allows conclusive results for all parentage testing cases. The father or the mother of the child can be confirmed or ruled out by identifying these genetic markers.  These markers occur in pairs and are passed from each parent to the child.  For each pair of markers, one comes from the mother (the maternal marker or allele) and the other comes from the father (the paternal marker or allele). See the results explained section for paternity testing.

The limitations in sibling testing is located in the fact there are no obligatory sibling genes.  Therefore, even if none of the genes examined are shared by two siblings in a sibling analysis, it cannot be concluded that the two people are not true siblings (see Table 1). This is because the inheritance of genes from the parents are random events.  For example, lets call one chromosome A and the other B.  Whether you get the A chromosome or the B chromosome can not be predicted.  There is a 50% chance you will either get the A or B chromosome.  If you both siblings get the A chromosome it will score high, if you both siblings get the B chromosome, it will score high but if by chance one sibling gets the A chromosome and the other sibling gets the B chromosome it will score low.

Grandfather

Uncle

Male Child

Great Grandfather

The 17 Y-STR loci provides the greatest power of discrimination for paternity, sibling, grandparentage and other male kinship analysis.
* DYS385 can be polymorphic showing two distinct markers.