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Sibling Test-Full or Half

1. Sibling
2. Sibling

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     Test results:

     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.

     Likelihood ratio LESS than one=unlikely to be related

     Likelihood ratio GREATER then one=likely to be related. 
     (The range is from one to infinity).

     Full sibling: will indicated that you are likely or unlikely to share a common
     mother and father.

     Half sibling: will indicated that you are likely or unlikely to share a common
     parent (mother or father).

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 (see Table 1).

Understanding Sibling Genetics

Your unique DNA pattern is inherited from our parents.  Everybody has two sets of chromosomes, a total of 23 pairs or 46 chromosomes. You will inherit one chromosome from your mother and the other from you father.  Because the father has two copies of each of the 23 chromosomes, there is a 50% chance that he will randomly pass on a particular chromosome to his offspring.  Similarly, the mother also has two copies of each chromosome, and there is a 50% chance of her offspring getting either one of the two.  If two siblings share the same mother and father, theoretically, they should share 50% of their mother's chromosomes, and 50% of their father's chromosomes.  During a sibling test, many different chromosomes are analyzed.  If two people are full siblings, mathematically, 50% of the genes which are examined should be identical.  If two people are half siblings, 25% of their genes should be identical.  During a sibling test we analyze 16 different chromosomes are examined and compared.  The number of shared genes are analyzed and a sibling or kinship index is calculated. The sibling index indicates the probability that a random person in the population would have the shared genes examined.

Degrees of Relationship



Se Second


% genes in common






Identical twins



Great grandparent-Great grandchild


Full sibling

Half siblings

Great uncle-Great nephew



Half uncle-Half nephew



First cousins






Limitations of Sibling Testing

Table 1. Limitation of Sibling Testing









Children-Sharing Two Alleles
40.21% for true biological siblings and 10.16% for unrelated siblings

Child 1

F1 M1

Scores Very High

Child 2

F1 M1

Children-Sharing One Allele
50.48% for true biological siblings and 53.02% for unrelated siblings

Child 1

F1 M1

Scores High

Child 2

F1 M2

Children-Sharing No Alleles
9.31% for true biological siblings and 36.83% for unrelated siblings

Child 1

F1 M1

Scores Low

Child 2

F2 M2

In this example, the father has two alleles called F1 and F2.  The mother has two alleles M1 and M2. Each children they produce will receive one allele from each parent. Which allele each child will inherit is random.  The best case would be if the children share the same or one allele. Worst case is if they share no alleles.  The more alleles you test the greater the chance of finding a match. We are currently testing 16 alleles.

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 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). 

The limitations in sibling testing lie 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 is a random event.  Thus, it is possible that by chance, less than 50% of the genes are common or maybe even none of the genes are shared. Thus, even if two people do not share any of the genes examined, we cannot conclusively state that they are not true siblings.  This can be compared to the tossing of a coin. Theoretically, when a coin is tossed the chances of getting heads or tails would be 50/50. However, we do not always observe a 50/50 ratio. If a coin is tossed six times, theoretically, we should observe heads 3 of the six times and tails 3 of the six times. However, it is possible, though unusual that we would observe only heads or only tails all six times.

Increasing the Discrimination Power of Sibling Tests

A number of factors can drastically increase the discrimination power of sibling testing. For example, cases in which the siblings have the same mother but want to know if they have the same father, testing of the mother would greatly increase the discrimination power of the test and greatly increase the confidence level of the results on whether the siblings share the same father. Similarly, testing other relatives of the father such as grandparents, aunts and uncles would give valuable information and drastically increase the discrimination power of the test.  The best case would be to test an alleged father or fathers.  This would conclusively show whether the two children share the same father. 

Please feel free to call the office if you have any questions or concerns about sibling testing. Alternatively, you can e-mail a staff scientist at with your questions.

Male Siblings: If the two siblings are male and you are trying to determine if they share the same biological father than Y-chromosome testing my be an alternative approach. The Y-chromosome is the only genetic information that will be identical in both of the male siblings as long as they share the same father. If the male siblings have different biological fathers they will have two different Y-chromosomes.

Due to the lack of recombination, the allele combination of a number of Y chromosome specific STR loci has to be considered a haplotype. Except for mutation events, all male relatives of the paternal lineage will share the same allele combination. This means that the statistical significance of a Y-STR DNA match cannot be assessed by the product rule and estimation of the haplotype frequency is limited by the size of the haplotype database (1). This leads to reduced inclusion probabilities and a discrimination rate that is significantly lower than that for autosomal STR polymorphisms.  In other words, you should always proceed with the traditional STR analysis and you should use the Y-chromosome analysis to aid in the identification of siblings if the likelihood ratio is low.  For example, we had a recent case in which we tested two alleged brothers.  There test generated a likelihood ratio of 7.8 to 1.  This number indicated that the brothers were 7.8 times more likely related to each other.  7.8 is not a very convincing number but it was not less than one therefore it indicated that they were likely to be related.  We then proceed to generate a DNA profile using Y-STRs.  The Y-STR data that was generated indicated that the brother’s had different Y-chromosomes.  Therefore, they did not share the same biological father and that the likelihood ratio of 7.8 to 1 was generated just by random chance. 

1.  Jobling MA, Pandya A, Tyler-Smith C. The Y chromosome in forensic analysis and paternity testing. Int J Legal Med 1997;110:118-24.
2. Tzeng CH Lyou JY et al. Determination of sibship by PCR-amplified short tandem repeat analysis in Taiwan. Transfusion. 2000; 40: 840
3. Wenk RE Traver M Chiafari FA. Determination of sibship in any two persons. Transfusion. 1996; 36: 259-262.
4. Wenk RE Chiafari FA. Distinguishing full siblings from half-siblings in limited pedigrees. Transfusion. 2000; 40: 44-47.

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