Understanding Sibling Genetics

Siblings & DNA

Each person is made up of billions of cells, and most of the cells in our bodies contain a full set of genetic information in the form of DNA (deoxyribonucleic acid). DNA acts much like a genetic "blueprint", dictating how we look and develop. The DNA found in every person is as unique as a fingerprint and except for identical twins, and no two people share the same DNA pattern. Our unique DNA pattern is inherited from our parents. Each person has DNA in the form of 23 pairs of chromosomes. One chromosome of each pair is inherited from our mother, and the other chromosome of each pair is inherited from our father.

Unlike most of the cells in our bodies, the sex cells (sperm and egg) only contain a single copy of each of the 23 chromosomes. Thus, when a sperm and an egg unite, the 23 pairs of chromosomes come together to form a complete set of 23 pairs of chromosomes, one from each parent.

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 sibship test, many different chromosomes are analyzed. If two people are full siblings, statistically, approximately 50% of the genes which are examined should be identical. If two people are half siblings, approximately 25% of their genes should be identical.

During a sibship test, at least 16 different genes are examined and compared. The number of shared genes and the pattern in which they are shared are analyzed, and a sibship index is calculated. The sibship index indicates the probability that a random person in the population would have the shared genes examined.

Limitations of Sibship Testing

In paternity and maternity testing, there are certain obligatory paternal and maternal genes which must be present in both the child and his/her biological parents. Obligatory genes are genes which must be present in order for a positive relationship to be established. This allows conclusive results for all parentage testing cases. The limitations in sibship testing lie in the fact there are no obligatory sibship genes. Therefore, even if none of the genes examined are shared by two siblings in a sibship analysis, it cannot be concluded that the two people are not true siblings. 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 Sibship Tests

A number of factors can drastically increase the discrimination power of sibship testing. For cases in which the siblings have the same mother but want to know if they have the same father, testing of the mother would 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 additional children, grandparents, aunts and uncles would give valuable information and drastically increase the discrimination power of the test. If the parents are available for testing, the best case would be to test the parents directly, as this would conclusively show whether the two children share the same parents.

For more information or to see whether DNA testing can help you with your particular situation, please call Genex or contact us online.

Back to DNA Sibship Test »