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DNA Sibship Analysis
Analysis of the DNA of two
separate individuals can provide information as to whether they
are full siblings, half siblings, or not related. However, unlike
paternity testing which provides very conclusive results: 100% for
non-paternity and greater than 99.9% for paternity inclusion, the
answers provided by sibship testing are not always so conclusive.
If you are considering sibship testing, please read the following
information to understand how sibship is analyzed, its limitations,
and how the chances for getting meaningful results could be improved.
If you have further questions, please contact Genetrack directly
at 1-888-828-1899 to speak with a DNA advisor.
Understanding Sibling Genetics
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, 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, 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 sibship test, at least 16 different genes are examined and compared.
The number of shared genes 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 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. 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 Genetrack or
contact us online.
Information and Appointments:
By phone: 1-888-828-1899 or (604) 325-7282
By Fax: 1-888-655-8877 or (604) 325-2208
Email: lab@genetrack.bc.ca
Appointments can also be arranged online by using the secure
online appointment booking form.
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