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. |
