Preimplantation
genetic diagnosis (PGD) allows the embryologist and
physician to screen embryos for certain inherited genetic
diseases. These diseases include Fragile X Syndrome,
Sickle Cell Anemia, Tay Sachs, Cystic Fibrosis, Hemophilia,
Thalassemia, and others.
Each
cell in the body, and in an embryo, contains the full
genetic blueprint of a person and the chromosomes can
be examined for abnormalities. Therefore, some genetic
diseases can be identified by examining the chromosomes
in an embryonic cell.
Once
an embryo has fully matured and is ready for transfer,
a small hole is made in the outer membrane (zona pellucida)
using acid Tyrode's solution, a needle, or a laser and
a single cell is withdrawn. The embryo is not damaged
by this process and will develop normally.
The
chromosomes are analyzed using a process known as fluorescent
in situ hybridization (FISH) and the polymerase chain
reaction. The polymerase chain reaction "amplifies"
or "duplicates" sections of the chromosome
allowing examination of specific segments.
Some
diseases are "sex linked" meaning they are
contained on the X or Y chromosome. Hemophilia is carried
on the X chromosome and a mother with the disease or
who is a carrier of the gene abnormality has a very
high probability of passing it to a male child. Diseases
such as Down's syndrome are caused by an extra chromosome
(chromosome 21).
After PGD, only embryos that
are not carrying the genetic disorder are transferred
into the uterus. Many researcher hold great hope that
advances in PGD will allow for the screening of other
diseases and identify embryos that are at high risk
for miscarriage.
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