Genetic screening is the process of testing for a genetic disease in order to identify a subgroup of people that either have the disease or the potential to pass it on to their offspring. Preimplantation genetic diagnosis (PGD) helps to provide genetic information on embryos obtained through in vitro fertilization (IVF), thereby allowing implantation of only those embryos identified as unaffected with a given genetic or chromosomal disorder.
Preimplantation genetic diagnosis is recommended when couples are at risk of transmitting a known genetic abnormality to their children. Only healthy and normal embryos are transferred into the mother’s uterus, thus diminishing the risk of inheriting a genetic abnormality and late pregnancy termination. As the number of patients undergoing IVF increases, the use of Preimplantation Genetic Diagnosis broadens, and more mutations underlying genetic disorders are identified, PGD is becoming more widespread.
Preimplantation genetic diagnosis allows physicians to analyze an embryo formed outside the mother’s uterus for genetic defects that would prevent that embryo from implanting, produce a miscarriage or cause an inherited disease upon birth. Preimplantation genetic diagnosis refers specifically to when one or both genetic parents have a known genetic abnormality and testing is performed on an embryo to determine if it also carries a genetic abnormality. As the complete cure for many genetic diseases are not likely to be found soon; preventing the disease is preferable to waiting for a possible cure to eventually become available. Doing screening for genetic diseases prior to implantation will prevent termination of pregnancy following prenatal testing (chorionic villous testing, amniocentesis) which can be a difficult and traumatic decision to follow through with.
Preimplantation genetic diagnosis involves testing the early embryo after in vitro fertilisation. One or two cells (blastomeres) are removed at biopsy from the preimplantation embryo at the 6-10 cell stage (day 3 of development), thus allowing replacement into the uterus of unaffected embryos. The isolated cells are evaluated for specific genetic conditions. Embryos that are found to be unaffected are transferred back to the woman’s uterus on day five of embryo development. Preimplantation Genetic Diagnosis can be used to diagnose 3 major groups of disease:
Sex-linked disorders – Sex-linked diseases are passed down through families through one of the X or Y chromosomes. X and Y are sex chromosomes. Dominant inheritance occurs when an abnormal gene from one parent causes disease, even though the matching gene from the other parent is normal. The abnormal gene dominates. Examples of such disorders include Duchenne and Becker muscular dystrophy, haemophilia, red-green colour blindness, congenital night blindness, some high blood pressure genes.
Single gene defects – Single gene disorders are caused by DNA changes in one particular gene, and often have predictable inheritance patterns. Over 10,000 human disorders are caused by a change, known as a mutation in a single gene. Sickle cell anaemia, cystic fibrosis, Tay-Sachs disease and Huntington disease are some examples.
Chromosomal disorders – Chromosomal disorders are where chromosomes (or parts of chromosomes) are missing or changed. Examples of chromosomal abnormalities include Down syndrome, Trisomy 18 known as Edwards Syndrome, Trisomy 13, Klinefelter syndrome, XYY syndrome, Turner syndrome and triple X syndrome.
In the past, persons with a genetic disease or those who know that they are carriers frequently choose not to have children in order to avoid the risk of passing on the disease to future generations. Preimplantation Genetic Diagnosis now allows these couples the opportunity to have a child free of their particular disease.
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