Preimplantation genetic diagnosis is when tests are performed on a pregnant women to determine if the embryo carries a defective gene. The testing is especially valuable when one or both of the parents is carrying a known genetic abnormality.This is different from certain other testing known as preimplantation genetic screening, which specifically refers to the testing performed on embryos whose parents have not been diagnosed with any known genetic defect.
This screening is for aneuploidy, which is having a number of chromosomes that is different than what is normal for that species. Down Syndrome is one condition where the number of chromosomes differs. Every species, including humans, possess a certain number of chromosomes unique to them.
Self correction refers to studies that show embryos may have the ability to stop the growth or multiplication of abnormal cells and alter their identifying chromosomal structure into that of a normal genetic code.
The findings of self correction provides researchers with the opportunity to study embryonic development and stem cells without having to specially develop embryos first. These embryos would have been discarded by the couples seeking fertility treatments. The couples who screened for abnormalities by using the preimplantation genetic diagnosis would have been faced with an embroyo that failed to implant itself after the in vitro application, or would have aborted itself due to the abnormality or would have begun to grow despite the faulty chromosome.
The reason for the self corrective genetic makeover is unclear, but it has been speculated that somewhere between the time when the fertilized egg started to divide and the blastocyst stage of development, the embryo lost the faulty chromosome and started to grow normally. While the embryos are unable to grow into fetuses outside of the womb, the discovery of the potential for self correction has profound implications for stem cell research.