Children born with genetic diseases are often born to healthy couples who did not know they were carriers for diseases or conditions. There are steps a couple trying to conceive can take to minimize or eliminate passing the genetic mutation to their child which include genetic screening before pregnancy, preimplantation genetic diagnosis (PGD) and preimplantation genetic screening (PGS). Genetic screening before pregnancy includes a blood test to look for abnormal genes that could be passed to your child. PGD is utilized during an in vitro fertilization cycle to remove a cell or group of cells from an embryo, composed of DNA from both the egg and sperm, while it is still developing in the laboratory for genetic testing.
Genetic screening is most effective when completed before trying to conceive. Screening before pregnancy is important because carriers of a condition may not have any symptoms indicating they carry the abnormal gene.
There are several ways that a genetic condition can be passed on, they include:
- Autosomal dominant inheritance requires only one copy of the gene for the person to be affected
- Autosomal recessive inheritance requires two copies of the gene (one copy from each parent) to be affected
- X-linked dominant inheritance is passed only on the X chromosome, can affect both males and female, but generally more severely affects male offspring
- X-linked recessive inheritance is passed only on the X chromosome, and generally affect only the male offspring. Female offspring will carry a copy of the abnormal chromosome, but usually do not display symptoms
- Y-linked inheritance is passed only on the Y chromosome and every son will be affected
- Mitochondrial inheritance is also called maternal inheritance because only the egg contributes mitochondria to the developing embryo
PGD is important for people who have a genetic disease and want to transfer only embryos that are not affected with the mutation. PGD can also be done in the event pre-pregnancy genetic screening was not performed. Biopsy of the embryo occurs on day 3 or day 5 of embryonic development while the embryo is still developing in the laboratory. PGD can screen for a single gene defect, an array of potential defects or for sex selection.
PGS is available when chromosomally normal genetic parents are screened for aneuploidy. Aneuploidy occurs during meiosis, a form of cell division, when either the sperm or egg should each have half of the chromosomes needed for completion of an embryo. During meiosis, a cell divides into two cells and on occasion there will be an error in the division resulting in one receiving two copies of a gene and the other not receiving any copy of the gene. When the abnormal egg or sperm develops into an embryo it will have chromosomal abnormalities which are a common cause of birth defects. Many cases of aneuploidy are not compatible with life.
These tests are invaluable for women with a history of multiple miscarriages or older women whose eggs often have more chromosomal abnormalities as they age. Chromosomal errors can account for unsuccessful attempts at pregnancy or failing to maintain a pregnancy to term. Genetic testing, as described above, may allow a healthy pregnancy to develop.