Genetic disorders at a glance
- Genes control the development and function of the human body; a genetic disorder or disease is caused by one or more mutations in the genetic code.
- Genetic disorders can be inherited from parents, or develop sporadically.
- Genetic testing with in vitro fertilization (IVF) such as PGD can help determine if an embryo carries a gene mutation.
What are genetic disorders?
Genes carry instructions that control the development and function of the human body. Genes control all physical and medical characteristics, such as hair color, blood type and susceptibility to disease. A genetic disorder is a disease caused by a mutation in one gene, by mutations in multiple genes, by a combination of gene mutations and environmental factors, or by damage to chromosomes (the structures that carry genes).
Some diseases are inherited from parents and are present in an individual at birth, like sickle cell disease. Other diseases are caused by acquired mutations in a gene or group of genes that occur in the egg (or sperm). Such mutations are not inherited from a parent, but occur randomly. Thus, some genetic disorders show up spontaneously in one individual, with no previous family history. Descendants may inherit a disease or be carriers of the disease without being affected.
How genetic tests work with IVF
Five days after a woman’s egg is fertilized during IVF, a healthy embryo is at the Blastocyst or 80-100 cell stage. For preimplantation genetic testing, a few cells are removed from the embryo. This neither damages the embryo nor interferes with normal development. The embryos are frozen. In the lab, specialists examine the cells for genetic diseases and disorders. If the cells pass the tests, the embryos are cleared for transfer to the woman’s uterus in a second treatment cycle.
Aneuploidy screening requires harvesting of a woman’s eggs, fertilization, and sampling of embryonic cells. Using sophisticated new tools, cells from the embryo can be tested for the number of chromosomes. Ideally, there are 23 pairs of chromosomes, plus X and Y chromosomes if it’s a boy, or two X chromosomes if it’s a girl.
Aneuploidy is a term that describes a situation where there is only one of a certain chromosome instead of a pair (monosomy), or if there are three instead of two (trisomy). For instance, a common trisomy is when three number 21 chromosomes are present, which usually leads to pregnancy loss or a baby born with Down Syndrome.
If a specific single genetic disorder is known from the family history, Preimplantation Genetic Diagnosis (PGD) can be used to check for the disorder in every embryo. Only normal embryos are transferred to the woman’s uterus, reducing but not totally eliminating the chance of an abnormal embryo.
Does PGD replace prenatal testing?
Preimplantation genetic diagnosis is not 100 percent reliable and does not completely replaced prenatal testing. However, two common methods of diagnostic prenatal testing – chorionic villus sampling (CVS) and amniocentesis are typically recommended later in a woman’s pregnancy, and cannot detect genetic disorders before pregnancy.