Even before a child is conceived, much (but certainly not all) of the information needed to determine a future child's risk of having a genetic disease is already present in the genomes of his or her parents. For dominant diseases, the parent may be affected and the concept of screening does not apply. For recessive diseases, parents can carry a pathogenic mutation in one of their genes and will typically be unaffected and unaware that they carry a mutation. But if a child inherits both pathogenic mutations from two carrier parents, they are at risk of the disease (refer to Appendix 3 for more information on inheritance). For this reason, much of the advancement in Genomic and Precision Medicine has occurred in the field of prenatal and preconception carrier testing of prospective parents. This chapter will deal with these advancements and their practical use by nongeneticist physicians and practitioners such as primary care providers, obstetricians, pediatricians, midwives, and fertility specialists.
Since the 1970s, tests have been available to detect carrier parents who might have a child affected by a rare recessive disease if the reproductive partner is also a carrier. Diseases such as Tay–Sachs have been almost eliminated in select populations due to aggressive carrier screening. However, screening for a small number of disorders in a narrow segment of the population is a far cry from screening all prospective parents for all medicine, but medicine is indeed heading in that direction.
TRADITIONAL CARRIER SCREENING
Carrier screening has traditionally been heavily influenced by pretest risk. Individuals with a family history of a genetic disorder would be offered carrier testing. Prospective parents belonging to a unique ethnic group with a higher rate of a particular disorder would be screened for that disorder only. Screening beyond this was simply not technically or financially feasible.
Now, the cost of sequencing many genes is essentially the same as sequencing one, and medicine has swung away from paternalism and seeks to involve the patient to whatever extent is possible and desired. These two forces have combined to fertilize (pun intended) a robust industry providing a variety of carrier screening products that detect an ever-increasing number of disorders, and that are marketed to any and all prospective parents regardless of family or ethnic background.
Nevertheless, obtaining a complete family history remains essential for any provider caring for a prospective parent. As will be discussed below, many of the newer one-size-fits-all commercial preconception screening tests are not necessarily suited for a family with a clear history of a specific disease. Diseases that run in the family, especially in children, may require testing and counseling by a specialist. It is the responsibility of the primary practitioner to distinguish families that lack defined risk factors for inherited diseases and can thus appropriately undergo generic screening from those families that require more in-depth consideration.