Abstract
Family history is an essential tool for disease risk assessment, because it incorporates not only shared genetic susceptibilities but also shared environmental, behavioral and cultural factors. Family history data should be taken as part of routine patient care by all medical practitioners. This chapter discusses single-gene disorders, i.e. disorders for which there is a single cause, such as a gene mutation. Single-gene disorders cause the patterns of inheritance that obey the laws of segregation and independent assortment that Gregor Mendel first described (autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive and Y-linked). Because the laws that govern transmission of a single genetic factor are known, a physician or genetic counselor can give accurate estimates of the probability that other family members might possess the mutation. In this chapter we discuss the foundational principles that the reader must know to understand these principles, as well as guidelines on when to refer to a genetic specialist.
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Notes
- 1.
 The term “normal” is often used to indicate the wild-type, or most common, allele of the gene. It is possible, though, to have several different alleles of a gene produce isoforms of the protein that can support healthy development. All such alleles can be seen as “normal,” while any versions of the gene’s sequence that make a version of the protein that does not support healthy development can be considered “mutant.”
- 2.
 For our purposes, if you use “N” for a functional, or normal, version of the gene’s sequence, and “M” for a defective, or mutant, version of the gene’s sequence, there are three possible genotypes for an autosomal gene: NN, NM and MM. For an X-linked gene, using “Xn” to denote an X chromosome with a functional copy of the gene and “Xm” to denote an X chromosome with a mutant copy of the gene, the possible genotypes for a female are XnXn, XnXm or XmXm, while the possible genotypes for a male are XnY or XmY. For Y-linked genes, a male’s possible genotypes are XYn or XYm.
Selected References
American Society for Human Genetics (2004) Your Family History. http://www.ashg.org/genetics/ashg/educ/007.shtml
Bennett RL (1999) The Practical Guide to the Genetic Family History. New York: Wiley-Liss, Inc.
Centers for Disease Control and Prevention, Office of Genomics and Disease Prevention (2010) Using Family History to Promote Health. http://www.cdc.gov/genomics/public/famhistMain.htm
U.S. Department of Health and Human Services (2004) U.S. Surgeon General’s Family History Initiative. http://www.hhs.gov/familyhistory/
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Sweet, K.M., Michaelis, R.C. (2011). Making the Most of Family History Information, Single Gene Disorders and Mendelian Patterns of Inheritance, and When to Refer to a Genetic Specialist. In: The Busy Physician’s Guide To Genetics, Genomics and Personalized Medicine. Springer, Dordrecht. https://doi.org/10.1007/978-94-007-1147-1_2
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