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Summary Article: Genetic Testing and Counseling from Encyclopedia of Global Health

Genetic testing is the use of biochemical and cytological techniques to determine the carrier and disease status of an individual with regard to any number of genetic or chromosomal states which may manifest themselves in disease states. Genetic counseling is the provision of information and assistance to individuals (consultands) who are, or may be, affected by a disease of genetic origin. Consultands may themselves be suffering from a disease, may be a relative (usually a parent) of such an individual, or may have questions about their own carrier status (typically for purposes of reproductive planning). Advice given in genetic counseling includes calculation and interpretation of pretest and posttest probabilities (both for development of disease and of disease appearance in offspring) as well as provision of information and advice on follow-up actions individuals might take given their newfound knowledge.

COMMON GENETIC TESTS

Because genetic testing is neither universal nor inexpensive, genetic testing methods vary widely by geography and available resources. As a broad distinction, genetic testing may be divided into cytological and molecular techniques. The principle cytological method in genetic testing is the karyotype. Karyotyping examines the gross appearance of all 46 chromosomes. While not able to detect point mutations, this method can detect abnormalities caused by unequal crossing over or nondisjunction. The latter are a class of abnormalities caused when meiosis is abnormal and subsequent fertilization leads to cells with either more or less than two of a particular chromosome. Syndromes caused by nondisjunction which can be detected by karyotyping include Down syndrome and Turner’s syndrome.

Among molecular techniques, the most accurate is gene sequencing where various methods are used to determine the exact base sequence of a relevant locus. While sequencing allows precise determination of genotype, it is not always appropriate. Because sequencing is not cheap, it is not feasible to sequence an entire genome. Instead, counselors and/or physicians work with consultands to determine the region of DNA that should be sequenced. More commonly, when many different loci are of interest, tests are done that determine the presence or absence of specific alleles known to cause or cosegregate with disease. Such testing methods, unlike sequencing, cannot confirm the presence of a wildtype allele, only the absence of specified mutant alleles.

KEY PRINCIPLES AND EVENTS

Generally, patients are not told what decisions to make regarding the various testing and management options. Instead, they are given information and support and empowered to make their own decisions. This is known as the principle of nondirective counseling and has been widely adopted as standard practice, particularly in North America and western Europe. It should also be emphasized that although the activities of counseling are commonly described in relation to genetic testing, counseling requires the ability to identify and address complex psychosocial issues associated with both the disorder of interest and the family of the consultand.

National Human Genome Research Institute researcher Milton English, Ph.D., using a pipette to load DNA into a gel.

The events in genetic counseling case management can be broadly classified into four chronological phases: collection of information, assessment, counseling, and follow-up. Genetic counselors collect information on family history and medical history as well as work with consultands to determine which further tests to order. In assessment, this information is combined with a physical examination to validate or establish a diagnosis. The counseling phase can only begin when this information has been established either definitively or in a probabilistic manner. The nature and consequences of the diagnosis are explained with emphasis not just on the consultand, but on the consequences for relatives. In the case of potential parents, the risk to current and future children is also explored. Decisions about case management and referrals to other health professionals are also made in this phase. Finally, responsible genetic counseling entails follow-up for psychosocial support and to help assess and incorporate emergent information into improved case management.

    SEE ALSO:
  • American Society of Human Genetics (ASHG); Down Syndrome; Turner’s Syndrome.

BIBLIOGRAPHY
  • D. L. Baker; J. L. Schuette; W. R. Uhlmann, A Guide to Genetic Counseling (Wiley, 1998).
  • R.J. M. Gardner; G. R. Sutherland, Chromosome Abnormalities and Genetic Counseling, 2nd ed. (Oxford University Press, 1996).
  • R. L. Nussbaum, et al., Thompson & Thompson: Genetics in Medicine, 6th ed., revised reprint (Saunders, 2004).
  • Bimal P. Chaudhari
    Boston University
    Copyright © 2008 by SAGE Publications, Inc.

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