Index case

A clinically affected individual through whom attention is first drawn to a genetic disorder in a family.

Genetic counseling

A communication process that seeks to assist affected or at-risk individuals and families in understanding the natural history, disease risks, and mode of transmission of a genetic disorder; to facilitate informed consent for genetic testing when appropriate; to discuss options for risk management and family planning; and to provide for or refer individuals for psychosocial support as needed. The National Society of Genetic Counselors Task Force has also defined the term genetic counseling. (NCI Thesaurus)

Consultand

An individual who presents for genetic counseling. Also called counselee.

Counselee

An individual who presents for genetic counseling. Also called consultand.

Genome-wide association study

Genome-wide association studies are a way for scientists to identify genes involved in human disease. This method searches the genome for small variations, called single nucleotide polymorphisms or SNPs (pronounced “snips”), that occur more frequently in people with a particular disease than in people without the disease. Also called GWAS.

GWAS

GWAS are a way for scientists to identify genes involved in human disease. This method searches the genome for small variations, called single nucleotide polymorphisms or SNPs (pronounced “snips”), that occur more frequently in people with a particular disease than in people without the disease. Also called genome-wide association study.

X-linked recessive

X-linked recessive inheritance refers to genetic conditions associated with mutations in genes on the X chromosome. A male carrying such a mutation will be affected, because he carries only one X chromosome. A female carrying a mutation in one gene, with a normal gene on the other X chromosome, will be unaffected.

Mosaicism

The occurrence of 2 or more cell lines with different genetic or chromosomal make-up, within a single individual or tissue.

Carrier rate

The proportion of individuals in a population who have a single copy of a specific recessive gene mutation; also sometimes applied to the prevalence of mutations in dominantly acting genes such as BRCA1 and BRCA2. Also called carrier frequency.

Sensitivity

The frequency with which a test yields a true positive result among individuals who actually have the disease or the gene mutation in question. A test with high sensitivity has a low false-negative rate and thus does a good job of correctly identifying affected individuals. (NCI Thesaurus)

Microsatellite instability

MSI. A characteristic of cells that contain an abnormality in DNA mismatch repair (see microsatellite). For example, the presence of MSI in colorectal tumor tissue may be used as a marker for germline mutations in one of the DNA mismatch repair genes associated with HNPCC. MSI can also occur sporadically, and in these cases is related to gene hypermethylation. This is an issue in the differential diagnosis of HNPCC. Also called MSI.

MSI

Microsatellite instability. A characteristic of cells that contain an abnormality in DNA mismatch repair (see microsatellite). For example, the presence of MSI in colorectal tumor tissue may be used as a marker for germline mutations in one of the DNA mismatch repair genes associated with HNPCC. MSI can also occur sporadically, and in these cases is related to gene hypermethylation. This is an issue in the differential diagnosis of HNPCC. Also called microsatellite instability.

Indeterminate

A negative test result in an individual where a clearly deleterious mutation has not been found in any family members. The genetic risk status of such an individual must be interpreted in the context of his or her personal and family history. Also called uninformative and inconclusive.

Inconclusive

A negative test result in an individual where a clearly deleterious mutation has not been found in any family members. The genetic risk status of such an individual must be interpreted in the context of his or her personal and family history. Also called uninformative and indeterminate.

Genetic heterogeneity

The production of the same or similar phenotypes (observed biochemical, physiological, and morphological characteristics of a person determined by his/her genotype) by different genetic mechanisms. There are two types: (1) allelic heterogeneity – when different alleles at a locus can produce variable expression of a condition; and (2) locus heterogeneity – the term used to describe disease in which mutations at different loci can produce the same disease phenotype.