Version May 2024
| Term | Definition |
| Genotype | An individual's genetic blueprint. Can also be used to describe the nucleotide sequence or combination of variants at a specific position in the genetic sequence. |
| Phenotype | A physical or biochemical manifestation of a genotype. |
| Penetrance | The frequency with which the genotype results in a phenotypic change. |
| Variable expressivity | The phenomenon whereby a specific genotype confers range of phenotypic changes (eg, different ages of onset, severities of disease, combinations of signs and symptoms). |
| Variant | A change in the nucleotide sequence that differs from a reference sequence. Variants are classified into one of five categories of pathogenicity (pathogenic, likely pathogenic, uncertain significance [VUS], likely benign, and benign). The purpose of using "variant" with a classifier instead of "mutation" or "polymorphism" is to reduce confusion and avoid pejorative connotations. |
| Presymptomatic | Term used in predictive genetic testing for the purpose of identifying unaffected persons at increased risk for genetic disease. |
| Predispositional | Term used in predictive genetic testing that refers to detecting a genotype with less than 100% penetrance in an asymptomatic individual – ie, the person may or may not develop the condition. |
| Types of chromosomes | |
| Autosomes | 22 paired sets of chromosomes that are not sex chromosomes (not X or Y). |
| Homozygous | Refers to a genotype in which both of the alleles in a pair have the same sequence (wild-type [according to a reference sequence] or a specified variant). |
| Heterozygous | Refers to a genotype in which only one allele in a pair has the variant of interest. |
| Compound heterozygous | Refers to a genotype with two different pathogenic variants in the same gene or heterozygous for two different pathogenic variants in two different genes. |
| Double heterozygote | Individual with two different pathogenic variants in two different genes that both contribute to the same phenotype and are each heterozygous for a variant of interest (usually a pathogenic variant). |
| Sex-chromosomes | The X and Y chromosomes. Females are typically XX; males are typically XY. |
| Modes of inheritance | |
| Mendelian inheritance | Pattern of inheritance described by Gregor Mendel. Generally refers to a trait or condition in which variants in a single gene are responsible for the phenotype. |
| Autosomal dominant | Inheritance pattern in which a pathogenic variant on one allele (ie, heterozygosity for a pathogenic variant) can cause the phenotype. Does not imply that individuals with a pathogenic variant will always develop the phenotype. A child born to an affected individual usually has a 50% chance of inheriting the variant. |
| Autosomal co-dominant | Inheritance pattern in which each of two alleles contributes equally to the phenotype. Generally, individuals with one variant allele generally have a milder phenotype than individuals with two variant alleles. A child born to an affected individual usually will inherit one of the variants. |
| Autosomal recessive | Inheritance pattern in which a pathogenic variant on both alleles is usually required for the phenotype to develop. A child born to an affected individual is an obligate carrier of one of the variants. The parents of an affected individual are both obligate carriers, and the siblings of an affected individual have a 25% chance of inheriting both pathogenic variants. |
| X-linked | Inheritance pattern in which a pathogenic variant in a gene on the X chromosome is responsible for the phenotype. Many X-linked conditions demonstrate recessive inheritance (both alleles must be affected for a female to develop the phenotype; one affected allele is sufficient for males to develop the phenotype). Thus, in most X-linked conditions, males are more frequently and/or more severely affected than females. |
| Non-Mendelian inheritance | Pattern of inheritance that is not strictly autosomal dominant, autosomal recessive, or X-linked. |
| Multifactorial | Pattern of inheritance in which a mixture of genetic and environmental factors contributes to the phenotype. |
| Polygenic | Pattern of inheritance in which more than one gene is involved in producing the phenotype. |
| Mitochondrial | Inheritance pattern in which a variant in the mitochondrial DNA causes the phenotype. Mitochondrial inheritance is transmitted exclusively from the egg (eg, from the mother). |
| Types of genetic testing | |
| Diagnostic genetic testing | Assessment of the DNA sequence or chromosomal changes (copy number variant, structural alteration) used for diagnostic (or disease characterization) purposes. |
| Carrier screening | Assessment of DNA sequence or chromosomal changes in an at-risk population. Carrier screening for autosomal recessive conditions is common in the prenatal setting. |
| Prenatal testing | Assessment of fetal DNA sequence or chromosomal changes for specific disease-causing variants or chromosomal abnormalities. |
| Preimplantation testing | Assessment of a blastocyst for DNA sequence or chromosomal changes for the purpose of selecting embryos for implantation that are free of genotypes known to cause a disease. |
| Newborn screening | A variety of genetic and metabolic tests performed in the newborn period for the purposes of early identification of children with disease-associated genotypes. |
| Direct-to-consumer (DTC) testing | Genetic testing marketed directly to individuals who wish to be tested for pathogenic variants (risk alleles) in disease genes. May also be combined with ancestry testing and genotyping for other traits. Testing is not always preceded by a risk assessment (eg, personal or family history, physical examination, other laboratory testing). |
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