FAQs: Glossary

Answers for providers who have questions about genetics terminology.

Allele: The alternative form or version(s) of a gene. People inherit one allele for each autosomal gene from each biological parent.

Amino Acids: The fundamental molecules that serve as the building block for proteins. There are 20 different amino acids. A protein consists of one or more chains of amino acids (called polypeptides) whose sequence is encoded in a gene.

Autosomal: A specific gene that is not on a sex chromosome and is a numbered chromosome.

Benign/Likely Benign: The variant is not, or probably not, responsible for causing disease. Likely benign refers to a variant there is not enough scientific research on to be certain.

Carrier: A carrier is someone who has inherited a recessive allele for a genetic condition but does not show traits or symptoms of that condition.

Chromosome: Chromosomes are threadlike structures made of proteins and DNA that carry genomic information from cell to cell.

De Novo Variant: A genetic change that is present for the first time in one family member as a result of a variant (or mutation) in a germ cell (egg or sperm) of one of the parents, or a variant that arises in the fertilized egg itself during early embryogenesis. Also called de novo variant, new mutation, and new variant.

Deletion: A type of genetic change that involves the absence of a segment of DNA. It may be as small as a single base or as large as a major chromosomal region.

DNA: Deoxyribonucleic acid (DNA) is the molecule inside cells that contains the genetic information responsible for the development and function of an organism. DNA molecules allow this information to be passed from one generation to the next.

Dominant Inheritance: This occurs when an abnormal gene inherited from one parent can cause disease. The disease occurs even when the matching gene from the other parent is normal.

Duplication: Refers to a type of genetic change in which one or more copies of a DNA segment (which can be as small as a few bases or as large as a major chromosomal region) are produced. Duplications occur in all organisms.

Exome: An exome is the sequence of all the exons in a genome, reflecting the protein-coding portion of a genome.

Exon: The sequence of a DNA or RNA molecule containing information coding for a protein or peptide sequence.  Most genes have multiple exons with introns between them.

Gene: The basic unit of inheritance. Genes are passed from parents to offspring and contain the information needed to specify physical and biological traits. Most genes code for specific proteins, or segments of proteins, which have different functions within the body. Humans have approximately 20,000 protein-coding genes.

Genetic Counseling: Genetic counseling is the process of helping people understand the medical, psychological, and familial implications of genetic contributions to disease. This process integrates the following:

  • Interpretation of family and medical histories to assess the chance of disease occurrence or recurrence.
  • Education about inheritance, testing, management, prevention, resources, and research.
  • Counseling – to promote informed choices and adaptation to the risk or condition.

Genetic Testing: The sequencing of human DNA to discover genetic differences, anomalies, or variants that may cause disease.

Genome: The complete set of DNA (genetic material) in an organism. In humans, almost every cell in the body contains a complete copy of the genome. The genome contains all the information needed for a person to develop and grow.

Intron: The sequence of noncoding DNA found between exons (or coding regions) in a gene that does not remain in the final mature mRNA molecule following transcription of that gene and does not code for amino acids that make up the protein encoded by that gene. Most protein-coding genes in the human genome consist of exons and introns.

Known Familial Variant (KFV): When genetic testing reveals the cause of an inherited disease in an affected family member, the genetic change is called a ‘known familial mutation’ (KFM) or ‘known familial variant’ (KFV).

Mosaic: Mosaicism or genetic mosaicism is a condition in multicellular organisms in which a single organism possesses more than one genetic line as the result of genetic mutation.

Mutation: A change in the DNA sequence of an organism. Mutations can result from errors in DNA replication during cell division, exposure to mutagens, or a viral infection. Germline mutations (that occur in eggs and sperm) can be passed on to offspring, while somatic mutations (that occur in body cells) are not passed on.

Next-Generation Sequencing (NGS): A high-throughput method used to determine a portion of the nucleotide sequence of an individual’s genome. This technique utilizes DNA sequencing technologies that are capable of processing multiple DNA sequences in parallel. Also called massively parallel sequencing and NGS.

Pathogenic/Likely Pathogenic: A genetic variation that alters the structure and function of a protein in a way that leads to disease.

Pedigree: A diagram of family history that uses standardized symbols. A pedigree shows relationships between family members and indicates which individuals have certain genetic pathogenic variants, traits, and diseases within a family. A pedigree can be used to determine disease inheritance patterns within a family

Proband: A person serving as the starting point for the genetic study of a family.

Protein: A protein is made up of one or more long, folded chains of amino acids (each called a polypeptide), whose sequences are determined by the DNA sequence of the protein-encoding gene.

Recessive Inheritance: This occurs when both genes in a pair must be abnormal to cause disease. People with only one abnormal gene in the pair are called carriers. These people typically do not exhibit signs or symptoms of the condition.

Syndrome: A group of traits or conditions that tend to occur together and characterize a recognizable disease. Some syndromes have a genetic cause.

Trinucleotide Repeat (TNR): Trinucleotide repeats (TNR) are repetitive genomic sequences that can occur at various lengths within a gene or region of the genome. When these repeated sequences expand beyond the normal range for that region it can disrupt the normal function of the protein that results from the gene containing the TNR. This expansion can cause a TNR disorder in an individual.

Variant: An alteration in the most common DNA nucleotide sequence. The term variant can be used to describe an alteration that may be benign, pathogenic, or of unknown significance. The term variant is increasingly being used in place of the term mutation.

Variant Classification: The process of categorizing a particular variant’s likelihood to cause or predispose an affected individual to a certain disease or disorder.

Variant Of Uncertain Significance (VUS): A genetic change for which the impact on an individual’s risk for disease is not yet known. As more scientific information becomes available, a VUS may change and be associated with a disease or not associated with a disease.

X-Inactivation: In embryonic development in females, one of the two X chromosomes is randomly and permanently inactivated in cells other than egg cells. This phenomenon is called X-inactivation or lyonization. X-inactivation ensures that females, like males, have one functional copy of the X chromosome in each cell.

X-Linked Inheritance: This occurs when a genetic trait or condition is the result of a gene located on the X chromosome. In males (who only have one X chromosome), a variant in the copy of the gene on the single X chromosome causes the condition.

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