Division of Genetics and Genomics Genetics Glossary

alleles: Different forms of the same gene. Many genes have two alleles. A person may have two of the same allele (homozygous) or two different alleles (heterozygous).

amino acid: The building blocks of proteins. There are 20 different amino acids. A mutation in a gene may cause a change in the amino acid sequence. This change in the sequence causes the protein not to function properly.

amniocentesis: A prenatal diagnosis method in which a needle is inserted into the amniotic sac that surrounds the baby under the guidance of ultrasound. Some of the amniotic fluid is drawn up through the needle. Within the amniotic fluid are cells from the baby. These cells contain the baby's DNA which is used for genetic testing, including testing for single gene disorders and chromosome analysis. Amniocentesis is generally performed between 15 and 18 weeks gestation.

carrier: A carrier is someone who has one working allele and one non-working allele of a particular gene. Carriers are usually healthy and show no signs of the disorder they are “carrying.” Carriers are at risk for having children with a genetic disorder if their partner is a carrier of the same disorder. The term carrier may also refer to someone who has a balanced (no genes are duplicated or missing) chromosome rearrangement.

cell: Cells are the basic units of life. A human being is made up of at least 100 trillion cells. All of our cells (except our red blood cells) contain the entire human genome— all of the genetic instructions necessary to build a human being.

chorionic villus sampling (CVS): A prenatal diagnosis method in which a needle is inserted into the small finger-like projections of the placenta under the guidance of ultrasound. These finger-like projections are called chorionic villi. The chorionic villi are made up of the same genetic material as the baby and are used for genetic testing including testing for single gene disorders and chromosome analysis. CVS is usually performed between 10 and 12 weeks’ gestation.

chromosome: Structures found in the nucleus of each cell, each containing thousands of genes and made up of tightly packed DNA. Chromosomes come in pairs, with most people having 23 pairs, or 46 chromosomes. The first 22 pairs are called autosomes because they are the same in both males and females. The 23rd pair is referred to as the sex chromosomes because they determine a person's gender. Males have an X and a Y chromosome, and females have two X chromosomes.

chromosomal microarray analysis (CMA): A genetic test that looks for missing and extra pieces of chromosomes by comparing a patient’s DNA to a control (normal) sample of DNA. The test covers areas of the chromosomes known to be associated with genetic conditions, but can also cover other areas. CMA is generally ordered for patients with unexplained intellectual disability or developmental delay, unusual physical characteristics, and/or multiple congenital anomalies. It is usually done from a blood sample, although prenatal CMA is also available.

copy number variation (CNV): A gain or loss of genetic material. A person can have a CNV without it causing a genetic condition.

deletion: The absence of a piece of DNA. A deletion can be as small as one base pair (the basic unit of DNA) within a gene or as large as multiple genes.

deoxyribonucleic acid (DNA): The molecule that contains all of our genetic information. It is made up of two long, twisting paired strands (called a double helix). Each strand is made of four chemical units called nucleotide bases (A, C, G, T). These bases are strung together in a precise order, similar to letters being strung together to make words. Our genes are made up of DNA, and each gene has its own unique sequence of DNA. The structure of the DNA makes it possible for genetic information to be passed from generation to generation.

duplication: The presence of extra DNA. Duplication results in extra copies of part of a gene, an entire gene, or a series of genes.

enzyme: A protein that speeds up a chemical reaction in the body. Enzymes are usually required to break down (or metabolize) substances in our body.

exome: The part of our genetic code (genome) that codes for proteins, accounting for about 1 to 2 percent of our entire genome. Individual genes are made up of exons (protein-coding sequences) and introns (sequences that do not encode proteins). The exome is made up of all the exons of our genes. 

gene: A specific segment of DNA whose sequence contains the information needed to make a specific molecule, usually a protein. Genes are the instructions that tell our body how to work.

gene expression: Not all of our genes are actively working all the time. When a gene is turned “on,” it is said to be expressed. Gene expression is the process by which a gene is activated and its instructions carried out.

genome: The complete DNA sequence of an individual.  It is made up of the exome (the protein-coding DNA) and the non-coding DNA.

genome-wide association study (GWAS): An approach of rapidly scanning the genomes of many people to find DNA variations associated with a disease. Such studies are particularly useful in finding genetic variations that contribute to common, complex diseases, such as asthma, cancer, diabetes, heart disease and mental illnesses.

genomics: A field within genetics that applies multiple technologies to sequence, assemble and analyze the function and structure of the genome.

karyotype: A term to describe the number and appearance of a person’s chromosomes. Performing a karyotype, also referred to as a chromosome analysis, allows geneticists to determine if there are large deletions and duplications or chromosomal rearrangements.

mitochondria: A compartment within the cell that acts as the cell's power plant. Mitochondria make and supply energy to carry out all of the cell's jobs. They have a small amount of their own DNA, called mitochondrial DNA, passed down from mother to child. Mutations in the mitochondrial DNA disrupt the production of energy in the cell.

mosaicism: A condition in which some of a person’s cells have a different genetic makeup than other cells. Mosaicism can occur with chromosome abnormalities or with single-gene abnormalities, resulting in some cells having a normal genetic makeup and other cells having a chromosome abnormality or mutation in a gene.

mutation: A change in the DNA sequence. Mutations can prevent proteins from functioning properly, resulting in medical problems, developmental delay or mental retardation, emotional problems or certain physical characteristics. Mutations can also be considered normal variants within the population.

non-coding DNA: Genetic sequences that do not code for proteins. Non-coding DNA makes up more than 98 percent of our genomes. It was once referred to as “junk” DNA, but it’s now recognized that many non-coding DNA sequences have functions in the body, such as regulating activity of other genes.

nucleus: A small compartment within the cell that contains our DNA.

pharmacogenomics: The study of the role of genetics in predicting individual responses to medications or classes of medications. Some patients may have better or worse responses due to genetic differences in their metabolism.

preimplantation genetic diagnosis (PGD): A procedure used to decrease the chance of having a child with a particular genetic disorder. Families who decide to pursue PGD must undergo in vitro fertilization (IVF). Before the embryos are implanted, genetic testing is performed on one cell removed from the embryo. Only embryos that do not have the mutation in question are transferred to the mother's uterus. PGD is not available for all disorders, and prenatal diagnosis via CVS or amniocentesis may be offered to confirm the results of PGD.

protein: A large, complex molecule made up of amino acids. Proteins perform a variety of functions and do most of the cell's work. The function of a protein depends on its shape, which is determined by the gene that codes for it.

ribonucleic acid (RNA): A single-strand “photocopy” of our DNA that helps make proteins and plays an important role in regulating our cells’ activities. RNA has three main types: messenger RNA, transfer RNA and ribosomal RNA. Messenger RNA (mRNA) is transcribed (or copied) from a cell’s DNA. This is then translated into the amino acids that make up proteins. Transfer RNA (tRNA) carries the amino acids to the ribosome, a structure in the cell that reads the mRNA and assembles the proteins. Ribosomal RNA (rRNA) is a component of the ribosomes that helps with the assembly.

somatic mutation: A mutation that occurs after conception.  Somatic mutations can occur in any cell of the body, except for the egg or sperm cells.  Because they do not occur in the egg or sperm cells, they are not passed on to children.  Somatic mutations are often seen in tumor cells but can be important causes for some genetic conditions.

variant of unknown significance (VUS): A mutation, deletion or duplication of DNA whose effects, if any, are not yet known.

whole genome sequencing: A test that looks throughout a person’s entire genome for genetic changes. It is often done to try to find an explanation for a patient’s symptoms when tests for known causative genes come back negative.

whole exome sequencing: A test that looks throughout a person’s entire exome for genetic changes. It is similar to whole genome sequencing but only looks at the protein-coding genes and is faster and less expensive.