Genetics Program Genetics Glossary

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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 for which they are carriers. Carriers are at risk for having children with a genetic disorder if they have children with a carrier of the same genetic 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.

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

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 at 10-12 weeks gestation.

Chromosome: Structures found in the nucleus of the cell. Each chromosomes contains thousands of genes and is 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.

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 4 different chemical units called nucleotide bases. 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 the genetic information to be transmitted 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 breakdown (or metabolize) substances in our body.

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.

Mitochondria: A specific compartment within the cell that acts as the cell's power-plant. Mitochondria make and supply energy to the cell to carry out all of the cell's jobs. They have a small amount of DNA called mitochondrial DNA. Mutations in the mitochondrial DNA disrupt the production of energy in the cell.

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.

Newborn Screening: Testing involving a heel prick that is performed in newborns as part of state public health programs. Newborn screening is intended to identify infants at risk for certain genetic disorders for which early diagnosis and treatment are available. This kind of testing does not identify whether a child has one of these disorders; it tries to identify those who are most at risk for having one. If a newborn screen comes back positive, further diagnostic testing is usually required to confirm or specify the results of the screening. Counseling is often offered to parents after a positive screen.

Pre-implantation 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.

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