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Words you may meet

Some technical words appear in the sections above and in the details below. Here is what they mean, in words from a public genetics glossary.

Gene

The gene is considered 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 differing functions within the body. Humans have approximately 20,000 protein-coding genes. NHGRI Talking Glossary of Genomic and Genetic Terms

Chromosome

Chromosomes are threadlike structures made of protein and a single molecule of DNA that serve to carry the genomic information from cell to cell. In plants and animals (including humans), chromosomes reside in the nucleus of cells. Humans have 22 pairs of numbered chromosomes (autosomes) and one pair of sex chromosomes (XX or XY), for a total of 46. Each pair contains two chromosomes, one coming from each parent, which means that children inherit half of their chromosomes from their mother and half from their father. Chromosomes can be seen through a microscope when the nucleus dissolves during cell division. NHGRI Talking Glossary of Genomic and Genetic Terms

Autosomal Recessive Disorder

Autosomal recessive is a pattern of inheritance characteristic of some genetic disorders. “Autosomal” means that the gene in question is located on one of the numbered, or non-sex, chromosomes. “Recessive” means that two copies of the mutated gene (one from each parent) are required to cause the disorder. In a family where both parents are carriers and do not have the disease, roughly a quarter of their children will inherit two disease-causing alleles and have the disease. By contrast, an autosomal dominant disorder requires only a single copy of the mutated gene from one parent to cause the disorder. Sickle cell anemia is an example of an autosomal recessive genetic disorder. NHGRI Talking Glossary of Genomic and Genetic Terms

Autosomal Dominant Disorder

Autosomal dominant is a pattern of inheritance characteristic of some genetic disorders. “Autosomal” means that the gene in question is located on one of the numbered, or non-sex, chromosomes. “Dominant” means that a single copy of the mutated gene (from one parent) is enough to cause the disorder. A child of a person affected by an autosomal dominant condition has a 50% chance of being affected by that condition via inheritance of a dominant allele. By contrast, an autosomal recessive disorder requires two copies of the mutated gene (one from each parent) to cause the disorder. Huntington’s disease is an example of an autosomal dominant genetic disorder. NHGRI Talking Glossary of Genomic and Genetic Terms

Mitochondrial DNA

Mitochondrial DNA is the circular chromosome found inside the cellular organelles called mitochondria. Located in the cytoplasm, mitochondria are the site of the cell’s energy production and other metabolic functions. Offspring inherit mitochondria — and as a result mitochondrial DNA — from their mother. NHGRI Talking Glossary of Genomic and Genetic Terms

Genetic Imprinting

Genomic imprinting is the process by which only one copy of a gene in an individual (either from their mother or their father) is expressed, while the other copy is suppressed. Unlike genomic mutations that can affect the ability of inherited genes to be expressed, genomic imprinting does not affect the DNA sequence itself. Instead, gene expression is silenced by the epigenetic addition of chemical tags to the DNA during egg or sperm formation. Epigenetic tags on imprinted genes usually stay in place for the life of the individual. NHGRI Talking Glossary of Genomic and Genetic Terms

Protein

Proteins are large, complex molecules that play many important roles in the body. They are critical to most of the work done by cells and are required for the structure, function and regulation of the body’s tissues and organs. 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. NHGRI Talking Glossary of Genomic and Genetic Terms

Mutation

A mutation is 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. NHGRI Talking Glossary of Genomic and Genetic Terms

Pathogenic Variant

A pathogenic variant is a genomic variant that may increase a person’s risk of developing a condition, disorder or disease. In many cases, carrying a pathogenic variant does not guarantee that a person will develop the specific condition but increases the risk for it. Pathogenic variants can be inherited from a parent or can occur spontaneously due to factors such as environmental exposure and aging. NHGRI Talking Glossary of Genomic and Genetic Terms

Genetic Testing

Genetic testing is the use of a laboratory test to examine an individual’s DNA for variations, typically performed in the context of medical care, ancestry studies or forensics. In a medical setting, the results of a genetic test can be used to confirm or rule out a suspected genetic disease. Results may also be used to determine the likelihood of parents passing on a genetic mutation to their offspring. Genetic testing may be performed prenatally or after birth. Genetic testing is also used to study the genomes of tumors in cancer cases. NHGRI Talking Glossary of Genomic and Genetic Terms

Carrier

A carrier, as related to genetics, is an individual who “carries” and can pass on to its offspring a genomic variant (allele) associated with a disease (or trait) that is inherited in an autosomal recessive or sex-linked manner, and who does not show symptoms of that disease (or features of that trait). The carrier has inherited the variant allele from one parent and a normal allele from the other parent. Any offspring of carriers is at risk of inheriting a variant allele from their parents, which would result in that child having the disease (or trait). NHGRI Talking Glossary of Genomic and Genetic Terms

Allele

An allele is one of two or more versions of DNA sequence (a single base or a segment of bases) at a given genomic location. An individual inherits two alleles, one from each parent, for any given genomic location where such variation exists. If the two alleles are the same, the individual is homozygous for that allele. If the alleles are different, the individual is heterozygous. NHGRI Talking Glossary of Genomic and Genetic Terms

Mendelian Inheritance

Mendelian inheritance refers to certain patterns of how traits are passed from parents to offspring. These general patterns were established by the Austrian monk Gregor Mendel, who performed thousands of experiments with pea plants in the 19th century. Mendel’s discoveries of how traits (such as color and shape) are passed down from one generation to the next introduced the concept of dominant and recessive modes of inheritance. NHGRI Talking Glossary of Genomic and Genetic Terms