Human Genetic Variation
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gene expression: Highly specific process in which a gene is switched on at a certain time and begins production of its protein.

gene mapping: Determining the relative positions of genes on a chromosome and the distance between them.

gene pool: Sum total of genes, with all their variations, possessed by a particular species at a particular time.

gene therapy: Evolving technique used to treat inherited diseases. The medical procedure involves replacing, manipulating, or supplementing nonfunctional genes with healthy genes.

gene transfer: Insertion of unrelated DNA into the cells of an organism. There are many different reasons for gene transfer, for example, attempting to treat disease by supplying patients with therapeutic genes. There are also many possible ways to transfer genes. Most involve the use of a vector, such as a specially modified virus that can take the gene along when it enters the cell.

genetic code: Instructions in a gene that tell the cell how to make a specific protein. A, T, G, and C are the "letters" of the DNA code; they stand for the chemicals adenine, thymine, guanine, and cytosine, respectively, that make up the nucleotide bases of DNA. Each gene's code combines the four chemicals in various ways to spell out three-letter "words" that specify which amino acid is needed at every step in making a protein.

genetic counseling: Short-term educational counseling process for individuals and families who have a genetic disease or who are at risk for such a disease. Genetic counseling provides patients with information about their condition and helps them make informed decisions.

genetic map: Chromosome map of a species that shows the position of its known genes and/or markers relative to each other, rather than as specific physical points on each chromosome.

genetic marker: Segment of DNA with an identifiable physical location on a chromosome and whose inheritance can be followed. A marker can be a gene, or it can be some section of DNA with no known function. Because DNA segments that lie near each other on a chromosome tend to be inherited together, markers are often used as indirect ways of tracking the inheritance pattern of a gene that has not yet been identified, but whose approximate or exact location is known.

genetic screening: Testing a population group to identify a subset of individuals at high risk for having or transmitting a specific genetic disorder.

genetics: Study of inherited variation.

genome: All the DNA contained in an organism or a cell, which includes both the chromosomes within the nucleus and the DNA in mitochondria.

genotype: Genetic identity of an individual that does not show as outward characteristics.

germ line: Sequence of cells, each descended from earlier cells in the lineage, that will develop into new sperm and egg cells for the subsequent generation.

haploid: Number of chromosomes in a sperm or egg cell; one half the diploid number.

heterozygous: Possessing two different forms of a particular gene, one inherited from each parent.

highly conserved sequence: DNA sequence that is very similar in several different kinds of organisms. Scientists regard these cross-species' similarities as evidence that a specific gene performs some basic function essential to many forms of life and that evolution has therefore conserved its structure by permitting few mutations to accumulate in it.

homozygous: Possessing two identical forms of a particular gene, one inherited from each parent.

Human Genome Project (HGP): International research project to map each human gene and to completely sequence human DNA.

hybridization: Base pairing of two single strands of DNA or RNA.

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