Human Genetic Variation
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codon: Three bases in a DNA or RNA sequence that specify a single amino acid.

cystic fibrosis (CF): Hereditary disease whose symptoms usually appear shortly after birth. They include faulty digestion, breathing difficulties and respiratory infections due to mucus accumulation, and excessive loss of salt in sweat. In the past, cystic fibrosis was almost always fatal in childhood, but treatment is now so improved that patients commonly live to their 20s and beyond.

cytogenetic map: Visual appearance of a chromosome when stained and examined under a microscope. Particularly important are visually distinct regions, called light and dark bands, that give each of the chromosomes a unique appearance. This feature allows a person's chromosomes to be studied in a clinical test known as a karyotype, which allows scientists to look for chromosomal alterations.

deletion: Particular kind of mutation: loss of a piece of DNA from a chromosome. Deletion of a gene or part of a gene can lead to a disease or abnormality.

deoxyribonucleic acid (DNA): Chemical inside the nucleus of a cell that carries the genetic instructions for making living organisms.

diploid: Number of chromosomes in most cells except the gametes. In humans, the diploid number is 46.

DNA-microchip technology: Technology that identifies mutations in genes. It uses small glass plates that contain synthetic single-stranded DNA sequences identical to those of a normal gene.

DNA replication: Process by which the DNA double helix unwinds and makes a copy of itself.

DNA sequencing: Determining the exact order of the base pairs in a segment of DNA.

dominant: Gene that almost always results in a specific physical characteristic (for example, a disease) even though the patient's genome possesses only one copy. With a dominant gene, the chance of passing on the gene (and therefore the disease) to children is 50-50 in each pregnancy.

double helix: Structural arrangement of DNA, which looks something like an immensely long ladder twisted into a helix, or coil. The sides of the "ladder" are formed by a backbone of sugar and phosphate molecules, and the "rungs" consist of nucleotide bases joined weakly in the middle by hydrogen bonds.

duplication: Particular kind of mutation: production of one or more copies of any piece of DNA, including a gene or even an entire chromosome.

electrophoresis: Process in which molecules (such as proteins, DNA, or RNA fragments) can be separated according to size and electrical charge by applying an electric current to them. The current forces the molecules through pores in a thin layer of gel, a firm, jelly-like substance. The gel can be made so that its pores are just the right dimensions for separating molecules within a specific range of sizes and shapes. Smaller fragments usually travel farther than large ones. The process is sometimes called gel electrophoresis.

enzyme: Protein that encourages a specific biochemical reaction, usually speeding it up. Organisms could not function if they had no enzymes.

exon: Region of a gene that contains the code for producing the gene's protein. Each exon codes for a specific portion of the complete protein. In some species (including humans), a gene's exons are separated by long regions of DNA (called "introns" or sometimes "junk DNA") that have no apparent function.

fluoresence in situ hybridization (FISH): Process that vividly paints chromosomes or portions of chromosomes with fluorescent molecules. This technique is useful for identifying chromosomal abnormalities and gene mapping.

gene: Functional and physical unit of heredity passed from parent to offspring. Genes are pieces of DNA, and most genes contain the information for making a specific protein.

gene amplification: Increase in the number of copies of any particular piece of DNA. A tumor cell amplifies, or copies, DNA segments naturally as a result of cell signals and, sometimes, environmental events.

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