Human Genetic Variation
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oncogene: Gene that is capable of causing the transformation of normal cells into cancer cells.

pedigree: Simplified diagram of a family's genealogy that shows family members' relationships to each other and how a particular trait or disease has been inherited.

pharmacogenomics: Study of genetic variation underlying differential response to drugs.

phenotype: Observable traits or characteristics of an organism, for example, hair color, weight, or the presence or absence of a disease. Phenotypic traits are not necessarily genetic.

physical map: Chromosome map of a species that shows the specific physical locations of its genes and/or markers on each chromosome. Physical maps are particularly important when searching for disease genes by positional cloning strategies and for DNA sequencing.

polymerase chain reaction (PCR): Fast, inexpensive technique for making an unlimited number of copies of any piece of DNA. Sometimes called "molecular photocopying," PCR has had an immense impact on biology and medicine, especially genetic research.

polymorphism: Gene that exists in more than one version (allele) and where the rare allele can be found in more than 2 percent of the population.

recessive: Genetic trait that appears only in people who have received two copies of a mutant gene, one from each parent.

restriction enzyme: Enzyme that recognizes specific nucleotide sequences in DNA and cuts the DNA molecule at these points.

ribonucleic acid (RNA): Chemical similar to a single strand of DNA. In RNA, the letter U, which stands for uracil, is substituted for T (thymine) in the genetic code. RNA delivers DNA's genetic message to the cytoplasm of a cell where proteins are made.

ribosome: Cellular organelle that is the site of protein synthesis.

sequence tagged site (STS): Short DNA segment that occurs only once in the human genome and whose exact location and order of bases are known. Because each is unique, STSs are helpful for chromosome placement of mapping and sequencing data from many different laboratories. STSs serve as landmarks on the physical map of the human genome.

sex chromosome: One of the two chromosomes that specify an organism's genetic sex. Humans have two kinds of sex chromosomes, one called X and the other, Y. Normal females possess two X chromosomes and normal males, one X and one Y.

sex-linked: Located on the X chromosome. Sex-linked (or X-linked) diseases are generally seen only in males.

silent mutation: Mutation that results in an unchanged amino acid sequence and thus in a protein with normal function.

single-nucleotide polymorphism (SNP): Difference in a single base of DNA.

somatic cell: Any of the body's cells except the reproductive cells.

suicide gene: Strategy for making cancer cells more vulnerable to chemotherapy. One approach has been to link parts of genes expressed in cancer cells to other genes for enzymes not found in mammals that can convert a harmless substance into one that is toxic to the tumor.

tamoxifen: Drug that, when tested in clinical trials, reduced by about half the development of breast cancer in women taking the drug as compared with women taking a placebo.

transgenic: Experimentally produced organism in which DNA has been artificially introduced and incorporated into the organism's germ line, usually by injecting the foreign DNA into the nucleus of a fertilized embryo.

translocation: Breakage and removal of a large segment of DNA from one chromosome, followed by the segment's attachment to a different chromosome.

trisomy: Possessing three copies of a particular chromosome instead of the normal two copies.

tumor-suppressor gene: Protective gene that normally limits the growth of tumors. When a tumor suppressor is mutated, it may fail to keep a cancer from growing. BRCA1 and p53 are well-known tumor-suppressor genes.

vector: Agent that transfers material from one organism to another. For example, a virus can be a vector for the transfer of a gene.

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