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Glossary

Acquired mutations:
gene changes that arise within individual cells and accumulate throughout a person's lifetime; also called somatic mutations. (See Hereditary mutation.)

Alleles:
variant forms of the same gene. Different alleles produce variations in inherited characteristics such as eye color or blood type.

Alzheimer's disease:
a disease that causes memory loss, personality changes, dementia and, ultimately, death. Not all cases are inherited, but genes have been found for familial forms of Alzheimer's disease.

Amino acid:
any of a class of 20 molecules that combine to form proteins in living things.

Amyotrophic lateral sclerosis:
an inherited, fatal degenerative nerve disorder; also known as Lou Gehrig's disease.

Autosome:
any of the non-sex-determining chromosomes. Human cells have 22 pairs of autosomes.

Base pairs:
the two complementary, nitrogen-rich molecules held together by weak chemical bonds. Two strands of DNA are held together in the shape of a double helix by the bonds between their base pairs. (See Chemical base.)

BRCA1:
a gene that normally helps to restrain cell growth.

BRCA1 breast cancer susceptibility gene:
a mutated version of BRCA1, which predisposes a person toward developing breast cancer.

Carrier:
a person who has a recessive mutated gene, together with its normal allele. Carriers do not usually develop disease but can pass the mutated gene on to their children.

Carrier testing:
testing to identify individuals who carry disease-causing recessive genes that could be inherited by their children. Carrier testing is designed for healthy people who have no symptoms of disease, but who are known to be at high risk because of family history.

Cell:
small, watery, membrane-bound compartment filled with chemicals; the basic subunit of any living thing.

Chemical base:
an essential building block. DNA contains four complementary bases: adenine, which pairs with thymine, and cytosine, which pairs with guanine. In RNA, thymine is replaced by uracil.

Chromosomes:
structures found in the nucleus of a cell, which contain the genes. Chromosomes come in pairs, and a normal human cell contains 46 chromosomes, 22 pairs of autosomes and two sex chromosomes.

Clone:
a group of identical genes, cells, or organisms derived from a single ancestor.
Cloning:
the process of making genetically identical copies.

Contig maps:
types of physical DNA maps that consist of overlapping segments of DNA (contigs) that, taken together, completely represent that section of the genome. (See Physical maps.)

Colonoscopy:
examination of the colon through a flexible, lighted instrument called a colonoscope.

Crossing over:
a phenomenon, also known as recombination, that sometimes occurs during the formation of sperm and egg cells (meiosis); a pair of chromosomes (one from the mother and the other from the father) break and trade segments with one another.

Cystic fibrosis:
an inherited disease in which a thick mucus clogs the lungs and blocks the ducts of the pancreas.

Cytoplasm:
the cellular substance outside the nucleus in which the cell's organelles are suspended.

Dementia:
severe impairment of mental functioning.

DNA:
the substance of heredity; a large molecule that carries the genetic information that cells need to replicate and to produce proteins.

DNA repair genes:
certain genes that are part of a DNA repair pathway; when altered, they permit mutations to pile up throughout the DNA.

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

Dominant allele:
a gene that is expressed, regardless of whether its counterpart allele on the other chromosome is dominant or recessive. Autosomal dominant disorders are produced by a single mutated dominant allele, even though its corresponding allele is normal. (See Recessive allele.)

Enzyme:
a protein that facilitates a specific chemical reaction.

Familial adenomatous polyposis:
an inherited condition in which hundreds of potentially cancerous polyps develop in the colon and rectum.

Familial cancer:
cancer, or a predisposition toward cancer, that runs in families.

Functional gene tests:
biochemical assays for a specific protein, which indicates that a specific gene is not merely present but active.

Gene:
a unit of inheritance; a working subunit of DNA. Each of the body's 50,000 to 100,000 genes contains the code for a specific product, typically, a protein such as an enzyme.

Gene deletion:
the total loss or absence of a gene.

Gene expression:
the process by which a gene's coded information is translated into the structures present and operating in the cell (either proteins or RNAs).

Gene markers:
landmarks for a target gene, either detectable traits that are inherited along with the gene, or distinctive segments of DNA.

Gene mapping:
determining the relative positions of genes on a chromosome and the distance between them.

Gene testing:
examining a sample of blood or other body fluid or tissue for biochemical, chromosomal, or genetic markers that indicate the presence or absence of genetic disease.

Gene therapy:
treating disease by replacing, manipulating, or supplementing nonfunctional genes.

Genetic linkage maps:
DNA maps that assign relative chromosomal locations to genetic landmarksNeither genes for known traits or distinctive sequences of DNA - on the basis of how frequently they are inherited together. (See Physical maps.)

Genetics:
the scientific study of heredity: how particular qualities or traits are transmitted from parents to offspring.

Genome:
all the genetic material in the chromosomes of a particular organism.

Genome maps:
charts that indicate the ordered arrangement of the genes or other DNA markers within the chromosomes.

Genotype:
the actual genes carried by an individual (as distinct from phenotypeNthat is, the physical characteristics into which genes are translated).

Germ cells:
the reproductive cells of the body, either egg or sperm cells.

Germline mutation:
(See Hereditary mutation.)

Hereditary mutation:
a gene change in the body's reproductive cells (egg or sperm) that becomes incorporated in the DNA of every cell in the body; also called germline mutation. (See Acquired mutations.)

Human genome:
the full collection of genes needed to produce a human being.

Human Genome Project:
an international research effort (led in the United States by the National Institutes of Health and the Department of Energy) aimed at identifying and ordering every base in the human genome.

Huntington's disease:
an adult-onset disease characterized by progressive mental and physical deterioration; it is caused by an inherited dominant gene mutation.

Imprinting:
a biochemical phenomenon that determines, for certain genes, which one of the pair of alleles, the mother's or the father's, will be active in that individual.

Inborn errors of metabolism:
inherited diseases resulting from alterations in genes that code for enzymes.

Leukemia:
cancer that begins in developing blood cells in the bone marrow.

Li-Fraumeni syndrome:
a family predisposition to multiple cancers, caused by a mutation in the p53 tumor-suppressor gene.

Linkage analysis:
a gene-hunting technique that traces patterns of heredity in large, high-risk families, in an attempt to locate a disease-causing gene mutation by identifying traits that are co-inherited with it.

Melanoma:
a cancer that begins in skin cells called melanocytes and spreads to internal organs.

Molecule:
a group of atoms arranged to interact in a particular way; one molecule of any substance is the smallest physical unit of that particular substance.

Mutation:
a change in the number, arrangement, or molecular sequence of a gene.

Newborn screening:
examining blood samples from a newborn infant to detect disease-related abnormalities or deficiencies in gene products.

Nucleotide:
A subunit of DNA or RNA, consisting of one chemical base plus a phosphate molecule and a sugar molecule.

Nucleus:
the cell structure that houses the chromosomes.

Oncogenes:
genes that normally play a role in the growth of cells but, when overexpressed or mutated, can foster the growth of cancer.

p53:
(See Tumor-suppressor genes.)

Penetrance:
a term indicating the likelihood that a given gene will actually result in disease.

Phenylketonuria (PKU):
an inborn error of metabolism caused by the lack of an enzyme, resulting in abnormally high levels of the amino acid phenylalanine; untreated, PKU can lead to severe, progressive mental retardation.

Physical maps:
DNA maps showing the location of identifiable landmarks, either genes or distinctive short sequences of DNA. The lowest resolution physical map shows the banding pattern on the 24 different chromosomes; the highest resolution map depicts the complete nucleotide sequence of the chromosomes. (See Contig maps.)

Precancerous polyps:
growths in the colon that often become cancerous.

Predictive gene tests:
tests to identify gene abnormalities that may make a person susceptible to certain diseases or disorders.

Prenatal diagnosis:
examining fetal cells taken from the amniotic fluid, the primitive placenta (chorion), or the umbilical cord for biochemical, chromosomal, or gene alterations.

Probe:
a specific sequence of single-stranded DNA, typically labeled with a radioactive atom, which is designed to bind to, and thereby single out, a particular segment of DNA.

Proofreader genes:
(See DNA repair genes.)

Prophylactic surgery:
surgery to remove tissue that is in danger of becoming cancerous, before cancer has the chance to develop. Surgery to remove the breasts of women at high risk of developing breast cancer is known as prophylactic mastectomy.

Protein:
a large, complex molecule composed of amino acids. The sequence of the amino acidsNand thus the function of the proteinNis determined by the sequence of the base pairs in the gene that encodes it. Proteins are essential to the structure, function, and regulation of the body. Examples are hormones, enzymes, and antibodies.

Protein product:
the protein molecule assembled under the direction of a gene.

Recessive allele:
a gene that is expressed only when its counterpart allele on the matching chromosome is also recessive (not dominant). Autosomal recessive disorders develop in persons who receive two copies of the mutant gene, one from each parent who is a carrier. (See Dominant allele.)

Recombination:
(See Crossing over.)

Renal cell cancer:
a type of kidney cancer.

Reproductive cells:
egg and sperm cells. Each mature reproductive cell carries a single set of 23 chromosomes.

Restriction enzymes:
enzymes that can cut strands of DNA at specific base sequences.

Retinoblastoma:
an eye cancer caused by the loss of a pair of tumor-suppressor genes; the inherited form typically appears in childhood, since one gene is missing from the time of birth.

RNA:
a chemical similar to DNA. The several classes of RNA molecules play important roles in protein synthesis and other cell activities.

Sarcoma:
a type of cancer that starts in bone or muscle.

Screening:
looking for evidence of a particular disease such as cancer in persons with no symptoms of disease.

Sex chromosomes:
the chromosomes that determine the sex of an organism. Human females have two X chromosomes; males have one X and one Y.

Sickle-cell anemia:
an inherited, potentially lethal disease in which a defect in hemoglobin, the oxygen-carrying pigment in the blood, causes distortion (sickling) and loss of red blood cells, producing damage to organs throughout the body.

Somatic cells:
all body cells except the reproductive cells.

Somatic mutations:
(See Acquired mutations.)

Tay-Sachs disease:
an inherited disease of infancy characterized by profound mental retardation and early death; it is caused by a recessive gene mutation.

Transcription:
the process of copying information from DNA into new strands of messenger RNA (mRNA). The mRNA then carries this information to the cytoplasm, where it serves as the blueprint for the manufacture of a specific protein.

Translation:
the process of turning instructions from mRNA, base by base, into chains of amino acids that then fold into proteins. This process takes place in the cytoplasm, on structures called ribosomes.

Tumor-suppressor genes:
genes that normally restrain cell growth but, when missing or inactivated by mutation, allow cells to grow uncontrolled.

Wilms' tumor:
a kidney cancer (tumor) that occurs in children, usually before age 5.

X chromosome:
a sex chromosome; normal females carry two X chromosomes.

Y chromosome:
a sex chromosome; normal males carry one Y and one X chromosome.