Amino acids
The basic building blocks of proteins. There are 20 different amino acid types. Each protein consists of a different sequence of amino acids linked together according to the genetic information encoded in DNA.

Autosome
Any paired chromosome that is alike in the cells of males and females, as distinguished from the sex chromosomes X or Y. Humans have 22 pairs of autosomes.

Base pairs
Pairs of complementary nitrogenous bases that interact to form each rung of DNA's double helix. Adenine (A) pairs with thymine (T); cytosine (C) pairs with guanine (G).

C. elegans
A small worm that is used as a model organism to study genetics and development.

Cell
The basic subunit of any living organism; the simplest unit that can exist as an independent living system.

Chromosome
A structure of DNA and protein found in the cell nucleus. Each chromosome contains hundreds or thousands of the genes that form our hereditary blueprint. Humans have 23 pairs of chromosomes (each parent contributes one chromosome in each pair), containing a total of 50,000 to 100,000 genes from each parent.

Chromosome jumping
A research technique developed to move rapidly up or down a chromosome in search of a particular gene. It enables researchers to make large jumps over uninformative regions of DNA.

Chromosome walking
A research technique in which overlapping fragments of a chromosome are checked one at a time in search of a particular gene. It covers smaller regions than chromosome jumping.

Complementary DNA (cDNA)
A single strand of DNA synthesized in the lab to complement the bases in a given strand of messenger RNA. Complementary DNA represents the parts of a gene that are expressed in a cell to produce a protein.

Conserved DNA sequences
Sequences of DNA that have remained largely unchanged throughout evolution. Conserved DNA sequences are believed to be involved in important functions that are similar in different species.

Cystic fibrosis (CF)
A genetic disease involving a sticky buildup of mucus in the lungs (which makes breathing difficult and leads to infections), as well as pancreatic insufficiency (which leads to digestive problems). It is a recessive disease, occurring only when a child inherits two mutated copies of the CF gene--one from each parent.

Cystic Fibrosis Foundation
Organized to help patients and families with information and referrals about cystic fibrosis. Headquarters: 6931 Arlington Road, Bethesda, MD 20814. Toll-free phone: (800)FIGHT-CF. Or call (301)951-4422. Email: info@cff.org. Home page: www.cff.org.

Dominant
Refers to a characteristic that is apparent even when the relevant gene is present in only one copy. A dominant disorder can be inherited from only one parent.

DNA (deoxyribonucleic acid)
The genetic material of living organisms; the substance of heredity. It is a large, double-stranded, helical molecule that contains genetic instructions for growth, development, and replication. The rungs of this double helix are made of base pairs.

DNA probe
A specific sequence of single-stranded DNA used to seek out a complementary sequence of single-stranded DNA. Probes are usually labelled radioactively or chemically to make them easier to detect, thus revealing the target DNA sequence.

Drosophila melanogaster
A type of fruit fly used as a model organism to study genetics and development.

Duchenne muscular dystrophy
A genetic disease that prevents muscles from developing properly in early childhood and may be crippling. Primarily affecting males, it is caused by a mutant gene on the X chromosome.

Embryonic stem (ES) cells
Early embryonic cells (usually mouse cells) that can be genetically manipulated by researchers and then implanted into embryos to produce transgenic animals (usually mice) with predetermined genetic mutations.

Exon
A segment of a gene that contains instructions for making a protein. In many genes the exons are separated by "intervening" segments of DNA, known as introns, which do not code for proteins; these introns are removed by splicing to produce messenger RNA.

Gene
A sequence of DNA that represents a fundamental unit of heredity. Most genes encode proteins, but some code for RNA molecules.

Gene cloning
Isolating a gene and producing many identical copies of it so that it can be studied in detail.

Gene expression
The process by which the information encoded in a gene is converted into protein or some form of RNA. The DNA sequence is first transcribed into RNA and then usually--but not always--translated into protein.

Gene linkage map (also called linkage map)
A map of the relative positions of genetic loci on a chromosome. Genes that are generally inherited together are located close to each other on the chromosome and said to be linked.

Gene targeting
A method scientists use to alter or turn off a particular gene in animals used in research.

Gene therapy
Treatment of a disease by introducing a new gene into a cell. The new gene may be used to replace a function that is missing because of a defective gene.

Gene transcription
The process by which a gene's DNA sequence is copied into messenger RNA; the first step in gene expression.

Gene translation
The process by which transcribed messenger RNA directs the synthesis of proteins from amino acids.

Genetic disease
A disease caused by a genetic mutation that is either inherited or arises spontaneously.

Genetic marker
A landmark that identifies a unique location on a chromosome.

Genetic screening
Testing of a population to identify people at risk for a specific genetic disease or at risk for transmitting such a disease to their children.

Geneticist
A scientist who studies genes, the basic units of heredity, and their variations.

Genome
The total genetic information of a particular organism. The normal human genome consists of 3 billion base pairs of DNA in each set of 23 chromosomes from one parent.

Germ cells
Cells that give rise to sperm or egg.

Hereditary Disease Foundation
Dedicated to research and information on Huntington's disease. Headquarters: 1427 7th Street, #2, Santa Monica, CA 90401. Telephone: (310) 458-4183. Fax: (310) 458-3937. Email: cures@hdfoundation.org. Home page: www.hdfoundation.org.

Homologous recombination
A process by which one DNA segment can replace another DNA segment that has a similar sequence. Homologous recombination is used by researchers to create "knockout" mice in which the function of a specific gene is disrupted.

Human Genome Project
A large international project to map and sequence all the DNA in human chromosomes.

Huntington's disease
A genetic disease of the central nervous system that produces progressive dementia and involuntary movements. It usually starts between ages 30 and 50 and slowly progresses to death. Since it is a dominant disease, a child of a parent with Huntington's disease has a 50-50 chance of inheriting it.

Introns
Noncoding DNA sequences that interrupt the sequences containing instructions for making a protein (exons). Introns are not represented in messenger RNA; only the exons are translated into protein. The function of introns is still being explored.

In vitro fertilization
Combination of an egg and a sperm outside of the mother's body to create an embryo that is implanted in her uterus.

"Knockout" mice
Experimental mice created by disrupting the function of a specific gene.

Messenger RNA
RNA transcribed from the DNA making up a gene. It transmits genetic information from the nucleus to the cytoplasm, where it directs the synthesis of proteins.

Mutation
A permanent transmissible change in DNA sequence. It can be an insertion or deletion of genetic information, or an alteration in the original genetic information.
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National Organization for Rare Disorders (NORD)

A federation of voluntary health organizations dedicated to helping people with rare diseases and to assisting the organizations that serve them. Headquarters: P.O. Box 8923, New Fairfield, CT 06812. Toll-free phone: (800) 999-6673. Or call (203) 746-6518. Fax: (203) 746-6481. Home page: www.pcnet.com/~orphan/

Nucleotide
A building block of DNA or RNA, consisting of one nitrogenous base, one phosphate molecule, and one sugar molecule (deoxyribose in DNA, ribose in RNA).

Oligonucleotide
A short string of nucleotides; a single-stranded segment of DNA, often used as a probe to find a matching sequence of DNA or RNA.

Oncogene
A gene that is capable of causing the transformation of normal cells into cancer cells.

Peptide
Two or more amino acids linked together.

Polymorphisms
Naturally occurring variations in DNA sequence. Polymorphisms are useful as genetic markers because they allow researchers to distinguish between DNA of different origins.

Positional cloning
Locating the gene for a specific trait by comparing the DNA of people who have that trait to the DNA of relatives who don't have the trait.

Promoter
A segment of DNA that acts as a controlling element in the expression of a gene. It marks the site of initiation of gene transcription.

Protein
A molecule composed of amino acids linked together in a particular order specified by a gene's DNA sequence. Proteins perform a wide variety of functions in the cell; these include serving as enzymes, structural components, or signaling molecules.

Proto-oncogene
A normal gene that can be altered (either through mutation or through nearby insertion of viral DNA) to become an active oncogene.

Recessive
Refers to a characteristic that is apparent only when two copies of the gene encoding it are present--one from the mother and one from the father.

Recombinant DNA technology
Techniques for cutting apart, splicing together, and producing pieces of DNA from different sources.

Restriction fragment length polymorphism (RFLP)
A variation in DNA sequence that is easily recognized because it occurs at a site where a restriction enzyme cuts a specific sequence, producing DNA fragments of varying lengths. RFLP's often serve as genetic markers.

RNA (ribonucleic acid)
A single-stranded nucleic acid (similar to the double-stranded nucleic acid DNA) that contains the sugar ribose (DNA contains deoxyribose). There are many forms of RNA, including messenger RNA, transfer RNA, and ribosomal RNA (all involved in protein synthesis).

Sex chromosome
One of the chromosomes (X or Y) involved in sex determination. Normal human females have two X chromosomes in each cell, while normal males have one X and one Y.

Sickle cell anemia
A genetic disease in which the hemoglobin protein is mutated, giving red blood cells a twisted shape that may painfully block circulation. This often leads to medical crises and may cause an early death. The recessive disease occurs in children who have inherited the mutated gene from both their parents, mostly among people of African or Mediterranean origin.

Single-stranded DNA
DNA normally exists in two complementary strands that form a double helix, but it can be separated into two strands of single-stranded DNA. Separation becomes necessary when genes are being transcribed or when DNA is replicated prior to cell division.

Somatic cell
In multicellular organisms, a cell of the soma, or tissues, as opposed to a cell of the germ line, or sex cell.

Transcription
see gene transcription.

Transgenic mice
Mice that have a foreign gene introduced into their cells.

Translation
see gene translation.

Tumor-suppressor gene
A gene that helps prevent the transformation of normal cells into cancer cells. When both copies of a tumor-suppressor gene are mutated in a cell, however, the lack of suppression may lead to the development of cancer.

X chromosome
One of the chromosomes involved in sex determination. Normal human females have two X chromosomes in each somatic cell, while normal males have one X and one Y chromosome in each somatic cell. X-linked--Refers to any gene found on the X chromosome or traits determined by such genes. Refers also to the specific mode of inheritance of such genes.

Y chromosome
One of the chromosomes involved in sex determination. Normal human males carry one X chromosome and one Y chromosome in each somatic cell, while normal females have two X chromosomes in each somatic cell.

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