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Abortion: premature expulsion or loss of an embryo; may be induced or spontaneous (miscarriage).

Allele: any one of a number of alternative forms of the same gene.

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.

Altered nuclear transfer (ANT): theroretical procedure where the nucleus from an egg is removed and an altered genome is inserted so that, in theory, a human clone is not created and human pluripotent stem cells can still be harvested; an proposed alternative to somatic cell nuclear transfer (SCNT) or cloning.

Amniocentesis: sampling of amniotic fluid by needle aspiration for genetic analysis of a fetus.

Artificial Insemination (AI): a procedure where sperm is placed in a woman’s uterus, cervix, or vagina by means other than sexual intercourse. 

Asexual reproduction: reproduction without the meeting of egg and sperm, usually creating an exact genetic copy of the original; cloning is asexual reproduction.

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

Blastocyst: early stage in embryonic development (5-7 days after fertilization) before implantation; consists of outer trophoblast and inner stem cell mass.

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

Carrier testing: testing to identify individuals who carry disease-causing recessive genes that can 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.

Chromosomes: structures found in the nucleus of a cell that 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, acheived in mammals by somatic cell nuclear transfer.  (See also Reproductive cloning and Therapeutic cloning.)

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

Diabetes: a disease associated with the absence or reduced levels of insulin.  (Insulin is a hormone essential for the transport of glucose to cells.)

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.)

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

Enzyme: a protein that facilitates a specific chemical reaction.

Embryo: early stages of fetal growth, from conception to the eighth week of pregnancy.

Fetus: an organism in the stage of development that follows the embryonic stage; in humans, an unborn baby from the eighth week of pregnancy to birth.

Gamete: a haploid sex cell, egg, or sperm, that contains a single copy of each chromosome.

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 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 engineering: the manipulation of an organism's genome by introducing or eliminating specific genes through modern molecular biology techniques.  A broad definition of genetic engineering also includes selective breeding and other means of artificial selection.

Gene modification: the chemical repair of a gene's defective DNA sequence.

Gene pool: the totality of all alleles of all genes of all individuals in a particular population.

Genetic disease: a disease that has its origin in mutations to the genetic material, DNA.

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

Genotype: the structure of DNA that determines the expression of a trait.  (See also Phenotype.)

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

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

Human Genome Project: a project coordinated by the National Institute of Health (NIH) and the Department of Energy (DOE) to determine the entire sequence of DNA of the human chromosomes.

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.

Insulin: a peptide hormone secreted by the islets of Langerhans of the pancreas that regulates the level of sugar in the blood.

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

Mitochondria: normal structures called organelles in cells,  located in the cell's cytoplasm outside the nucleus.

Mitochondrial DNA:  DNA found in the mitochondria of a cell, the mitochondrial chromosome comes from the egg so it is inherited only from the mother.

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

Natural selection. The differential survival and reproduction of organisms with genetic characteristics that enable them to better utilize environmental resources.

Nucleus: the cell structure that houses the chromosomes.

Ovum: a female gamete or egg, also called an oocyte.

Phenotype: the observable characteristics of an organism; the expression of gene alleles (genotype) as an observable physical or biochemical trait.  (See Genotype.)

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.

Prenatal diagnosis: examining fetal cells taken from the amniotic fluid or the umbilical cord for evidence of disease.

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.)

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

Reproductive cloning: somatic cell nuclear transfer (SCNT), where the resulting cloned organism is implanted into a surrogate for continued development.  In humans, it is cloning to produce a child.

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

Sexual reproduction: the process where two cells (gametes) fuse to form one hybrid, fertilized cell.

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.

Species: a classification of related organisms that can freely interbreed.

Somatic cells: all body cells except the reproductive cells.

Somatic cell nuclear transfer (SCNT): also known as cloning, a procedure where the nucleus of an egg is removed and the nucleus of a somatic cell is placed in the egg.  The egg is then "shocked" and begins to grow as if it were fertilized.  The resulting embryo is a genetic clone of the donor of the somatic cell.  Often called therapeutic cloning.

Stem Cells: cells that continue to grow and proliferate, and can differentiate into particular specialized cell types.  Typically divided into two groups: embryonic stem cells (ECS) that are harvested from embryos, and adult stem cells (ASC) that come from tissues other than embryonic tissues.

Therapeutic Cloning: somatic cell nuclear transfer (SCNT) where the resulting cloned embryo is allowed to develop to the blastocyst stage or farther, and then destroyed for stem cells or other tissue.

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

Transgenic animal: genetically engineered animal or offspring of genetically engineered animals.  The transgenic animal usually contains material from at least one unrelated organism, such as a virus, plant, or other animal.

Zygote: the first cell formed by the fusion of egg and sperm (and the combination of their separate genomes) or by the implantation of a somatic cell nucleus into an egg in somatic cell nuclear transfer, or cloning.