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N O P Q R S T U V W X Y Z
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Allele: alternative forms of a gene or DNA sequence at a given location on a chromosome.
Amino Acid: the basic building block of proteins. There are 20 different amino acids that link together in various order to form proteins. The order of amino acids is determined by the genetic sequence.
Amplification: the creation of numerous copies of a given DNA sequence, generally through the polymerase chain reaction (PCR).
Annealing: the pairing of complementary single strands of DNA to form a double helix.
Anticipation: the phenomenon where dominantly inherited diseases have an earlier age of onset and increasing severity in successive generations.
Autosome: any chromosome other than the sex chromosomes (X and Y) and mitochondrial chromosomes. Humans have 22 pairs of autosomes.
Base pairs: pairs of complementary bases that interact to form the runs of the DNA double helix. Adenine pairs with thymine; guanine pairs with cytosine.
BRCA1 or 2: two genes that have been linked to an increased risk for familial breast cancer
Carrier: an individual who is heterozygous for a mutant allele that only causes disease in the homozygous state.
CGEP: the Community Genetics and Ethics Project.
Chromosome: a structure of DNA, coiled around proteins, which is found in the nucleus. Humans have 23 pairs of chromosomes (a set is inherited from each parent). Each chromosome has hundreds to thousands of genes arranged linearly along the chromosome.
Codon: a triplet of nucleotides in DNA or RNA that specify a specific single amino acid.
Compementary: the specificity of base pairing in the DNA helix. Guanine pairs with cytosine and adenine pairs with thymine. Thus guanine is said to be complementary to cytosine and adenine is complementary to thymine.
Crossing over: The reciprocal breaking and rejoining of homologous chromosomes during meiosis that results in exchange of chromosomal segments.
Cystic Fibrosis: a genetic disease transmitted by recessive inheritance. The disease is characterized by a buildup of mucus in the lungs and pancreatic insufficiency.
Cytoplasm: the substance of the cell surrounding the nucleus. The majority of organelles are found in this compartment of the cell.
Cytosine: one of the nitrogenous bases found in DNA and RNA. Cytosine (C) pairs with guanine (G) to form a rung in the DNA helix.
Deletion: a genetic mutation involving the loss of DNA. This may be small, affecting only a portion of a single gene, or it may be large, such as partial or complete loss of a chromosome, affecting many genes.
Denaturation: conversion of double stranded DNA into single strands, generally accomplished by heating.
Dominant: conditions or traits that are observable in individuals who have just one copy of the mutant gene (heterozygotes).
DNA (deoxyribonucleic acid): the genetic material of living organisms, the molecule of heredity. A double helix containing the sugar deoxyribose.
DOE: United States Department of Energy, one of the funding agencies of the Human Genome Project.
ELSI: Ethical, Legal and Social Implications
Enzyme: a protein that acts as a catalyst in a biological reaction or process. Enzymes help speed up biological reactions, but are not consumed or destroyed in the reaction.
Eugenics: literally meaning "well born." Eugenics is a genetic and social theory whereby the human race is improved by selective reproduction whereby desireable characteristics are propogated and undesireable traits are eliminated.
Exon: the portion of a gene that contains the code for a protein.
Expressivity: refers to the nature and severity of a phenotype. Autosomal dominant traits frequently have variable expressivity.
Extension: elongation of a growing chain of newly synthesized DNA.
Frameshift mutation: a genetic mutation that involves a deletion or insertion that is not an exact multiple of three nucleotides. A frameshift mutation changes the reading frame of a gene and generally results in a premature stop codon.
Gametes: the sex cells (eggs in females and sperm in males). Gametes differ from other cells in that they contain only one of each chromosome (instead of two).
Gel electrophoresis: a technique where DNA migrates through a gel matrix in reponse to an applied electrical current. Gel electrophoresis allows DNA to be separated based on size.
Gene: a sequence of DNA that represents a basic unit of heredity. Genes function by being expressed in RNA and proteins.
Gene expression: the process where the information found in a gene is converted into a functional molecule (RNA or protein).
Gene therapy: the treatment of a disease by introducing a new gene into a cell or cells.
Genetic code: the nucleotide triplets that specify the twenty different amino acids.
Genetic marker: an indentifiable segment of DNA that can be mapped to a region of a chromosome.
Genetic counseling: a process of assessing and communicating risk for genetic disease, providing information to individuals affected with a genetic disease regarding treatment and reproductive options.
Genetic screening: testing a population to identify subsets of individuals at risk for a given genetic condition or at risk for transmitting a condition to their offspring.
Genetic testing: the analysis of DNA, RNA, chromosomes or metabolites to identify genetic alterations related to a heritable condition.
Genome: the complete DNA sequence (total genetic information) of an organism.
Genomic imprinting: the differential expression of a gene depending on whether it was inherited from the father or mother.
Genotype: the genetic makeup of an individual.
Germ cells: the cells that give rise to the reproductive cells (sperm and egg).
Germline gene therapy: the introduction of genetic material into reproductive cells (sperm and egg) such that the introduced genetic material is then incorporated into the host genome and passed onto subsequent generations.
Germline mutation: a mutation present in the reproductive cells, and thus is transmissible to offspring.
Guanine: one of the nitrogenous bases found in DNA and RNA. Guanine (G) pairs with cytosine (C) to form a rung in the DNA helix.
HGP: Human Genome Project. A project to map and sequence all the DNA in the human genome.
Hemizygous: having only a single copy of a gene. For instance, genes located on the Y-chromosome or in the case where a gene or portion of DNA has been deleted.
Heterozygote: an individual who has two different alleles at a given genetic locus (is heterozygous).
Heterozygous: having two different alleles at a given genetic locus.
Homozygote: an individual who has the same alleles at a given genetic locus.
Homozygous: having the same alleles at a given genetic locus.
Insertion: a genetic mutation where nucleotides are added (inserted) into a DNA sequence. This may be small, affecting only a single gene, or may involve portions of a chromosome.
Intron: the non-coding regions of a gene that interrupt the protein coding regions (exons).
Inversion: a chromosomal abnormality where a segment of a chromosome is rotated 180° and reinserted.
Karyotype: the chromosome makeup of an individual. Also used to describe the photograph of chromosomes of an individual.
Linkage: the tendency of genes or DNA sequences to be inherited together as a result of their proximity on the same chromosome.
Locus: a specific position or location on a chromosome.
Loss of heterozygosity (LOH): loss of one allele in a tumor from a locus for which the individual's normal cells are heterozygous (have two alleles).
Marker: a tag near or within a gene of interest. Genetic markers are generally very polymorphic.
Meiosis: a type of cell division that occurs in the germ cells. The end result is the production of gametes (eggs and sperm) with just one copy of the chromosomes.
Messenger RNA (mRNA): the template used for protein synthesis. mRNA is generated by transcription of a gene (DNA).
Microsatellite: a polymorphic sequence of DNA consisting of tandemly repeated units of DNA where the repeat unit is usually 1-4 nucleotides long.
Missense mutation: a genetic mutation where a single base substitution changes the codon thereby specifying a different amino acid.
Mitochondrial DNA: the DNA found in the chromosomes of the mitochondria. Mitochondria are organelles in the cell which possess their own unique DNA and are maternally inherited.
Multifactorial inheritance: traits that result from the interplay of environmental and genetic factors.
Mutation: a permanent, transmissible change in the DNA sequence. This can be loss or addition of genetic material (deletion or insertion) or a change in the original gene sequence.
NIH: National Institutes of Health, one of the funding agencies of the Human Genome Project.
Nonsense mutation: a genetic mutation where a single nucleotide substitution creates a stop codon.
Nucleotide: the building block of DNA and RNA. A nucleotide consists of a nitrogenous base, a sugar, and a phosphate molecule all linked together.
Nucleus: the portion of the cell that contains the genetic material (DNA is the form of chromosomes).
Oncogene: a gene normally found in a cell, which when mutated contributes to the transformation from normal cell to tumor cell. Oncogenes act in a dominant manner, so only one allele needs to be mutated to see an effect on function.
Organelle: a structure within a cell that is specialized for a particular function.
Pedigree: a diagram of a family history that shows relationships and individuals affected with a given hereditary condition.
Penetrance: the fraction of persons who have a deleterious gene who develop the disease. In complete penetrance, 100% of individuals who have the mutated gene will develop the condition.
Pharmacogenetics: the study of the genetic basis for differing responses to drugs.
Phenotype: the observed characteristics of an individual that is a result of the interaction of his or her genes with the environment.
Point mutation: the substitution of one nucleotide for another in the genetic sequence.
Polygenic: describes traits or diseases caused by the interaction of multiple genes, each of which has a relatively small effect.
Polymerase Chain Reaction (PCR): a technique in which multiple copies of a specific DNA sequence are created (amplified).
Polymorphic: referring to gene sequences that have variant forms.
Polymorphism: a naturally occurring variation in the DNA sequence. Polymorphisms are generally considered to be non-deleterious variations in the genetic sequence (i.e. they do not cause disease).
Primer: in PCR, a short segment of DNA (usually 18-22 nucleotides long) used to target and originate replication of a specific DNA sequence.
Protein: a molecule made of amino acids linked together in an order specified by a gene sequence. Proteins are involved in a variety of cellular functions including acting as: enzymes, structural components, or signaling molecules.
Reading frame: the possible ways to read a nucleotide sequence as a series of triplets translated to codons for protein synthesis.
Recessive: a condition or trait that is only observed in individuals who have two copies of the variant allele (are homozygous).
Recombinatin: the formation of new combinations of genes caused by the exchange of genetic material between homologous chromosomes.
Restriction enzyme: a protein isolated from bacteria that recognizes specific DNA sequences and cuts the DNA at those sites.
Restriction fragment length polymorphism (RFLP): a variation in the DNA sequence that affects a recognition site for a given restriction enzyme. The genetic change alters the length the DNA fragment following cutting with the enzyme.
Ribosome: an organelle, composed of protein and ribosomal RNA, which is the site of protein synthesis in the cytoplasm.
RNA, ribonucleic acid: a single stranded nucleic acid containing the sugar ribose. There are several forms of RNA (messenger RNA, ribosomal RNA, transfer RNA) all of which are involved in protein synthesis and ribozymes which are RNA molecules which have enzymatic activity.
Sex chromosome: the chromosomes involved in sex determination; the X and Y chromosomes in humans.
Silent mutation: a change in the genetic sequence that does not alter the protein sequence.
Somatic: referring to the non-reproductive parts of the body (not germ cells).
Somatic gene therapy: the insertion of new genetic material into a cell or tissue other than the reproductive cells. Thus the new genetic material will not be passed onto subsequent generations.
Somatic mutation: a mutation occurring in a somatic cell (non-reproductive cell). Because it does not occur in reproductive cells, somatic mutations are not passed on to offspring.
Splicing: the removal of introns from messenger RNA and the fusion of exons to form the final template for protein synthesis.
Syndrome: a pattern of multiple anomalies believed to be pathologically related.
Thymine: one of the nitrogenous bases found in DNA. Thymine (T) pairs with adenine (A) to form a rung in the DNA helix.
Transcription: the process where a gene's DNA sequence is copied into messenger RNA.
Translation: the process where messenger RNA directs the creation of proteins from amino acids.
Translocation: a type of chromosome alteration where part of a chromosome is transferred to another part of a different chromosome.
Trisomy: the condition of having three copies of a chromosome instead of two.
Tumor suppressor gene: a gene found normally in cells, which contributes to tumor development when both copies of the gene are deleted or inactivated. The name is derived from their normal function which would block tumor development.
Uniparental disomy: the phenomenon where two copies of a given chromosome are inherited from one parent (no copy of that chromosome is inherited from the other parent).
Uracil: one of the nitrogenous bases found in RNA. Uracil (U) replaces the nitrogenous base, which is found in DNA but not RNA.
X chromosome: one of the chromosomes involved in sex determination in humans. Females have two X chromosomes, while males have one X chromosome and one Y chromosome.
X-linked disease: a disease caused by a gene located on the X-chromosome.