March 2, 1999

Adenine A base pair or nucleotide of DNA and RNA. Pairs with thymine in DNA, and pairs with uracil in RNA.

Allele The various sequences of DNA nucleotides for a locus or gene. E.g., the ABO blood system locus has the A, B, and O alleles and the Rhesus (Rh) blood group locus has positive (+) and negative (-) alleles. A gene or locus that has more than one common allele is called a polymorphism.

Amino acids The basic building block of proteins and enzymes. There are twenty amino acids, each of which is coded for by a codon (three adjacent nucleotides).

Amniocentesis Prenatal diagnostic technique in which a needle is inserted into the abdomen of a pregnant woman (usually in the 16th week of pregnancy) and a small amount of amniotic fluid is extracted. Fetal cells in the amniotic fluid are then analyzed for genetic abnormalities.

Anticipation Term used to denote a disorder that has an earlier age of onset and greater severity in more recent generations of a pedigree.

Autosome A pair of chromosomes that are alike in males and females; any chromosome other than the X and Y sex chromosomes.

Base pairs (bp) Also called nucleotides, base pairs comprise the information of DNA and RNA. They are linearly arranged with each nucleotide pairing with a complementary nucleotide. There are four base pairs for DNA. Adenine (A) pairs with thymine (T) and cytosine (C) pairs with guanine (G). For RNA, uracil (U) takes the place of thymine.

bp Abbreviation for base pair.

cDNA Abbreviation for complementary DNA.

centiMorgan (cM) Named after Thomas Hunt Morgan who discovered linkage and recombination. A unit used to measure the statistical distance between two loci. One cM is equivalent to the recombination frequency of 1% between the two loci.

Chromosome A structure of DNA and protein found in the cell nucleus. Each chromosome contains hundreds or thousands of the genes. Humans have 23 pairs of chromosomes, one of each pair being contributed by the mother and the other by the father. Genes are linearly arranged on the chromosome.

Cloning Cloning has two different meanings. (1) Two or more organisms with identical genotypes, usually with the implication that one organism has been derived from the genetic material of the other organism. (2) Gene cloning refers to the isolation (and often sequencing) of a gene) and the subsequent production of many copies of the gene to study it.

cM Abbreviation for centiMorgan.

Codon A sequence of three nucleotides that code for an amino acid. E.g., the sequence adenine-adenine-adenine (AAA) codes for the amino acid phenylalanine. Some codons also work as punctuation marks denoting the start and stop of a peptide chain.

Complementary DNA (cDNA) A single strand of DNA that is synthesized in the laboratory to have a complementary sequence to a particular messenger RNA.

Congenital A trait or disorder that is present from birth; may be due to genes and/or prenatal environment.

Congenital adrenal hyperplasia (CAH) Sometimes referred to as adrenogenital syndrome, CAH is a genetically heterogeneous collection of Mendelian recessive disorders where androgenizing substance build up in a developing fetus and masculinize the organism. The degree of masculinization is usually incomplete and dependent upon the actual Mendelian disorder. Individuals who are chromosomally XX are given gender corrective surgery, hormone replacement therapy, and are raised as girls.

Conserved DNA sequences Sequences of DNA that have changed very little over evolution. They are believed to code for proteins, enzymes, or regulatory regions that are especially important for life.

Contig (contig map) Short for "contiguous." Cloned sections of DNA that are contiguous and overlap each other. A number of contigs permit the mapping of a large section of a chromosome.

Crossing over (recombination) The pairing up of homologous chromosomes and the exchange of genetic material.

Cystic fibrosis A recessive genetic disorder that involves an accumulation of mucous in the lungs, creating difficulty in breathing and providing a ripe medium for infections. The most common Mendelian disorder among individuals of European ancestry.

Cytosine A base pair or nucleotide of DNA and RNA. Pairs with guanine.

DNA (deoxyribonucleic acid) The molecule that contains the genetic code for all earthly life forms except for a few types of virus. The molecule resembles a spiral staircase. The sides of the staircase are composed of sugars and phosphates and the rungs consist of a pair of nucleotides (or base pairs). The particular ordering of the nucleotides is responsible for the genetic instructions behind growth and development and behind the regulations of many physiological functions.

DNA fingerprint A series of DNA polymorphisms typed in a single individual; usually encountered in forensic applications and paternity testing.

DNA probe A single-stranded DNA (or RNA) sequence labeled with special chemicals. The sequence of nucleotides for the probe is engineered to complement another single-stranded DNA (the target sequence). The probe binds with the target and then techniques are used to "light up" the special chemical to see exactly where the target is located.

Dominance One of Mendel's original laws which he thought applied to all hereditary traits but is now know to apply to only some. Mendel defined it as the fact that the phenotype for a heterozygote is the same as the phenotype for one of the two homozygotes. Today dominance is defined at the fact that a heterozygote does not have a phenotypic value that is the exact average of the phenotype value of the two homozygotes.

Editing The process during which the introns in the transcripted RNA are cut out and the exons are spliced together to form messenger RNA.

Embryonic stem cells Embryonic cells, usually taken from a mouse, that are engineered by homologous recombination. The resulting gene(s) are then implanted into other embryos to produce transgenic organisms.

Endoplasmic reticulum A network-like structure scattered throughout the cell. Ribosomes, the locations for protein manufacture, are densely located on the endoplasmic reticulum.

Environment of evolutionary adaptation (EEA) The environment and ecology during which a species evolved as opposed to the environment and ecology in which we find the species today.

Enzyme A type of protein responsible for a chemical reaction in the body. A substrate binds to the enzyme, the enzyme performs the reaction, and the substrate is converted into a product. Enzymes typically end in the suffix "ase" with the body of the term denoting the effect. E.g., polymerase results in a polymer chain being created (as in splicing two sections of DNA together).

Epistasis Statistical interaction among different genes.

Exon A nucleotide sequence within a gene that contains part of the blueprint for a polypeptide chain. Often, the exons within a gene are interspersed with noncoding regions (or introns). After transcription of the DNA into RNA, the introns are spliced out and the exons are joined together to form messenger RNA. Exons are also called "coding regions."

Fitness See reproductive fitness.

Fragile sites A particular section of a chromosome where, under the microscope, the chromosome appears to break apart (or nearly break apart) when the chromosomes are chemically treated in an appropriate way. There are several fragile sites throughout the genome, the most widely reported of which is the site on the X chromosome responsible for Fragile X syndrome.

Fragile X syndrome An X-linked disorder due to an unstable trinucleotide repeat (CGG), fragile X is characterized by mental retardation, attention and conduct problems, abnormally long face with large ears, and macroorchidism (in postpubertal males). Normal individuals have between 6 and 50 CGG repeats while fragile X individuals have between 230 and 1000 repeats. The most common Mendelian disorder associated with mental retardation, fragile X has a prevalence of 1/1250 (males) and 1/2500 (females).

Gamete A cell containing half of the genetic information that is used for reproduction; a sperm or egg cell.

Gene A sequence of DNA packaged in a chromosome that contains the information to make a polypeptide chain or an RNA molecule.

Gene targeting Creating mutations in a single gene that is then implanted into an embryo for the purpose of creating transgenic animals. See also homologous recombination, embryonic stem cells, and knockout mice.

Genetic drift (random genetic drift, random drift, or drift) The change in allele or genotypic frequencies due to chance and to chance alone.

Genetic heterogeneity The same disorder (or trait) can be due to different loci. E.g., albinism, congenital adrenal hyperplasia (CAH).

Genetic marker A polymorphic section of DNA that has a known location on a chromosomes. Genetic markers are used in linkage analysis to test whether a trait allele cosegregates with a marker allele within pedigrees.

Genome The total genetic information for an individual organism or for a species. The human genome consists of about 3 billion nucleotides.

Genomic imprinting (parental imprinting or imprinting) A recently discovered phenomenon in which an allele may be differentially expressed in the phenotype depending on whether it was inherited from the father or the mother.

Germ cell A cell that gives rise to a sperm or egg cell; sometimes used to refer to the sperm or egg itself.

Guanine A base pair or nucleotide of DNA and RNA. Pairs with cytosine.

Haplotype (haploid genotype) A "genotype" consisting of the alleles for two or more linked loci; the alleles on a chromosome. For example, the haplotype Ab/aB denotes that a person has alleles A and b on one chromosome and alleles a and B on the other chromosome.

Heritability The degree to which observed (or phenotypic) individual differences can be attributed to genetic individual differences; the ratio of genetic variance to phenotypic variance. Broad-sense heritability is places all types of genetic variance in the numerator (additive genetic variance, dominance genetic variance, and all forms of epistatic genetic variance). Narrow-sense heritability places only the additive genetic variance in the numerator.

Heterozygote An organism or genotype that has two different alleles at a locus. E.g., for the ABO blood system, genotypes AB, AO, and BO are heterozygotes.

Homologous recombination A laboratory technique in which a section of DNA on a chromosome is replaced by another similar sequence. The purpose is usually to "knockout" the gene to determine what happens when the gene is disrupted.

Homozygote An organism or genotype that has two copies of the same alleles at a locus. E.g., for the ABO blood system, genotypes AA, BB, and OO are homozygotes.

Huntington's Disease (HD) A presenile dementia caused by an autosomal dominant gene located on the short arm of chromosome 4. The problem with the HD gene is a unstable trinucleotide repeat. The disorder involves gradual loss of motor coordination and dementia (a progressive and irreversible loss of cognitive functioning).

Imprinting see genomic imprinting

Independent Assortment One of MendelÌs original laws stating that the hereditary factor for one trait (e.g., pea shape) is independent of the hereditary factor for another trait (e.g., pea color).

Intron A section of DNA within a gene that does not code for a polypeptide chain. When a gene is transcribed into RNA the transcript RNA contains both the introns and the sections that contain the code for the peptide (exons). Introns are cut out and exons are spliced together to form messenger RNA.

Karyotype Pictures of stained chromosomes ordered by size.

kb Abbreviation for kilobase.

Kilobase (kb) 1,000 base pairs or nucleotides

Knockout mice Mice genetically engineered by homologous recombination so that a particular gene does not function and hence is "knocked out." Has nothing to do with Mike Tyson.

Linkage (linked genes) Genes that are close together on a chromosome.

Linkage analysis A procedure used to find the approximate chromosomal location of a gene. Linkage analysis traces the cosegregation of a trait allele with one or more marker alleles within pedigrees.

Locus Usually, another name for a gene; a continuous section of DNA on a chromosome.

Lyonization (Lyon hypothesis, X inactivation) The inactivation of one of the X chromosomes in the somatic cells of a female. Hence, females are genetic mosaics with respect to their X chromosomes.

Major gene (major locus) A gene that is part of a polygenic or oligogenic system but has a large influence on the phenotype.

Marker gene (marker locus or markers) A continuous sequence of DNA with a known chromosomal location. Markers are used in linkage analysis to find a gene for a trait or disease.

Meiosis Process of cell division during which a germ cell containing the full chromosome complement of an organism produces the sperm or egg cells with one-half of the chromosome complement of the organism.

Mendelian disorder or trait (monogenic disorder or trait) A disorder or trait due to a single gene.

Messenger RNA (mRNA) RNA that has been transcribed from DNA and then edited to contain the information for making a polypeptide chain.

Methylation In genetics, the attachment of methyl groups to nucleotides, especially cytosine, resulting in a reduced transcription of genes.

Mitochondria Organelles scattered through the cell that are responsible for oxidative metabolism. Mitochondria contain mitochondrial DNA (mtDNA) that is inherited through only the mother.

Mitosis Process of cell division that ends in two identical cells.

Monogenic disorder or trait (Mendelian disorder or trait) A disorder or trait due to a single gene.

Mosaic The existence of two genetically different cell lines in the same individual.

Mutagen Something that can cause a mutation (e.g., UV radiation, X-rays).

Mutation A irregular change in the DNA. Germinal mutations occur in the production of sperm or egg and are transmitted to the next generation. Somatic mutations, far more common than germinal mutations, influence all other cells of the organism. Mutations may affect only a single nucleotide (point mutation) or large sections of DNA up to a whole chromosome.

Natural selection Evolutionary process initially described by Charles Darwin and Alfred Wallace. Differential reproduction and subsequent transmission of heritable traits as a function of adaptation to the environment.

Nucleotide see base pair.

Neutral gene (or neutral locus) A section of DNA that does not contribute to reproductive fitness.

Nucleus The part of a cell that containing the chromosomes. With few exceptions (e.g., red blood cells), all cells have a nucleus.

Oligogenic disorder or trait Polygenic transmission but the number of genes influencing the disorder or trait is small.

Oligonucleotide A short nucleotide sequence of single-stranded DNA or RNA often used as a probe to find a matching sequence of DNA or RNA. Oligonucleotide primers are used in the polymerase chain reaction to make multiple copies of the gene of interest.

Oncogene A gene that can transform cells into cancer cells.

Parental imprinting see genomic imprinting.

Path analysis A visual, pictorial, and diagramatic method used in genetics to test mathematical models of genetic and familial transmission of traits.

PCR Abbreviation for polymerase chain reaction.

Penetrance The probability that an organism will exhibit a disorder (or trait) in the phenotype given that the organism has a genotype that allows the disorder (or trait) to be exhibited. If the probability is 1.0, the locus is termed fully penetrant. If the probability is less than 1.0, the locus is termed incompletely or partially penetrant. Retinoblastoma, a dominant disorder leading to cancer of retinal tissue, is 90% penetrant; hence, 90% of those who carry the gene will exhibit the trait. Modern methods of genotyping have demonstrated that many Mendelian disorders previously thought to be fully penetrant actually have very high penetrance but are not completely penetrant (e.g., cystic fibrosis).

Peptide chain (polypeptide chain) A sequence of two or more amino acids linked together. A gene contains the code for the sequence of amino acids in the polypeptide chain.

Pleiotropy (pleiotropism) The fact that a single gene influences more than a single phenotype.

Polygenic More than a single locus influences the phenotype. Sometimes used to denote the possibility that a large number of loci influence the phenotype.

Polymerase chain reaction (PCR) A technique for making many copies of a DNA sequence from only a small amount of DNA.

Polymorphism A gene that has more than one common allele.

Population structure A nonstandard term used in this text to refer to the spatial, temporal, dispersal, and mating aspects of a population (or species) as they effect the population's (or species') evolution.

Positional cloning Locating the precise position of a gene for a trait by comparing the DNA of individuals who have the trait with the DNA sequence of their family members who do not have the trait.

Preparedness (prepared learning) A genetic and/or biological predisposition that makes it easy for members of species to learn some behaviors but hard to learn others.

Probe A single stranded section of DNA that is labelled (radioactively or florescently). The probe will bind to its complementary DNA sequence and the label "lights it up."

Promoter region (promotor gene) A section of DNA usually upstream from a structural gene. The promoter region (gene) is where transcription enzymes bind and initiate transcription of the structural gene.

Protein One or more polypeptide chains taking on a three dimensional configuration. Proteins serve as structural components of a cell (structural proteins), assist in chemical reactions (enzymes), and serve as signaling molecules.

Pseudogene A gene with a nucleotide sequence very similar to a structural gene, but because of mutations can no longer be transcribed or translated.

Recombination A fancy term for crossing over.

Reproductive fitness The number of gametes or offspring that an leaves to the next generation. Absolute fitness refers to the raw number of gametes or offspring. Relative fitness divides absolute fitness by the absolute fitness of a reference indvidual. Fitness may also be expressed in terms of genotypes or phenotypes instead of individuals.

Restriction enzyme (technically, restriction endonuclease) An enzyme that recognizes a certain section of DNA and cuts the DNA at that spot.

Restriction fragment length polymorphism (RFLP) Different alleles for a locus are determined by cutting the DNA with a restriction enzyme and analyzing the lengths of the resulting sections. A RFLP is usually used as a genetic marker.

Ribonucleic acid (RNA) A single-stranded sequence of nucleotides comprised of adenine, uracil, cytosine, and guanine. There are several species of RNA, the most important of which for protein synthesis are messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA).

Sequence Tagged Sites (STS) A relatively short segment of DNA that occurs only once in the human genome, has a known chromosomal location, and has a known nucleotide sequence. STS serve as "landmarks" of the genome and assist in the development of further chromosomal mapping and DNA sequencing.

Segregation One of Mendel's original laws stating that of the two parental alleles at a gene, only one taken at random is transmitted to an offspring.

Shifting balance theory A theory developed by Sewall Wright to explain the dynamic interaction among the different forces of evolution.

Somatic cell Any cell of the organism that is not a germ cell.

Structural gene A gene containing the code for making a polypeptide chain, a protein, or an enzyme.

STS Abbreviation for sequence tagged sites.

Tay Sachs Disease A disorder of lipid (fat) storage caused by an autosomal recessive allele. Infants with Tay Sachs fail to develop normally and usually die within a few months of birth. The Tay Sachs allele is found with high frequency among Askenazie Jews (Jews of eastern European ancestry), and a concerted effort in this community to identify heterozygotes (carriers) has dramatically reduced the frequency of the disorder.

Thymine A nucleotide of DNA only. In RNA thymine is replaced by uracil. Pairs with adenine.

Transcription The process whereby a strand of RNA is synthesized from the DNA resulting in a "mirror image" of the DNA.

Transfer RNA (tRNA) A molecule of that contains, among other things, a recognition site of 3 nucleotides (called an anticodon) and an amino acid. In protein synthesis, the anticodon pairs with the codon from messenger RNA and the amino acid is transferred into the growing polypeptide chain.

Translation. The process in which the sequence of information on the messenger RNA is read and a chain of amino acids is constructed from the information.

Translocation The process in which a segment of one chromosome breaks off and attaches to anther chromosome.

Trisomy Having three chromosomes instead of two homologous chromosomes. Trisomy 21 is the most frequent cause of Down's Syndrome.

Tumor-suppression gene A gene whose normal function is to prevent the transformation of a cell into a cancer cell. If a tumor-suppression gene mutates, cancer may result.

Uracil A nucleotide of RNA only. In DNA, thymine takes the place of uracil. Pairs with adenine.

Variable expressivity (variable expression) A single locus can result is a different degrees of expression of a phenotypic trait. E.g., untreated phenylketonuria results in intelligence starting from severe mental retardation but going into the normal range.

Variable number of tandem repeats (VNTR) A polymorphic section of DNA where the polymorphism consists of the number of times that a nucleotide sequence is repeated. E.g., one allele may have the sequence CAGAT repeated 6 times while another allele may have the same sequence repeated 10 times.

X inactivation See lyonization.

X-linked A gene located on the X chromosome.

zygote A fertilized egg.