Allele: An alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome.

Amniocentesis: A procedure in which amniotic fluid is removed from the uterus for testing or treatment. Amniotic fluid is the fluid that surrounds and protects a baby during pregnancy. This fluid contains fetal cells and various chemicals produced by the baby.

Amplification: The production of multiple (>millions) copies of a sequence of DNA. Repeated copying of a piece of DNA.

Anemia: Decrease in number of red blood cells (RBCs) or less than the normal quantity of hemoglobinin the blood. It can include decreased oxygen-binding ability of each hemoglobin molecule. Hemolytic anemia occurs when red blood cells are being destroyed prematurely.

Aneuploidy: The condition of having less than or more than the normal diploid number of chromosomes.

Anticipation: A phenomenon whereby the symptoms of a genetic disorder become apparent at an earlier age as it is passed on to the next generation. Anticipation is common in trinucleotide repeat disorders such as Huntington's disease and myotonic dystrophy where a dynamic mutation in DNA occurs.

Autoimmunity: Process whereby the immune system reacts against the body's own tissues. Autoimmunity may produce or be caused by autoimmune diseases.

Autosome: A chromosome not involved in sex determination. The diploid human genome consists of a total of 46 chromosomes: 22 pairs of autosomes, and 1 pair of sex chromosomes (the X and Y chromosomes).


Carrier: A carrier is an individual who carries and is capable of passing on a genetic mutation associated with a disease and may or may not display disease symptoms. Carriers are associated with diseases inherited as recessive traits. In order to have the disease, an individual must have inherited mutated alleles from both parents. An individual having one normal allele and one mutated allele does not have the disease. Two carriers may produce children with the disease.

cDNA: The abbreviation ofcomplementary DNA. It is DNA synthesized from an mRNA template in a reaction catalyzed by the enzyme reverse transcriptase.

Cell culture: The maintenance and growth of the cells of multicellular organisms outside the body in specially designed containers and under precise conditions of temperature, humidity, nutrition, and freedom from contamination. In a broad sense, cells, tissuses, and organs that are isolated and maintained in a laboratoryare considered the objects od tissue culture.

Centromere: The clear constricted portion of the chromosome at which the chromatids are joined and by which the chromosome is attached to the spindle during cell division.

Chimerism: The occurrence in an individual of two or more cell populations of different chromosomal constitutions, derived from different individuals. This contrasts with mosaicism in which the different cell populations are derived from a single individual.

Chromosome: An organized package of DNA found in the nucleus of the cell.Chromosomes come in pairs, and a normal human cell contains 46 chromosomes.

Cloning: The process of making identical copies of an organism, cell, or DNA sequence. Molecular cloning is a process by which scientists amplify a desired DNA sequence. The target sequence is isolated, inserted into another DNA molecule (known as a vector), and introduced into a suitable host cell. Then, each time the host cell divides, it replicates the foreign DNA sequence along with its own DNA.

Compound heterozygote: An individual who has two different abnormal alleles at a particular locus, one on each chromosome of a pair; usually refers to individuals affected with an autosomal recessive disorder.

Congenital: Present from birth, but not necessarily genetic.

CVS: The abbreviation of chorionic villus sampling. The removal of a small piece of placenta tissue (chorionic villi) from the uterus during early pregnancy to screen the baby for genetic defects.


Deformation: Congenital anomaly resulting from external compression of a normally formed part of the fetus.

Deletion: A type of mutation (a genetic aberration) in which a part of a chromosome or a sequence of DNA is missing. Deletion is the loss of genetic material.

De novo mutation: Analteration in agene that is present for the first time in one family member as a result of a mutation in a germ cell (egg or sperm) of one of the parents or in the fertilized egg itself.

Dominant: Alleles that determine the phenotype displayed in a heterozygote with another (recessive) allele.

Duplication: The presence of an extra segment of DNA, resulting in redundant copies of a portion of a gene, an entire gene, or a series of genes, usually caused by unequal crossing-over during gene replication when gametes are formed in meiosis.

Euchromatin: The genetically active portion of chromatin that is largely composed of genes.

Exon: Coding sequence of DNA present in mature mRNA. DNA initially transcribed to mRNA consists of coding sequences (exons) and non-coding sequences (introns). Introns are spliced out of the messenger RNA prior to translation, leaving only the exons to ultimately encode the amino acid product.

Fetus: An unborn or unhatched vertebrate especially after attaining the basic structural plan of its kind; specifically: a developing human from usually two months after conception to birth.

FISH: Fluorescence in situ hybridization (FISH) is a laboratory technique for detecting and locating a specific DNA sequence on a chromosome. The technique relies on exposing chromosomes to a small DNA sequence called a probe that has a fluorescent molecule attached to it. The probe sequence binds to its corresponding sequence on the chromosome.

Fibroblast: Connective tissue cells which secrete an extracellular matrix rich in collagen and other macromolecules.


Gametes: Mature male or female reproductive cell (sperm or ovum) with a haploid set of chromosomes (23 for humans).

Gene: The gene is the basic physical unit of inheritance. Genes are passed from parents to offspring and contain the information needed to specify traits. Genes are arranged, one after another, on structures called chromosomes. A chromosome contains a single, long DNA molecule, only a portion of which corresponds to a single gene.

Gene mutation: A mutation due to an intramolecular reorganization of a gene.

Genetics: The branch of biology that deals with heredity, especially the mechanisms of hereditary transmission and the variation of inherited characteristics among similar or related organisms.

Genetic mutation: Changes in the nucleotide sequence of the genetic material (i.e. DNA, or RNA, in the case of viruses), which are usually caused by copying errors during replication that further lead to base substitution, insertion, or deletion of one or more base pairs.

Genetic counseling: A process, involving an individual or family, comprising: evaluation to confirm, diagnose, or exclude a genetic condition, malformation syndrome, or isolated birth defect; discussion of natural history and the role of heredity; identification of medical management issues; calculation and communication of genetic risks; provision of or referral for psychosocial support.

Genetic code: The instructions in a gene that tell the cell how to make a specific protein. A, C, G, and T are the "letters" of the DNA code; they stand for the chemicals adenine (A), cytosine (C), guanine (G), and thymine (T), respectively, that make up the nucleotide bases of DNA. Each gene's code combines the four chemicals in various ways to spell out three-letter "words" that specify which amino acid is needed at every step in making a protein.

Genetic predisposition: see “Predisposition”

Genetic susceptibility: see “Predisposition”

Genome: The genome is the entire set of genetic instructions found in a cell. In humans, the genome consists of 23 pairs of chromosomes, found in the nucleus, as well as a small chromosome found in the cells' mitochondria. These chromosomes, taken together, contain approximately 3.1 billion bases of DNA sequence.

Genomic imprinting: The process by which maternally and paternally derived chromosomes are uniquely chemically modified leading to different expression of a certain gene or genes on those chromosomes depending on their parental origin. For instance, both Prader-Willi and Angelman syndromes are inherited when the same part of chromosome 15 is missing. When the father's complement of 15 is missing, the child has Prader-Willi, but when the mother's complement of 15 is missing, the child has Angelman syndrome.

Genotype: The genetic constitution of an organism or cell; also refers to the specific set of alleles inherited at a locus.

Germ cell: see “Gametes”

Germline mosaicism: see “Gonadal mosaicism”

Gonad: The part of the reproductive system that produces and releases eggs (ovary) or sperm (testicle/testis).

Gonadal mosaicism: Two or more genetic or cytogenetic cell lines confined to the precursor (germline) cells of the egg or sperm.

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

Haploid: A single set of chromosomes (half the full set of genetic material) present in the egg and sperm cells of animals and in the egg and pollen cells of plants. Human beings have 23 chromosomes in their reproductive cells.

Haplotype: A group of alleles of different genes on a single chromosome that are closely enough linked to be inherited usually as a unit.

Hemizygous: Describes an individual who has only one member of a chromosome pair or chromosome segment rather than the usual two; refers in particular to X-linked genes in males who under usual circumstances have only one X chromosome.

Heredity: The passing of traits to offspring (from its parent or ancestors).

Heterochromatin: Densely staining chromatin that appears as nodules in or along chromosomes and contains relatively few genes.

Heterozygous: With respect to a particular trait or condition, an individual who has inherited two different alleles, usually one normal and the other abnormal, at a particular locus.

HLA: One of a group of proteins found on the surface of white blood cells and other cells that play an important part in the body's immune response to foreign substances. These antigens vary from person to person, and an HLA test is done before organ transplantation to find out if tissues match between a donor and a recipient.

Homolog chromosomes: The two chromosomes from a particular pair, normally one inherited from the mother and one from the father, containing the same genetic loci in the same order.

Homozygous: A genetic condition where an individual inherits the same alleles for a particular gene from both parents.

Ideogram: Ideograms are a schematic representation of chromosomes. They show the relative size of the chromosomes and their banding patterns.

In situ: To examine the phenomenon exactly in place where it occurs (i.e. without moving it to some special medium).

In situ hybridization: A type of hybridization that uses a labeled complementary DNA or RNA strand (i.e., probe) to localize a specific DNA or RNA sequence in a portion or section of tissue or, if the tissue is small enough, in the entire tissue. DNA ISH can be used to determine the structure of chromosomes.

In vitro: Outside a living organism, in the laboratory (outside the body). The opposite of in vivo (in the body).

In vivo: Inside a living organism, occurring in the body. The opposite of in vitro (outside the body).

Index case: The affected individual through whom a family with a genetic disorder is ascertained; may or may not be the consultand (the individual presenting for genetic counseling).

Infertility: The inability to produce children.

Informative family: A family having at least one parent contributing to the variance of the offspring genotype during transmission.

Intron: Non-coding sequence of DNA removed from mature mRNA prior to translation.DNA initially transcribed to mRNA consists of coding sequences (exons) and non-coding sequences (introns). Introns are spliced out of the messenger RNA prior to translation, leaving only the exons to ultimately encode the amino acid product.

Inversion: A chromosomal rearrangement in which a segment of genetic material is broken away from the chromosome, inverted from end to end, and re-inserted into the chromosome at the same breakage site. Balanced inversions (no net loss or gain of genetic material) are usually not associated with phenotypic abnormalities, although in some cases gene disruptions at the breakpoints can cause adverse phenotypic effects, including some known genetic diseases; unbalanced inversions (loss or gain of chromosome material) nearly always yield an abnormal phenotype.

Isochromosome: A chromosome produced by transverse splitting of the centromere so that both arms are from the same side of the centromere, are of equal length, and possess identical genes.


Karyotype: A photographic representation of the chromosomes of a single cell, cut and arranged in pairs based on their banding pattern and size according to a standard classification.

Linkage: The tendency of certain loci or alleles to be inherited together.Genetic loci that are physically close to one another on the same chromosome tend to stay together during meiosis, and are thus genetically linked.

Locus: The physical site or location of a specific gene on a chromosome.


Malformation: Irregular, anomalous, abnormal, or faulty formation or structure (congenital malformations).

Maternal inheritance: Inheritance of characters transmitted through extranuclear elements (as mitochondrial DNA) in the cytoplasm of the egg.

Metaphase: A stage in mitosis or meiosis during which the chromosomes are aligned along the equatorial plane of the cell.

Monosomy: The presence of only one chromosome from a pair; partial monosomy refers to the presence of only one copy of a segment of a chromosome.

Mosaic: An organism or one of its parts composed of cells of more than one genotype.

Mosaicism: The post-fertilization occurrence of two or more cell lines with different genetic or chromosomal constitutions within a single individual or tissue.

Multifactorial inheritance: The combined contribution of one or more often unspecified genes and environmental factors, often unknown, in the causation of a particular trait or disease.

Mutant: An individual, organism, or new genetic character, arising or resulting from an instance of mutation, which is a base-pair sequence change within the DNA of a gene or chromosome of an organism resulting in the creation of a new character or trait not found in the wild type.

Mutation: a change in a DNA sequence. N:

Nondisjunction: The failure of homologous chromosomes to separate in meiosis I, or the failure of sister chromatids to separate during meiosis II or mitosis. The result of this error is a cell with an imbalance of chromosomes.

Nucleotide: A nucleotide is the basic building block of nucleic acids. RNA and DNA are polymers made of long chains of nucleotides. A nucleotide consists of a sugar molecule (either ribose in RNA or deoxyribose in DNA) attached to a phosphate group and a nitrogen-containing base. The bases used in DNA are adenine (A), cytosine (C), guanine (G), and thymine (T). In RNA, the base uracil (U) takes the place of thymine.

Oligonucleotide: Polymers made up of a few (2-20) nucleotides. In molecular genetics, they refer to a short sequence synthesized to match a region of interest.

Oncogene: A gene, one or more forms of which is associated with cancer. Many oncogenes are involved, directly or indirectly, in controlling the rate of cell growth.


PCR: Polymerase chain reaction. A laboratory technique used to amplify DNA sequences. The method involves using short DNA sequences called primers to select the portion of the genome to be amplified. The temperature of the sample is repeatedly raised and lowered to help a DNA replication enzyme copy the target DNA sequence. The technique can produce a billion copies of the target sequence in just a few hours.

Pedigree: A pedigree is a genetic representation of a family tree that diagrams the inheritance of a trait or disease though several generations. The pedigree shows the relationships between family members and indicates which individuals express or silently carry the trait in question.

Penetrance: The proportion of individuals with a mutation causing a particular disorder who exhibit clinical symptoms of that disorder; a condition (most commonly inherited in an autosomal dominant manner) is said to have complete penetrance if clinical symptoms are present in all individuals who have the disease-causing mutation, and to have reduced or incomplete penetrance if clinical symptoms are not always present in individuals who have the disease-causing mutation.

Pharmacogenetics: The study or clinical testing of genetic variation that gives rise to differing response to drugs.

Polygenic disorder: Genetic disorder resulting from the combined action of alleles of more than one gene (e.g., heart disease, diabetes, and some cancers). Although such disorders are inherited, they depend on the simultaneous presence of several alleles; thus the hereditary patterns usually are more complex than those of single-gene disorders.

Polymorphism: Natural variations in a gene, DNA sequence, or chromosome that have no adverse effects on the individual and occur with fairly high frequency in the general population.

Polyploidy: An increase in the number of haploid sets (23) of chromosomes in a cell. Triploidy refers to three whole sets of chromosomes in a single cell (in humans, a total of 69 chromosomes per cell); tetraploidy refers to four whole sets of chromosomes in a single cell (in humans, a total of 92 chromosomes per cell).

Population: A group of individuals united by a common factor (e.g., geographic location, ethnicity, disease, age, gender).

Predisposition: Increased susceptibility to a particular disease due to the presence of one or more gene mutations, and/or a combination of alleles (haplotype), not necessarily abnormal, that is associated with an increased risk for the disease, and/or a family history that indicates an increased risk for the disease

Probe: A specific, pre-fabricated sequence of DNA or RNA, labeled by one of several methods, used to detect the presence of a complementary sequence by binding (hybridizing) to that site.

Prognosis: A prediction of the probable outcome of a disease based on a individual's condition and the usual course of the disease as seen in similar situations.

Promoter: A specific region just upstream from a gene that acts as a binding site for transcription factors and RNA polymerase during the initiation of transcription.

Proto-oncogene: A gene having the potential for change into an active oncogene.
Pseudogene: A pseudogene is a DNA sequence that resembles a gene but has been mutated into an inactive form over the course of evolution. A pseudogene shares an evolutionary history with a functional gene and can provide insight into their shared ancestry.

Recessive: A gene which will be expressed only if there are 2 identical copies or, for a male, if one copy is present on the X chromosome.

Recombinant DNA: DNA artificially constructed by combining genes from different organisms or by cloning chemically altered DNA, usually for the purpose of genetic manipulation.

Repeat sequence: The length of a nucleotide sequence that is repeated in a tandem cluster.

Reverse transcriptase: A DNA polymerase enzyme that transcribes single-stranded RNA into single-stranded DNA. It also helps in the formation of a double helix DNA once the RNA has been reverse transcribed into a single strand cDNA.

RFLP: Restriction fragment length polymorphism. Variation between individuals in DNA fragment sizes cut by specific restriction enzymes; polymorphic sequences that result in RFLPs are used as markers on both physical maps and genetic linkage maps. RFLPs usually are caused by mutation at a cutting site.

Ring chromosome: Aberrant chromosomes with no ends, i.e., circular.

Robertsonian translocation: The joining of two acrocentric chromosomes at the centromeres with loss of their short arms to form a single abnormal chromosome; acrocentric chromosomes are the Y chromosome and chromosome numbers 13, 14, 15, 21, and 22.

Satellite: The part of chromosome beyond the nuclear organiser is very short and appears like sphere.

Sex chromosome: see “Gonosome”

Syndrome: The group or recognizable pattern of symptoms or abnormalities that indicate a particular trait or disease.

Southern blot: The separation on an electrophoretic gel of sequences or fragments of genomic or complementary DNA, partially digested by endonucleases, The fragments are then 'blotted' onto a membrane and allowed to hybridize with a specific, labeled probe in order to detect which bands contain the fragment or gene of interest. Southern blot is particularly useful in detecting large gene rearrangements/deletions and large trinucleotide repeat expansions.

Structural gene: A gene that encodes a protein with a chief function as part of a physical structure within a cell. This is in contrast to genes that encode proteins with a function that is enzymatic or regulatory.

Susceptibility: see “Predisposition”


Tandem duplication: The occurrence of two identical sequences, one following the other, in a chromosome segment.

Teratogen: An agent that causes the production of physical defects in the developing embryo.

Terminal deletion: A deletion involving the terminal part of a chromosome and leading to an adhesive terminus.

Transcription: The process of making an RNA copy of a gene sequence. This copy, called mRNA molecule, leaves the cell nucleus and enters the cytoplasm, where it directs the synthesis of the protein, which it encodes.

Translation: The process of translating the sequence of an mRNA molecule to a sequence of amino acids during protein synthesis.

Translocation: A mutation in which a large segment of one chromosome breaks off and attaches to another chromosome.

Triploidy: The presence of three haploid sets of chromosomes, instead of two, in all cells; usually results in fetal death.

Trisomy: The presence of a single extra chromosome, yielding a total of three chromosomes of that particular type instead of a pair. Partial trisomy refers to the presence of an extra copy of a segment of a chromosome.

Tumor suppressor gene: Genes in the body that can suppress or block the development of cancer.

Ultrasound: High frequency sound waves used to identify and examine internal organs and structures without the invasive hazards of X xays, dyes, or fluoroscopy.

Uniparental disomy (UPD): The situation in which both members of a chromosome pair or segments of a chromosome pair are inherited from one parent and neither is inherited from the other parent. Uniparental disomy can result in an abnormal phenotype in some cases.


Wild-type allele: The normal, as opposed to the mutant, gene or allele.

Zygote: The cell that results from fertilization. It is the union of a spermatozoon and an ovum.