Glossary Of Terms

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

ALLELE: Variant forms of the same gene. Different alleles produce variations in inherited characteristics such as eye color or blood type.

ALLELIC HETEROGENEITY: A single disorder, trait, or pattern of traits caused by different mutations within a gene.

ALLELIC VARIANT: An alteration in the normal sequence of a gene, the significance of which may be unclear. Complete gene sequencing often identifies numerous allelic variants (sometimes hundreds) for a given gene.

AMINO ACID: Any of a class of 20 molecules that are combined to form proteins in living things. The sequence of amino acids in a protein, and therefore protein function, is determined by the genetic code.

ANEUPLOIDY: The occurrence of one or more extra or missing chromosomes leading to an unbalanced chromosome complement.

ANTICIPATION: The tendency in certain genetic disorders for individuals in successive generations to present at an earlier age and/or with more severe manifestations; often observed in disorders resulting from the expression of a trinucleotide repeat mutation that tends to increase in size and have a more significant effect when passed from one generation to the next.

AUTOSOMAL: Any of the chromosomes other than the sex-determining chromosomes (X and Y) or the genes on these chromosomes.

AUTOSOMAL DOMINANT: Describes a trait or disorder requiring the presence of only one copy of a gene mutation at a particular locus in order to express observable phenotype; specifically refers to genes on one of the 22 pairs of non-sex chromosomes.

AUTOSOMAL RECESSIVE: Describes a trait or disorder requiring the presence of two copies of a gene mutation at a particular locus in order to express observable phenotype; specifically refers to genes on one of the 22 pairs of non-sex chromosomes.  

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B

BACKGROUND RISK: The proportion of individuals in the general population who are affected with a particular disorder or who carry a certain gene; often discussed in the genetic counseling process as a comparison to the patient's personal risk given his or her family history or other circumstances.

BAND LEVEL: Terminology used in reference to cytogenetic analysis. Refers to the total number of stripes, or bands, elicited on each chromosome with staining techniques. Band level is the total number of bands estimated to be present in a haplotype set (23) of chromosomes. When analysis is performed at an early stage of mitosis (prometaphase), chromosomes appear longer, with approximately 700-1200 bands. At a later stage of mitosis (metaphase), chromosomes are more condensed, with approximately 300-600 bands. At higher band levels, the greater resolution increases the ability to identify more subtle chromosomal abnormalities and their breakpoints.

BASE PAIR: In DNA, two complementary, nitrogenrich molecules held together by weak chemical bonds. Two strands of DNA are held together in the shape of a double helix by the bonds between their base pairs.

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C

CARRIER: An individual who has a recessive, disease-causing allele at a particular locus on one chromosome of a pair and a normal allele at that locus on the other chromosome. Carriers are usually asymptomatic.

CARRIER DETECTION OR CARRIER TESTING: Testing used to identify usually asymptomatic individuals who have a gene mutation for an autosomal recessive or X-linked recessive disorder.

CARRIER FREQUENCY: The proportion of individuals in a population who have inherited a single copy of a specific recessive gene mutation.

CELL CYCLE:The complete series of events from one cell division to the next.

CENTROMERE: The constricted portion of the chromosome at which the chromatids are joined and to which the spindle attaches during mitosis and meiosis.

CHROMOSOME: Structure found in the nucleus of a cell, which contains the genes. Chromosomes come in pairs, and a normal human cell contains 46 chromosomes (23 pairs).

CODING REGION: Sequence of DNA consisting of a series of nucleotide bases (code) giving rise to the mature messenger RNA that will be translated into the specific amino acids of the protein product.

CODOMINANCE: Situation in which two different alleles for a genetic trait are both expressed.

CODON: In DNA or RNA, a sequence of three nucleotides that codes for a certain amino acid or signals the termination of translation (stop or termination codon).

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, not necessarily genetic.

CONSULTAND: The individual (not necessarily affected) who presents for genetic counseling and through whom a family with an inherited disorder comes to medical attention.

CROSS OVER (MEIOSIS): A specialized cell division in which a single diploid cell undergoes two nuclear divisions following a single round of DNA replication in order to produce four daughter cells, each with half the number of chromosomes as the original diploid cell. Meiosis occurs during the formation of gametes from diploid organisms and at the beginning of haplophase in those organisms that alternate between diploid and haploid generations.

CROSSING OVER: The exchange of a segment of DNA between two homologous chromosomes during meiosis leading to a novel combination of genetic material in the offspring.

CYTOGENETICS: The study of the structure, function, and abnormalities of human chromosomes.
 

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D

DE NOVO GENE MUTATION: Alteration in a gene present for the first time in one family member as a result of a mutation in the egg or sperm cell leading to that person’s conception.

DELETION: Absence of a segment of DNA; may be as small as a single base or large enough to encompass one or more entire genes. Large deletions involving a whole segment of a chromosome may be detected by routine examination of the chromosomes; intermediate deletions involving a few genes may be detected by using fluorescent in situ hybridization (FISH); smaller deletions involving a portion of a gene may only be detected through DNA analysis.

DEOXYRIBONUCLEIC ACID: DNA. The molecules inside cells thatmake up the genes, that carry genetic information from one generation to the next.

DERIVATIVE CHROMOSOME: Term used to denote an abnormal chromosome consisting of segments from two or more chromosomes joined together as the result of a translocation, insertion, or other rearrangement.

DIPLOID: The normal number of chromosomes in a somatic cell; in humans, 46 chromosomes (22 pairs of autosomes plus two sex chromosomes).

DIRECT DNA ANALYSIS: Molecular genetic testing used to detect a genetic alteration associated with a specific disorder; direct DNA analysis is possible only when the gene, genes, or genomic region associated with a disorder is known.

DISEASE CAUSING MUTATION: A gene alteration that causes or predisposes an individual to a specific disease.

DNA BANKING: The process through which DNA is extracted from any of a number of possible cell sources and stored indefinitely by freezing or refrigerating for future testing; done when a specific test is not presently available or when the decision to have testing has not been made.

DOUBLE HETEROZYGOTE: An individual who has two different gene mutations at two separate genetic loci.

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E

EMBRYO: An animal in the early prenatal stages of growth and differentiation, specifically refers to the developing human during the first trimester of pregnancy.

EUPLOID: Any chromosome number that is a multiple of the haploid number.

EXON: The protein-coding DNA sequence of a gene. Each exon codes for a specific portion of the complete protein. In some species (including humans), a gene's exons are separated by long regions of DNA (called introns) that have no apparent function.

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F

FALSE PATERNITY: The situation in which the alleged father of a particular individual is not the biological father.

FALSE POSITIVE: A test result which indicates that an individual is affected and/or has a certain gene mutation when he or she is actually unaffected and/or does not have the mutation; i.e., a positive test result in a truly unaffected individual.

FAMILIAL: Describes a trait that is observed with higher frequency within the same family, whether the etiology is genetic or environmental, or a combination of the two.

FAMILY HISTORY: The genetic relationships and medical history of a family; when represented in diagram form using standardized symbols and terminology, usually referred to as a pedigree. A record of a person's current and past illnesses, and those of his or her parents, brothers, sisters, children, and other family members. A family history shows the pattern of certain diseases in a family, and helps to determine risk factors for those and other diseases.

FERTILITY: The capacity to conceive or to induce conception. May refer to either the male or female.

FETUS: An animal in the early prenatal stages of growth and differentiation; specifically refers to the developing human during the second and third trimesters of pregnancy.

FIBROBLAST: A connective tissue cell that makes and secretes collagen proteins.

FIRST-DEGREE RELATIVE: Parent, sibling, or offspring of an individual. A relative that shares one half of the person’s genes.

FLANKING MARKER: An identifiable, polymorphic region of DNA located to the side of a gene, but not within the gene itself. Used in linkage analysis to track the coinheritance of the gene in question.

FLUORESCENT IN SITU HYBRIDIZATION: (FISH) A cytogenetic technique used to identify the presence of specific chromosomes or chromosomal regions.  

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G

GAMETOGENESIS: The meiotic process by which mature
eggs and sperm are formed. Oogenesis refers specifically to the production of ova (eggs) and spermatogenesis to the production of sperm.

GENE: The functional and physical unit of heredity passed from parent to offspring. Genes are pieces of DNA, and most genes contain the information for making a specific protein.

GENE EXPRESSION: The detectable effect of a gene.

GENE PRODUCT: Genes are transcribed into segments of RNA (ribonucleic acid), which are translated into proteins. Both RNA and proteins are products of the expression of the gene.

GENE THERAPY: Treatment of a genetic disorder usually by the insertion of genetically altered genes into cells especially to replace defective genes or to provide a specialized disease fighting function (as the destruction of tumor cells).

GENETIC 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. Also refers to a family history that suggests an increased risk for the disease.

GENETIC TESTING: Analysis of DNA to look for a genetic alteration.

GENOMICS: The study of the sequence, structure, and function of the genome. The complete DNA sequence in an individual or species.

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

GENOTYPE-PHENOTYPE CORRELATION: The association between the presence of a certain mutation or mutations (genotype) and the resulting physical trait, (phenotype). GERMLINE: The cell line from which egg or sperm cells (gametes) are derived.

GERMLINE MOSAICISM: Two or more genetic or cytogenetic cell lines confined to the precursor cells of the egg or sperm; also called gonadal mosaicism.

GERMLINE MUTATION: The presence of an altered gene within the egg or sperm (germ cell) such that the altered gene can be passed to subsequent generations

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H

HAPLOID: Half the diploid or normal number of chromosomes in a somatic cell; the number of chromosomes in a gamete (egg or sperm) cell, which in humans is 23. HETEROZYGOTE: 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.

HOMOLOGOUS CHROMOSOMES: A pair of particular chromosomes, normally one inherited from the mother and one from the father, containing the same genetic loci in the same order.

HOMOZYGOTE: With respect to a particular trait or condition, an individual who has inherited identical alleles at a particular locus.

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I

IMPRINTING: A phenomenon in which the disease phenotype depends on which parent passed on the disease gene. This is determined by 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.

INFERTILITY: Incapable of or unsuccessful in achieving pregnancy over a considerable period of time (as a year) in spite of determined attempts.

INHERITANCE PATTERN: The manner in which a particular genetic trait or disorder is passed from one generation to the next. Autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, multifactorial, and mitochondrial inheritance are examples.

INSERTION: A chromosome abnormality in which material from one chromosome is inserted into another chromosome; or a mutation in which a segment of DNA is inserted into a gene or other segment of DNA, potentially disrupting the coding sequence.

INTRAFAMILIAL VARIABILITY: Variation in clinical presentation of a particular disorder among affected individuals within the same family.

INTRON: Non-coding sequence of DNA removed from mature 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. These may be balanced, with no loss or gain of genetic material, or unbalanced, with missing or extra genetic material.

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K

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

KINDRED: An extended family; term often used in linkage studies to refer to large families.

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L

LINKAGE ANALYSIS: Indirect DNA analysis. The use of several DNA sequence polymorphisms (normal variants) that are near or within a gene of interest to track the inheritance of a disease-causing mutation in that gene within a family.

LINKAGE DISEQUILIBRIUM: In a population, co-occurrence of a specific DNA marker and a disease at a higher frequency than would be predicted by chance alone.

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

LOCUS HETEROGENEITY: A single disorder, trait, or pattern of traits caused by mutations in genes at different chromosomal loci.

LYONIZATION: In females, the phenomenon in which one X chromosome (either maternally or paternally derived) is randomly inactivated in early embryonic cells, with fixed inactivation in all descendant cells; first described by the geneticist Mary Lyon.

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M

MAPPED GENE: A gene or phenotype whose relative position on a segment of DNA or on a chromosome has been established.

MARKER: An identifiable segment of DNA with enough variation between individuals that its inheritance and co-inheritance with alleles of a given gene can be traced; used in linkage analysis.

MARKER CHROMOSOME: A small chromosome, usually containing a centromere, occasionally seen in tissue culture, often in a mosaic state. A marker chromosome may be of little clinical significance or may create an imbalance for whatever genes are present. Clinical significance, particularly if found in a fetal karyotype, is often difficult to assess.

MATERNAL CONTAMINATION: The situation in which a fetal sample becomes contaminated with maternal cells, which can confound interpretation of the results of genetic analysis.

MEIOSIS: Specialized cell division in which a single diploid cell undergoes two nuclear divisions following a single round of DNA replication in order to produce four daughter cells, each containing half the number of chromosomes as the original diploid cell. Meiosis occurs during the formation of gametes from diploid organisms.

MESSENGER RNA: RNA that serves as a template for protein synthesis.

MICRODELETION SYNDROME: A syndrome caused by a chromosomal deletion spanning several genes that is too small to be detected under the microscope using conventional cytogenetic techniques. Other methods of DNA analysis can sometimes be employed to identify the deletion.

MITOCHONDRIAL INHERITANCE: Mitochondria, cytoplasmic organelles that produce the energy source ATP for most chemical reactions in the body, contain their own distinct genome; mutations in mitochondrial genes are responsible for several recognized syndromes and are always maternally inherited.

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.

MOSAICISM: The post-fertilization occurrence oftwo 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, in the causation of a particular trait or disease.

MUTATION: Any alteration in a gene from its natural state; may be disease causing or a benign, normal variant.

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N

NEW MUTATION: An alteration in a gene 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.

NON-DISJUNCTION: The failure of homologous chromosomes or chromatids to segregate during mitosis or meiosis, with the result that one daughter cell has both of a pair of parental chromosomes or chromatids, and the other has none.

NONSENSE MUTATION: A single base pair substitution that prematurely codes for a stop in amino acid translation (stop codon).

NUCLEOTIDE: A molecule consisting of a nitrogenous base (adenine, guanine, thymine, or cytosine in DNA; adenine, guanine, uracil, or cytosine in RNA), a phosphate group, and a sugar (deoxyribose in DNA; ribose in RNA). DNA and RNA are polymers of many nucleotides.

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O

OBLIGATE CARRIER OR OBLIGATE HETEROZYGOTE: An individual who may be clinically unaffected but who must carry a gene mutation based on analysis of the family history; usually applies to disorders inherited in an autosomal recessive or X-linked recessive manner.

ONCOGENE: A gene that normally directs cell growth. If altered, an oncogene can promote or allow the uncontrolled growth of cancer. Alterations can be inherited or caused by an environmental exposure to carcinogens.

ONCOSUPPRESSOR GENE: Also called a tumor suppressor gene. Genes in the body that can suppress or block the development of cancer.

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P

PARACENTRIC INVERSION: A chromosomal inversion in which the breakpoints are confined to one arm of a chromosome; the inverted segment does not span the centromere.

PARENTAGE TESTING: The process through which DNA sequences from a particular child and a particular adult are compared to estimate the likelihood that the two individuals are related; DNA testing can reliably exclude but cannot absolutely confirm an individual as a biological parent.

PEDIGREE: A diagram of the genetic relationships and medical history of a family using standardized symbols and terminology.

PENETRANCE: The proportion of individuals with a mutation causing a particular disorder who exhibit clinical symptoms of that disorder; a condition 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.

PERICENTRIC INVERSION: A chromosomal inversion in which the breakpoints occur on both arms of a chromosome and therefore the inverted segment spans the centromere.

PHENOTYPE: The observable physical and/or biochemical characteristics of the expression of a gene; the clinical presentation of an individual with a particular genotype.

PLEIOTROPY: Multiple, often seemingly unrelated, physical effects caused by a single altered gene or pair of altered genes.

POINT MUTATION: An alteration in DNA sequence caused by a single nucleotide base change, insertion, or deletion.

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

POLYMERASE CHAIN REACTION (PCR): A method of creating copies of specific fragments of DNA. PCR rapidly amplifies a single DNA molecule into many billions of molecules.

POLYMORPHISM: A natural variation in a gene, DNA sequence, or chromosome that has no adverse effect and occurs 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).

PREIMPLANTATION DIAGNOSIS: A procedure used to genetically test one cell removed from early embryos conceived by in vitro fertilization. Typically performed to identify specific genetic abnormalities, before transferring to the mother's uterus only those embryos determined not to have inherited the abnormality in question.

PREMUTATION: In disorders caused by trinucleotide repeat expansions, an abnormally large allele that is not associated with clinical symptoms but that can expand into a full mutation when transmitted to offspring (full mutations are associated with clinical symptoms of the disorder).

PRENATAL DIAGNOSIS: Testing performed during pregnancy to determine if a fetus is affected with a particular disorder. Chorionic villus sampling (CVS), amniocentesis, periumbilical blood sampling (PUBS), ultrasound, and fetoscopy are examples of procedures used either to obtain a sample for testing or to evaluate fetal anatomy.

PRESYMPTOMATIC TESTING: Testing of an asymptomatic individual in whom the identification of a gene mutation indicates the development of a specific condition at some future point in time. A negative result excludes the diagnosis.

PRIVATE MUTATION: A distinct gene alteration observed in a single family.

PROBAND: 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).

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R

REARRANGEMENT: A structural alteration in a chromosome, usually involving breakage and reattachment of a segment of chromosome material, and resulting in an abnormal configuration which may be balanced or unbalanced. Examples include inversion and translocation.

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

RECIPROCAL TRANSLOCATION: A chromosomal rearrangement in which a segment of one chromosome is exchanged with a segment of another chromosome of a different pair.

RECOMBINATION: The exchange of a segment of DNA between two homologous chromosomes during meiosis leading to a novel combination of genetic material in the offspring.

RECURRENCE RISK: The likelihood that a trait or disorder present in one family member will occur again in other family members.

REFLEX TESTING: Follow-up testing automatically initiated when certain test results are observed in the laboratory; used to clarify or elaborate on primary test results.

RESTRICTION FRAGMENT LENGTH POLYMORPHISM (RFLP) ANALYSIS: Fragment of DNA of predictable size resulting from cutting of a strand of DNA by a given restriction enzyme. DNA sequence mutations that destroy or create the sites at which a restriction enzyme cuts DNA change the size and number of DNA fragments resulting from digestion by a given restriction enzyme.

RING CHROMOSOME: Abnormal chromosomes in a circular configuration.

RISK ASSESSMENT: Calculation of an individual's chance of having inherited a certain gene mutation, of developing a particular disorder, or of having a child with a certain disorder based upon analysis of multiple factors including genetic test results, family medical history, and/or ethnic background.

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 chromosomes and the acrocentric chromosomes 13, 14, 15, 21, and 22.

 

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S

SCREENING: Testing designed to identify individuals in a given population who are at higher risk of having or developing a particular disorder, or carrying a gene for a particular disorder.

SECOND-DEGREE RELATIVE: A relative with whom one quarter of an individual's genes are shared (i.e., grandparent, grandchild, uncle, aunt, nephew, niece, half-sibling).

SEGREGATION: The separation of the homologous chromosomes and their random distribution to the gametes at meiosis.

SENSITIVITY: The frequency with which a test yields a positive result when the gene mutation in question is actually present in the individual being tested.

SOMATIC MUTATION: Alterations in DNA that occur after conception. Somatic mutations can occur in any of the cells of the body except the germ cells (sperm and egg) and therefore are not passed on to children.

SPECIFICITY: The frequency with which a test yields a negative result when the gene mutation in question is not present in the individual being tested.

SPORADIC: Denoting either a genetic disorder that occurs for the first time in a familydue to a new mutation or the chance occurrence of a disorder or abnormality that is not likely to recur in a family.

STEM CELLS: Relatively undifferentiated cells from which other types of cells can develop.

SUBSTITUTION: A type of mutation due to replacement of one nucleotide in a DNA sequence by another nucleotide or replacement of one amino acid in a protein by another amino acid.

 

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T

TELOMERE: The segment at the end of each chromosome which has a specialized structure and is involved in chromosomal replication and stability.

TRANSCRIPT: A sequence of RNA produced by transcription from a DNA template.

TRANSCRIPTION FACTOR: A protein that binds to regulatory regions and helps control gene expression.

TRANSFER RNA: Small RNA molecules that bond with amino acids and transfer them to the ribosomes, where proteins are assembled according to the genetic code carried by messenger RNA.

TRANSLATION: The process of forming a protein molecule at a ribosomal site of protein synthesis from information contained in messenger RNA.

TRANSLOCATION: A chromosome alteration in which a whole chromosome or segment of a chromosome becomes attached to or interchanged with another whole chromosome or segment. These may be balanced, with no loss or gain of genetic material, or unbalanced, with extra or missing genetic material.

TRINUCLEOTIDE REPEAT: Sequences of three nucleotides repeated in tandem on the same chromosome a number of times. A normal, polymorphic variation in repeat number with no clinical significance commonly occurs between individuals; however, repeat numbers over a certain threshold can, in some cases, lead to adverse effects on the function of the gene, resulting in genetic disease.

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.

TRISOMY RESCUE: The phenomenon in which a fertilized ovum initially contains 47 chromosomes but loses one of the trisomic chromosomes in the process of cell division such that the resulting daughter cells and their descendants contain the normal number of chromosomes.

 

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U

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.

UPD TESTING: Testing used to identify whether specific chromosomes are maternally or paternally derived; can aid in confirming the clinical diagnosis of certain disorders for which UPD is a possible underlying etiology.

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V

VARIABLE EXPRESSION: Variation in clinical features (type and severity) of a genetic disorder between individuals with the same gene alteration, even within the same family.

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X

X-LINKED DOMINANT: Describes a dominant trait or disorder caused by a mutation in a gene on the X chromosome.

X-LINKED LETHAL: A disorder caused by a dominant mutation in a gene on the X chromosome that is observed almost exclusively in females because it is almost always lethal in males.

X-LINKED RECESSIVE: A mode of inheritance in which a mutation in a gene on the X chromosome causes the phenotype to be expressed in males who are hemizygous for the gene mutation or in females who are homozygous for the gene mutation. Carrier females, who have only one copy of the mutation, do not usually express the phenotype, although differences in X-chromosome inactivation can lead to varying degrees of clinical expression.

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Z

ZYGOSITY TESTING: The process through which DNA sequences are compared to assess whether individuals born from a multiple gestation (twins, triplets, etc.) are monozygotic (identical) or dizygotic (fraternal); often used to identify a suitable donor for organ transplantation or to estimate disease susceptibility risk if one sibling is affected.