Acquired Mutations: Gene changes that arise within individual cells, due to various causes, and accumulate throughout a person’s lifetime. Acquired mutations are also called somatic mutations. (See Germline Mutation).

Adenocarcinoma: A type of cancer involving the cells from the lining or inner surface of an organ such as the colon. Most cancers of the colon, breast, and lung are adenocarcinomas.

Adenoma: A benign tumor involving cells from the lining or inner surface of an organ, such as the colon. If an adenoma becomes cancerous, it is then called an adenocarcinoma.

Agonist: A chemical that mimics the biochemical action of another chemical. Agonists often closely resemble naturally occurring substances and cause the same response in a cell that the naturally occurring substance would. The opposite is an antagonist, which blocks the action of an agonist.

Allele: A variant form or version of the same gene. Different alleles produce variations in inherited characteristics, which can be benign or disease-causing. An example of benign allele variations are those that cause different eye colors or blood types.

Amino Acid: Any of a class of 20 molecules that combine to form proteins.

Amniocentesis: A prenatal diagnosis method which uses cells from the amniotic fluid surrounding a developing fetus to determine the number and structure of its chromosomes, biochemical testing, or mutation detection. Amniocentesis is usually performed at 15 to 18 weeks into the pregnancy, calculated from the Last Menstrual Period. The procedure involves inserting a fine needle through the mother’s abdomen and uterus into the amniotic fluid surrounding the baby while using an ultrasound scan for guidance to prevent injury to the fetus. One to 2 tablespoons (20-30 ml) of the amniotic fluid are removed for analysis.

Amniocentesis is generally recommended when there is a family history of a genetic disorder or when the woman is over age 35 and therefore at a higher risk of having a baby with a chromosomal abnormality. It is a highly accurate test for chromosomal abnormalities, such as Trisomy 21 (Down Syndrome). The procedure is not very painful and has been described as a ‘cramping’ feeling. When performed at a qualified center, the risk of complications is ~0.5% and includes bleeding, cramping, and pregnancy loss. For other types of prenatal diagnosis, see Chorionic Villus Sampling, Ultrasound, Percutaneous Umbilical Blood Sampling,and Preimplantation Genetic Diagnosis.

Amyloid: A starchlike protein (known as a glycoprotein) that is deposited in tissues as a part of a disease process, called amyloidosis.

Amyloidosis: Disease resulting from the deposition of amyloid, a starchlike protein (known as a glycoprotein), in various tissues or organs of the body and impairing their function.

Aneuploidy: One or a few chromosomes above or below the normal chromosome number. In human beings, 23 pairs of chromosomes is the normal chromosome number in each non-reproductive cell. Disease can occur when there are more or less chromosomes than normal, or anueploidy. For example, three copies of chromosome number 21 or trisomy 21 (Down syndrome) is a form of aneuploidy.

Angiogenesis: blood vessel formation or growth. For cancer, it describes the formation of blood vessels between a tumor and the surrounding tissues. This process is necessary in order to keep the tumor alive and to allow it to grow larger.

Antagonist: A substance that acts against and blocks the biochemical action of a chemical. For example, a drug which binds to a cell receptor without eliciting the normal biochemical response is an antagonist. It is the opposite of an agonist.

Apoptosis: A form of cell death in which the cell uses specialized cellular machinery to kill itself. This is a normal cellular process and is necessary for development and health of the body by eliminating old cells, unnecessary cells, and unhealthy cells. Without an orderly process for getting rid of cells, we would double in size within a year. Cancerous cells do not go through apoptosis although they are unhealthy. Apoptosis is also called programmed cell death.

Aromatase: An enzyme involved in the production of estrogen. For women who have gone through menopause, aromatase produces all of the estrogen in the body.

Ashkenazi Jew: A person who has Jewish ancestors from Central and Eastern Europe including Germany, Austria, Poland, Lithuania, Russia, etc. Eighty percent of American Jews are of Ashkenazi descent.

Ataxia: A neurologic term describing muscular incoordination often manifesting as an unsteady gait and lack of postural balance.

Atypical Hyperplasia: An increase in the number of normal cells in a tissue. This has been seen in breast tissue and often leads to a mass, which a physician would biopsy to see if it is benign or malignant. See Factors Contributing to Breast Cancer Risk.

Autosomal Dominant Inheritance: A pattern of inheritance where an alteration in just one copy of a pair of genes causes a detectable trait. Individuals who possess one mutant copy of a gene for a dominant disorder are usually affected. A child conceived by an affected individual has a 50% chance of inheriting the abnormal copy of the gene, and thus the disorder or trait. Dominant disorders do not “skip” generations and most affected individuals have an affected parent.

There are two exceptions to this: 1) a new mutation in a dominant gene can cause the trait to appear for the first time in a family; 2) a few genes may have incomplete penetrance, i.e. not everyone who has the gene manifests the trait. Torsion dystonia is a dominantly inherited condition with reduced penetrance. For more information, see Introduction to Genetics – Autosomal Dominant Inheritance.

Autosomal Recessive Inheritance: A pattern of inheritance where a specific trait is manifest only when both copies of the gene are altered. A disorder is considered recessive when a single fully functional copy of a gene is sufficient to prevent disease.

In order for an individual to have two non-functional copies of a gene, a mutated copy must be inherited from each parent. It is expected that both parents of an affected child are entirely unaffected, even though they both carry an altered copy of the gene; they are called “carriers.” When both partners in a couple are carriers, they have a 25% (1-in-4) chance of having an affected child with each pregnancy. For more information, see Introduction to Genetics – Autosomal Recessive Inheritance. An example of an autosomal recessive disease is Tay-Sachs disease.

Autosome: Any of the non-sex-determining chromosomes. Human cells have 22 pairs of autosomes, numbered 1 to 22. (In addition to autosomes, human cells also contain a pair of sex chromosomes – XX in females and XY in males).

Axillary Lymph Node: Lymph nodes that are located in the cavity beneath the junction of the arm and the body (armpit). These lymph nodes are often biopsied in women with breast cancer to look for metastases.

Azoospermia: A condition in which there is a lack of sperm in the semen. Men with this condition often have cystic fibrosis transmembrane conductance regulator (CFTR) gene mutations. CFTR mutations can cause Cystic Fibrosis, in which the vast majority of men are infertile.


Base: Bases are molecules that are the essential building blocks of DNA and RNA, which in turn make up genes and proteins. There are four bases, often represented as letters that correspond to their chemical names. The code letters for the bases are A, T, G, and C, which stand for the chemicals adenine, thymine, guanine, and cytosine. The sequence of the bases, A, T, G, and C, encode the structure of proteins (and also determine their function).

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Beta-thalassema: A genetic blood disorder in which there is decreased production of hemoglobin. Hemoglobin is the protein that carries the oxygen in the blood to different tissues. People with the disorder have anemia and sometimes bone deformations. They also may require regular blood transfusions. Beta-thalassemia is found at a higher incidence in people of Mediterranean descent including Sephardic Jews. The disorder does not usually affect Ashkenazi Jews. This disorder is a different disease than sickle-cell anemia. For more information, see Beta-thalassemia.

Bloom Syndrome: A disorder which results in poor growth and an increased susceptibility to infection. There is a high rate of cancer from which individuals usually die before the age of 30. This disease occurs at a higher incidence in the Ashkenazi Jewish population than in the general population. For more information, see Bloom Syndrome.

Bone Marrow Transplantation: A procedure in which bone marrow is taken from a healthy donor and transplanted into a patient with deficient bone marrow function. Bone marrow is found in the soft, fatty tissue inside bones where blood cells (red blood cells, platelets, and white blood cells) are produced and developed. Deficient bone marrow function can be a result of chemical treatment (chemotherapy) or radiation treatment for cancer, as a result of hereditary blood diseases, or due to the accumulated damage of certain disorders. Healthy bone marrow may be taken from a patient before he/she undergoes chemotherapy or radiation treatment (autograft), or may be taken from a donor, especially in cases of hereditary blood disorders (allograft). Bone marrow transplantation may be considered in some of the non-curable, non-treatable Jewish genetic disorders such as Bloom Syndrome and Fanconi Anemia, and some cancers.

BRCA1 Breast Cancer Susceptibility Gene: BCRA1, like BRCA 2, is a tumor suppressor gene. This means that it is a gene that normally helps to control cell growth. Women who carry a genetic mutation in BRCA1 have an increased lifetime risk to develop breast cancer, and are at risk to develop it at an earlier age compared to the general population. For more information, see BRCA1.

BRCA2 Breast Cancer Susceptibility Gene: BRCA2, like BRCA1, is a tumor suppressor gene. This means that it is a gene that normally helps to control cell growth. Women who carry a genetic mutation in BRCA2 have an increased lifetime risk to develop breast cancer, and are at risk to develop it at an earlier age compared to the general population. For more information, see BRCA2.


CA-125: A blood protein that acts as a tumor marker associated with ovarian cancer. Eighty-five percent of women with clinically diagnosed ovarian cancer have been found to have elevated levels of CA125. Elevated levels of CA-125 have been linked to an increased likelihood of ovarian cancer even when a woman has not been diagnosed with ovarian cancer. HOWEVER, CA125 can also be elevated during the 1st trimester of pregnancy, during menstruation, in the presence of certain noncancerous illnesses, and with the presence of cancer in other tissues of the body. Therefore elevated CA125 levels do not guarantee the presence of ovarian cancer. Another problem with CA125 screening is that some women who have ovarian tumors may not have elevated CA125 levels. IN SUMMARY, CA125 levels are not accurate for detecting either the presence or absence of ovarian cancer.

Canavan Disease: A disorder characterized by progressive central nervous system degeneration. Infants appear normal at birth but then begin to lose skills and degenerate within the first year of life. Death usually occurs within the first 10-20 years of life. Canavan Disease occurs at a higher incidence within the Ashkenazi Jewish population than in the general population. For more information, see Canavan Disease.

Cancer: A general disease term for a condition characterized by uncontrolled, abnormal cell growth. It can involve any part of the body and can spread either locally or to another site in the body (Metastasis). Another term for cancer is malignancy.

Carrier: A person who has one mutated copy of a recessive gene and one non-mutated copy. Carriers do not usually develop disease but can pass the mutated gene on to their children. For autosomal recessive traits or diseases, two carriers have a 1-in-4 or 25% chance of having a child with the trait or disease in each pregnancy.

Carrier frequency: The numbers describing how many people in a population are carriers for an autosomal recessive disease. Carriers have a mutation on only one copy of their genes, and are usually unaffected.

Carrier Testing (Screening): Testing to identify individuals who carry one copy (out of two) of a disease-causing recessive gene that could 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 or ethnicity. For more information, see Screening.

Chorionic Villus Sampling (CVS): A prenatal diagnosis method which uses cells from the chorion (the earliest form of the placenta) surrounding a developing fetus to determine the number and structure of its chromosomes, biochemical testing, or mutation analysis. CVS is usually performed at 10 to 12 weeks into the pregnancy, calculated from the Last Menstrual Period. It involves the insertion of a needle through the cervix or the abdominal wall into the chorion (placenta) to remove cells for further testing. CVS is generally recommended when a patient has a family history of genetic disorders or when a woman is over age 35 and therefore at a higher risk of having a baby with a chromosomal abnormality. CVS is a highly accurate test for certain chromosomal abnormalities, such as Trisomy 21 (Down Syndrome). The procedure is not very painful and has been described as a ‘cramping’ feeling. When performed at a qualified center, the risk of complications is 1% and includes bleeding, cramping, and pregnancy loss. For other types of prenatal diagnosis, see Amniocentesis, Ultrasound, Percutaneous Umbilical Blood Sampling,and Preimplantation Genetic Diagnosis.

Chromosomes: Structures found in the nucleus of a cell, within which genes are packaged. Chromosomes come in pairs, and a normal human cell contains 46 chromosomes consisting of 22 pairs of autosomes and two sex-determining chromosomes.

CHRPE (congenital hypertrophy of the retinal pigment epithelium): Abnormal retinal pigmentation appearing as a “spot” on the eye but which does not interfere with vision. In an individual at risk for Familial Adenomatous Polyposis (FAP), the presence of multiple or bilateral CHRPE may be evidence of a gene mutation.

Clone: A genetically identical copy of an organism, gene, or cell resulting from manipulation of the egg in a laboratory. Although an organism can be genetically identical to another organism, the two copies may be very dissimilar based on minute differences in the environment in which they develop. The issues of clones and cloning are highly debated topics in genetics today, although not relevant to our discussion of genetic disorders.

Cloning: The process of making genetically identical copies. A highly debated topic in genetics today, although not relevant to our discussion of genetic disorders.

Colectomy: The surgical removal of all or part of the colon.

Colonoscopy: Examination of the colon through a flexible, lighted instrument called a colonoscope. The colonoscope transmits images of the colon and allows the physician to take tissue samples so they can diagnose irregularities including colon cancer. Colonoscopy lets the physician see the inside of the rectum, colon, and large intestine all the way up to the bottom of the small intestine. See Sigmoidoscopy.

Compound Heterozygotes: A person who has two different mutated forms (alleles) of a particular gene. In other words, both copies of the gene are mutated, but each copy has a different mutation.

Congenital: Any trait or condition that is present at birth. The condition or trait usually exists before birth, but can be caused by damage during the birthing process. An example is congenital adrenal hyperplasia, which is a genetic disorder that is present at birth and is characterized by a deficiency of the hormones aldosterone and cortisol and an overproduction of male sex hormones.

Connexin 26 GJB2: A condition characterized by congenital hearing loss with no other associated medical complications. This disorder has a higher incidence in the Ashkenazi Jewish population than in the general population. For more information, see Nonsyndromic Hearing Loss.

Contig (Contiguous) Map: A “map” depicting the order of several small overlapping clones, representing a segment of a chromosome. It is a chromosomal map demonstrating the location of adjacent DNA segments on a chromosome, which can be used to study a very large segment of the genome.

Crohn’s Disease: This disease is a form of inflammatory bowel disease and it causes inflammation of the gastrointestinal tract, most commonly in the ileum (the last section of the small intestine). Symptoms are intermittent and include abdominal pain, fever, nausea, vomiting, weight loss, and occasionally bloody diarrhea. Crohn’s disease can also cause inflammation of the joints, spine, eyes, and liver. It is diagnosed by an Xray or a colonoscopy. Treatment consists of anti-inflammatory drugs and possibly surgery.

Cyst: A closed sac lined by epithelial (skin) tissue and is usually filled with fluid or semisolid material.

Cystic Fibrosis: An autosomal recessive disease in which a thick mucus clogs the lungs and blocks the ducts of the pancreas. It also leads to infertility in males. At this time, the average life span is 30 years of age. It occurs at a higher incidence in Northern Europeans or individuals of Northern European ancestry than in other populations which includes Ashkenazi Jews. For more information, see Cystic Fibrosis.

Cytogenetics: The study of chromosomes.


Dentigerous Cyst: A cyst that contains fluid and one or more imperfect teeth (not fully developed). It occurs in relation to an erupted (grown in) tooth and it may very rarely become malignant.

Desmoid Tumor: A benign tumor that has the characteristics of a ligament and intertwines extensively around surrounding tissues. They may look like dense scar tissue. They are difficult to remove because of their close interaction with surrounding tissue and 70% recur over time.

DNA: A molecule that determines the structure, function, and behavior of the cell. It is the substance of heredity and it comprises genes. The characteristics or traits that each person has are due to the interaction of DNA and the environment.

DNA insertion-deletion loop: This DNA structure leads to a DNA copying error. It occurs most frequently in repetitive DNA sequences when the protein copying the DNA may “slip” resulting in more or less bases ordinarily seen in one strand. This leads to a “looping out of the resultant excess DNA bases. See below:

DNA Mutational Analysis: This type of genetic testing looks at an individual’s DNA to determine if there are mutations, or changes, in a specific gene. Individuals who receive testing are trying to determine if a mutated gene will result in production of a non-functional or poorly functional gene product (protein).

DNA Repair Genes: Genes that repair damaged DNA. When these genes are altered, they permit mutations to accumulate in the DNA, which can lead to disease. For example, Fanconi Anemia and Bloom Syndrome are caused by defects in DNA repair genes.

DNA Sequencing: A process that can determine the exact order of the bases in a segment of DNA. This allows for the determination of specific mutations in a gene by comparing changes of the sequence away from the sequence that is expected or is most commonly found.

Dominant Allele: An allele or gene that results in a trait or disease in a person who has only one mutated copy of that gene for the trait, regardless of the counterpart allele on the other chromosome. This is in contrast to a recessive gene in which a trait occurs only when both copies of the gene are nonfunctioning. For more information, see Autosomal Dominant Inheritance. For contrast, see Recessive Allele.

Down Syndrome: A condition caused by the presence of three copies of chromosome 21 (instead of the two copies, which normally occur). Affected children have characteristic facial features, mental retardation (moderate to severe), and may have heart defects and other problems. The average risk for Down syndrome to occur in a newborn in 1 in 800, but the specific risk increases with the increasing age of the mother. Triple marker testing results which show low levels of MSAFP (maternal serum alpha-fetoprotein) and uE3 (estriol), along with high levels of hCG (human chorionic gonadotropin) suggest an increased risk of Down syndrome. For more information, see Prenatal Screening.

Dystonia: A neurologic condition involving repeated involuntary twisting and movement of a variety of muscle groups. Torsion dystonia is a disorder that occurs at a higher incidence in the Ashkenazi Jewish population than in the general population. For more information, see Torsion Dystonia.


Encode: Containing coded information, such as the genetic blue print for making a protein, which is represented by a specific sequence of bases in DNA.

Endogamous: The description of a group in which reproduction occurs only within that particular group. Examples of how “endogamous groups” are defined would be religious groups, social class, or a specific culture. The Ashkenazi Jewish population has historically been an endogamous group and rarely married outside of the population.

Enzyme: A protein that facilitates a specific chemical reaction within a cell. Enzymes cause specific chemical changes in other substances without being changed themselves. Examples include the proteins which are necessary to break down (digest) food into smaller molecules that the body can use. A mutation in an enzyme can cause disease. For example, Tay-Sachs disease is caused by a mutation in an enzyme responsible for breaking down a specific type of fat, or lipid. The lipid then accumulates and causes a deterioration of the nervous system, resulting in the symptoms associated with Tay-Sachs disease.

Enzyme Assays: This type of testing measures the activity of a particular gene product (enzyme), in other words, how well the enzyme does its job. The level of enzyme activity is then compared to “normal” or “average” levels in individuals who have two normal copies of the gene. A person who is a carrier for a recessive disorder has one normal (non-mutated) copy of the gene and one mutated copy of the gene. Thus, they would be expected to produce about half the amount of functional product that a non-carrier individual would, or have 50% of normal enzyme activity. An individual who has two mutated copies of the gene will have much less gene product than is typically expected. An individual is not usually affected with symptoms of a disorder until his or her enzyme activity level is less than 10% of normal.

Epidermoid Cyst: A cyst containing fluid (oil or fatty semisolid material) and epidermal (skin) tissue. It appears as a small, nontender lump on the skin.

Epithelial: From the epithelium, which is the covering of the internal and external structures of the body. An example is the skin covering our bodies or the skin lining the digestive tract.

Ethics: The study of fundamental principles which define values and determine moral duty and obligation. See Bioethics.

Ethnicity: A person’s background or affiliation relating to a large group of people sharing a common and distinct national, racial, religious, linguistic, or cultural heritage. In almost every ethnic, racial, or demographic group, certain genetic diseases occur at higher frequencies among their members than in the general population.

Expressivity: The extent to which a genetic disease is expressed or how consistent the expression of a disease is. When there is variable expressivity of a disease, affected individuals may have symptoms ranging from mild to severe, but will always have some symptoms. Expressivity describes the degree of disease manifestation and can be contrasted to Penetrance in which the disease is either present or not (all-or-none expression).


False Negative: A test result indicating that a person does not have a mutation or disease when, in reality, they do.

False Positive: A test result indicating that a person has a mutation or disease when, in reality, they do not.

Familial: A trait that is more common in members of a family than in the general population. The trait can be caused by shared genes, shared environment, or both.

Familial Adenomatous Polyposis (FAP): An autosomal dominant condition in which hundreds of potentially cancerous polyps develop in the colon and rectum, and also sometimes in the stomach, duodenum, and terminal ileum. Polyps usually first appear at puberty and colon cancer is typically diagnosed by age 25. FAP is caused by a mutation in the APC gene. For more information, see Familial Adenomatous Polyposis.

Familial Cancer: Cancer, or a predisposition toward cancer, that runs in families. See Familial.

Familial Dysautonomia: A disorder caused by the abnormal development of the sensory and autonomic nervous systems. Common manifestations include difficulties with swallowing, temperature and blood pressure regulation, and pain perception. This disorder has a higher incidence in the Ashkenazi Jewish population than in the general population. For more information, see Familial Dysautonomia.

Fanconi Anemia: A disorder that is characterized by anemia, short stature, learning disabilities and mental retardation, increased cancer risk, and often congenital anomalies. This disorder has a higher incidence in the Ashkenazi Jewish population than in the general population. For more information, see Fanconi Anemia.

Fetus: The stage of human development from 10 weeks gestation until birth.

Fibroma: A benign tumor consisting of fibrous of connective tissue. It is also called a fibroid tumor or a fibroid.

First degree relative: A relative who is parent, sibling, or child of an individual. A second degree relative would be an uncle, aunt, grandparent, niece, nephew, or grandchild.

Founder Effect: This can occur when a new population is established by a few original founders, who only carry a small fraction of the total genetic variation of the original, larger population. If one or more of these founding members carry a disease mutation, a significant proportion of future descendents could also carry the disease mutation and the gene frequency would then be much higher than it was in the population that the settlers came from. For example, in the Ashkenazi Jewish population, some diseases are more prevalent than in the general population because Jewish communities were usually small and isolated.


Gaucher Disease: A disorder that leads to the progressive accumulation of a specific lipid or fat in the bone marrow, spleen, and liver. In Type I, there is no neurologic involvement, while Types II and III have involvement of the central nervous system. Type I has a higher incidence in the Ashkenazi Jewish population than in the general population, while Types II and III do not. For more information, see Gaucher Disease.

Gene: The physical unit of inheritance that occupies a specific location on a chromosome. A gene is comprised of DNA, and contains the code for a specific product, usually a protein such as an enzyme. The human genome contains between 30,000 to 35,000 genes.

Gene Deletion: The total loss or absence of a gene, which may lead to disease.

Gene Insertion: The addition of a DNA sequence into a gene that can range in size from a single nucleotide to thousands of nucleotides long. It can disrupt the normal structure and function of the gene that it is inserted into and may cause disease.

Gene Pool: All of the genes in all of the individuals in a breeding population. More precisely, it is the collective genotype of a population.

Gene Product: The biochemical material, either RNA or protein, that results from the expression of a gene. The amount of gene product is a measure of gene activity and abnormal amounts can be correlated with disease-causing alleles.

Gene Testing: The analysis of a sample of blood, or other tissue for genetic (DNA) markers that indicate the presence or absence of genetic disease.

Gene Therapy: Treating disease by replacing, manipulating, or supplementing nonfunctional genes with functional genes. An example would be the early studies of gene therapy for cancer in which researchers were trying to improve the body’s natural ability to fight the disease or to make tumors more sensitive to other kinds of therapy. Gene therapy to correct a hereditary disease may involve the addition of a functional gene or group of genes to a cell by gene insertion.

Genetic: Inherited; anything that has to do with information that is passed from parents to children through DNA in the genes.

Genetic Disorder: A condition resulting from an alteration, or mutation in one or both copies of a specific gene, resulting in disease symptoms.

Genetic Counseling: The educational process that helps individuals, couples, or families to understand genetic information and issues that may have an impact on them. It often deals with the determination of a genetic diagnosis in a person and/or the recurrence risks within that same generation or the next generation. Also see Genetic Counseling.

Genetic Drift: Genetic drift happens due to random changes in how often a gene appears in a population. This may explain why certain characteristics are more common among certain groups of people.

More technically, this can be explained as a change in the frequency of a gene, resulting from its transmission (by chance), to a higher or lower frequency than predicted. Genetic drift occurs most rapidly in small populations. In contrast, in large populations, random deviations in allele frequencies (how often a particular allele is seen) in one direction are more likely to be cancelled out by random changes in the opposite direction.

Genetics: The scientific study of heredity, or how particular qualities, traits, or genes are transmitted from parents to offspring.

Genetic Predisposition: When a person who has an inherited gene mutation has a greater risk than the general population of developing a disease associated with the mutation. For example, gene mutations in BRCA1 or BRCA2 can predispose a person to breast cancer, although they will not necessarily develop breast cancer.

Genetic Screening: Testing groups of individuals to identify defective genes capable of causing hereditary conditions. Genetic screening is used to refer to two different processes: 1) Testing individuals to see if they have a genetic trait or disease, and 2) Testing individuals to determine if they carry genes that could result in a genetic disorder in their children. Often referred to as population-based screening. For more information, see Genetic Testing.

Genetic Testing: Essentially the same as genetic screening because both use the same procedures. Often, the difference between testing and screening involves why an individual is being tested. Screening usually applies to an individual member of a group, while testing applies only to an individual. For example, an individual with a family history for (or family member with) a certain disease who suspects they may be a carrier would undergo genetic “testing.” In contrast, an individual member of a group who was at high risk for being a carrier, like an Ashkenazi Jew, would undergo genetic “screening.” For more information, see Genetic Testing.

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

Genome Maps: Charts that indicate the ordered arrangement of the genes or other DNA markers within the chromosomes.

Genotype: The actual genes that encode for the characteristics seen in an individual, or the genetic make-up of an individual. This is in contrast to the phenotype, which describes the physical characteristics of an individual.

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

Germline Mutation: A genetic change in the body’s reproductive cells (eggs or sperm) that, after fertilization, becomes incorporated in the DNA of every cell in the body; also called a hereditary mutation. (See Acquired Mutations.)

Gestation: The period of development in a fetus from the time of fertilization to birth. For humans, the entire gestation period is typically nine months long.

Granulocytes: A type of white blood cell in the immune system that destroys microorganisms.


Halachic: Relating to Jewish law (Halacha). Also spelled “Halakhic.”

Hamartoma: A growth that looks like a tumor but is disorganized and is benign.

Haplotypes: The representation of a set of adjacent or closely linked alleles on one chromosome.

HCG (human Chorionic Gonadotropin): A hormone that is produced by the developing placenta when a woman becomes pregnant. Levels increase over the first trimester and then begin to decline. Average levels over the course of the pregnancy have been calculated and are used in triple marker screening, which involves analysis of a blood sample from a pregnant woman. If the measured amount of hCG in a pregnant woman is very different than expected, it might indicate the presence of an abnormality in the fetus, such as Down Syndrome, Trisomy 18, or a neural tube defect. However, measurements of hCG levels during pregnancy are NOT diagnostic of fetal abnormality.

Hemizygous: When there is only one copy of a gene or group of genes in a cell, instead of the more typcial two. Males with the typical number of chromosomes are hemizygous for all genes on the X chromosome because they have only one copy of the X chromosome.

Hepatic: Referring to the liver.

Hepatosplenomegaly: Enlargement of the liver and spleen.

Hereditary Mutation: See Germline Mutation.

Hereditary Nonpolyposis Colorectal Cancer (HNPCC): A hereditary cancer predisposition syndrome caused by one of the genes involved in DNA repair (mismatch repair). It presents predominantly with early onset colon cancer but can also lead to endometrial (uterine) cancer and other cancers. It is estimated to account for 5% of all colorectal cancers.

Heredity: The transmission of a gene from a parent to a child.

Heterogeneity: When identical or similar diseases or symptoms are caused by different mutations in different genes. A general example of this is Hereditary non-polyposis colorectal cancer (HNPCC), a dominantly inherited cancer predisposition syndrome which can be caused by mutations in any one of several different genes.

Heterozygous: When the two alleles, or versions of a gene, at a single locus are different. A person who is heterozygous for an autosomal recessive disorder, (such as Tay-Sachs) with one fully functional copy and one non-functioning copy, is called a carrier and is unaffected with the disorder.

Heterozygote: A person possessing two different forms of a particular gene, at a single locus. A person who is a heterozygote for a gene associated with an autosomal recessive disorder has one functional copy of the gene and one nonfunctional copy. They would be called a carrier and would be unaffected.

Heterozygote Advantage: In certain specific environments, an individual who has one mutated copy of a gene may be healthier or more resistant to disease than either an individual who has two non-mutated copies of the same gene or an individual who has two mutated copies of the gene. This person can also be described as being heterozygous for the recessive mutation and an unaffected carrier. An example of this is an increased resistance to malaria in females who carry the gene for G6PD deficiency. This is important in areas of the world with a high incidence of malaria, and accounts for the higher-than-expected number of disease mutations. This is suggested as the reason that a high number of carriers have been maintained in the population for certain medically serious or lethal recessive disorders, instead of seeing a decrease of the mutation over time. For more information, see G6PD Deficiency.

Homozygous: Having two identical versions of the same gene, one on each chromosome. In autosomal recessive disorders a person who is homozygous for a mutation will manifest the disease (be affected) while a heterozygote will not.

Human Genome: The full collection of genes needed to produce a human being.

Human Genome Project: An international research effort (led in the United States by the National Institutes of Health and the Department of Energy) aimed at fully understanding the human genome.

Hyperplasia: A condition in which there is an abnormal increase in the number of normal cells in a tissue or organ.

Hysterectomy: Surgical removal of the uterus


Inflammatory Bowel Disease: A general term describing a chronic intestinal disease resulting in inflammation of the bowel, and either the large or small intestine. Two main subtypes exist and they are ulcerative colitis and Crohn’s disease. Symptoms include abdominal pain and diarrhea and these symptoms can be intermittent. It can occur at any age but most commonly occurs in young adults. Treatment involves medications and sometimes surgery.

Informed Consent: The communication between a patient and a health care provider about a procedure such as genetic tests. The patient is given information about the risks and benefits of genetic testing along with a description of the process and possible outcomes. The patient is then given an opportunity to ask questions and when the patient fully understands the information, the patient is asked to explicitly give consent for the testing to be performed. This allows the patient to exercise control over his or her medical care, and helps protect the health care provider from liability issues.

Inheritance: Transmission of genes from parents to offspring.


Currently, there are no definitions beginning with J.


Karyotype: The images of a set of stained chromosomes arranged in order from largest to smallest. On a karyotype, it is possible to see large scale duplications or deletions of genetic material. For example, Trisomy 21 (Down Syndrome) or Monosomy X (Turner Syndrome) can be seen on a karyotype.

Knudson’s “Two-hit” Hypothesis: A theory proposing that individuals who inherit a mutation in a tumor suppressor gene are more likely to develop cancer and at an earlier age than someone in the general population.

The theory proposes that hereditary cancer occurs via a multi-step pathway that begins with the inactivation of both copies of a tumor suppressor gene (“two-hits”). The “first hit” is an inherited mutation of a tumor suppressor gene, resulting in two different forms of the same gene within one cell (heterozygosity). The “second hit,” a mutation in the other copy of the tumor suppressor gene, then needs to occur before there is progression to cancer. The second hit results in two non-working copies of the tumor suppressor gene, and the resulting absence of the working copies contributes to the development of a tumor or abnormal growth.

Because an individual with an inherited mutation has a mutated copy of the gene in every cell, they are more likely to develop two mutations than someone who has to have a mutation first occur in a cell (“first hit”) and then have a second mutation occur in the other copy of the same tumor suppressor gene in the same cell (“second hit”). For the same reason, cancer is more likely to occur at an earlier age in an individual with an inherited (or germ line) mutation in a tumor suppressor gene.


Last Menstrual Period (LMP): A convention used for dating pregnancies in which day one is designated to be the first day of a woman’s last menstrual period. This is done to compensate for the often unknown conecption date. By this method, the embryological age of the fetus is typically two weeks younger than the date determined from the LMP.

Linkage Analysis: Tracing the inheritance of a gene or trait in family members by studying traits that are co-inherited (“linked”) with that gene or trait.

Locus: The specific place on a chromosome that a gene occupies; the location of a gene.

Lymph Node: One of numerous bean shaped bodies containing lymphocytes, arranged along the lymphatic system. The lymph node is an integral part of the immune system. It is also one of the most common sites for breast cancer metastases and lymph nodes in the region of lymphatic drainage from the breast are often removed and examined for metastasis at the time of breast cancer surgery.


Mammography: A screening process in which an X-ray is taken of the breasts. Mammography is used to detect breast cancer early, when lumps are less than 2 cm or smaller in size, and cannot be felt during physical examination. Currently, mammograms are considered routine health management for women over the age of 50, and may also be helpful for women between the ages of 40 and 50. Mammograms are not normally performed on women younger than 40 years, unless there is a clinical indication for it.

Manifesting Heterozygote: A female who manifests signs or symptoms of an x-linked recessive condition that ordinarily is only evident in males. See X-linked Recessive Inheritance.

Maternal Age Effect: As a woman gets older, the chance of nondisjunction occurring during the formation of an egg increases. Nondisjunction results in the egg having an extra chromosome or a missing chromosome, which results in birth defects or miscarriage.

Meconium Ileus: Obstruction of the intestine (ileus) by compacted, overly thick meconium, which is the medical term for a newborn’s first stools. Meconium ileus is a condition found in some babies with Cystic Fibrosis due to poor exocrine gland function and lack of digestive enzymes that normally break down meconium.

Mendelian Disorder: A disorder caused by a single gene defect, which can occur in either dominant or recessive inheritance patterns. The inheritance of these disorders follows the ratios first described by Gregor Mendel. A disorder that is not Mendelian would be one where the disorder was due to environmental causes or due to a combination of factors, also called Multifactorial Inheritance.

Mendelian Inheritance: A description of the inheritance of genes and traits from parents to their children, as first described by Gregor Mendel. See Mendelian Disorder.

Meningitis: Inflammation of the meninges, or the membranes that cover the brain and spinal cord.

Mastectomy: A general term for an operation in which breast tissue is removed. The operation may be a total mastectomy in which the entire breast is removed or a partial mastectomy in which only a part of the breast is removed. Axiallary lymph nodes may or may not also be removed during the surgery to look for metastases.

Maternal Serum Alpha Fetoprotein (MSAFP): A substance that is produced by the liver of the fetus, excreted into the amniotic fluid, and is then absorbed into the bloodstream of the mother. A blood sample can be taken from the mother and the level of MSAFP can be measured. The level increases gradually until late in pregnancy. The expected levels of this substance are known at specific times in pregnancy and variations from these amounts can indicate the presence of a problem in the fetus, such as a neural tube defect, Trisomy 18, or Down Syndrome. However, measurements of MSAFP levels during pregnancy are NOT diagnostic of fetal abnormality.

Menarche: The onset of menstruation, synonymous with female puberty.

Menopause: The complete cessation of menstruation, often occurring near age 50.

Metabolic Disorder: A disorder in which there is abnormality in any of the biochemical reactions that take place in the body. A metabolic disorder may affect the way food is digested, energy is produced, growth, cognitive function, and/or the elimination of waste.

Metachronous tumors: multiple separate tumors in the same tissue diagnosed within an interval greater than 6 months of each other.

Metastasis: The process by which cancer spreads from the primary site of the tumor to form another tumor at another site in the body. Cells that metastasize are still of the primary cell type. For example, breast cancer can metastasize to the liver; the cancerous cells in the liver tumor are breast cell descendents. This is important because breast cancer cells may be treated very differently than liver cancer cells.

Microsatellite Instability: Microsatellites are repeating DNA units of 2-4 nucleotides that are found throughout the genome. The repeats can be of various lengths, which means that there can be a variable number of 2-4 nucleotides repeated in a row in the DNA sequence. The number of repeats can increase or decrease during DNA replication and repair, a process described as “instability.” If this occurs in a gene, it can lead to disruption of the normal functioning of the gene, and disease. For example, this is often seen in Hereditary Nonpolyposis Colorectal Cancer.

Mucolipidosis IV: A disorder that causes central nervous system deterioration, usually within the first year of life. Symptoms of the disease are mild motor and mental retardation that slowly, progresses. Overall prognosis and estimated life span are not known. Mucolipidosis IV has a higher incidence in the Ashkenazi Jewish population than in the general population. For more information, see Mucolipidosis IV.

Multifactorial Inheritance: The inheritance pattern in which a trait or disease results from a combination of genetic factors and possibly some environmental influence. Many common traits are multifactorially inherited. Examples of this type of inheritance are skin coloring (melanocyte production determined by genes and sun exposure), Type 2 diabetes (predisposing genes and eating habits), and height (genes and food intake). For contrast, see Mendelian Inheritance.

Mutation: A process by which genes undergo a structural change. It can be a change in the size, arrangement, or molecular sequence of a gene. A mutation may not cause physical problems, for example a benign mutation that causes blue eyes or Type O blood, or a mutation may also be disease causing, such as mutations that cause Tay-Sachs disease.


Neural Tube Defect: Incomplete closure of a portion of the neural tube, during or prior to 24-27 days of embryonic development. The neural tube ultimately forms the brain, brainstem and spinal cord. The site of non-closure can be anywhere along the spinal cord (spina bifida) up to the brain and results in problems as minor as poor feet coordination to nearly complete lack of brain development (anencephaly). Its presence may be suggested by a high level of alpha-fetoprotein on a triple marker screen and it can be accurately detected on a high resolution ultrasound performed after 18 weeks. It occurs in approximately 1 in 2500 newborns. Many cases can be prevented by the intake of folic acid (a B vitamin) before pregnancy and during the 1st trimester.

Newborn screening: Examination of blood samples from a newborn infant to detect biochemical changes due to disease. Each state determines which disorders are included in its newborn screen. For information on this tests for each state, link to http://genes-r-us.uthscsa.edu.

Niemann-Pick Disease, Type A: A condition characterized by severe neurodegeneration beginning in infancy. Affected babies often have hepatosplenomegaly (enlarged liver and spleen), poor growth, and deterioration of physical and mental processes. Death usually occurs before 5 years of age. There are other types of Niemann-Pick disease, but only Type A is seen at a higher frequency in the Ashkenazi Jewish population compared to the general population. For more information, see Niemann-Pick Disease.

Non-classical Adrenal Hyperplasia: A disorder caused by an incomplete block in the pathway to convert cholesterol to cortisol. This block results in an overproduction of androgens, such as testosterone, and causes severe acne, excess facial and body hair, short stature, insulin resistance, menstrual problems, and reduced fertility in females. It does NOT cause female babies to have ambiguous or male genitals at birth and it is not life threatening (due to salt wasting), which are seen in congenital adrenal hyperplasia. This disorder has a higher incidence in the Ashkenazi Jewish population than in the general population. For more information, see Non-classical Adrenal Hyperplasia.

Nondisjunction: A condition in which chromosomes in a pair fail to separate during the process of cell division. When this occurs during the making of an egg or sperm cell, it can result in an egg or sperm with one too few or one too many chromosomes. For example, if this occurred such that a sperm had 24 chromosomes instead of 23, the embryo would have 47 chromosomes (after fertilizing an egg with 23 chromosomes), instead of 46. Down Syndrome is an example of a disorder resulting from having an extra chromosome (47 chromosomes) and Turner Syndrome is an example of a disorder resulting from having one less chromosome (45 chromsomes).

Non-Syndromic Hearing Loss: See Connexin 26 GJB2.

Nucleotide: One of the structural components making up DNA and RNA. A nucleotide consists of a base (one of four chemicals: adenine, thymine, guanine, and cytosine), a molecule of sugar, and a molecule of phosphoric acid.

Nucleus: A structure within a cell that houses the chromosomes (and the DNA that makes up the chromosomes). The nucleus is surrounded by a membrane, and it is the site where DNA replication and RNA synthesis take place.

Null Allele: An allele (or version of a gene) that has a mutation resulting in no functional protein production at all. This is in contrast to a mutation that still allows for some functional protein to be made.

Nulliparity: Never having been pregnant.


Odontomas: A tumor that differentiates and develops into tooth structures.

Oncogenes: Genes that normally regulate the growth of cells but, when overexpressed or mutated, can lose this control and foster the growth of cancer.

Oophorectomy: A general term for an operation to remove one or both ovaries.


Parental Age Effect: The phenomenon of increasing liklihood of having a child with a chromosome or genetic disorder as one, or both, of the parents get older. Advanced maternal age is considered to be 35 years at the time of delivery. Older women are more likely to produce egg cells with an extra chromosome, for example, which can lead to Down Syndrome or other chromosomal syndromes. Advanced paternal age, leading to an increase in incidence of certain autosomal dominant disorders such as Torsion Dystonia, is considered to be 45-50 years.

Pedigree: A family tree diagram often used to trace the heredity of a particular trait or disease through many generations of a family.

Penetrance: The likelihood that a given abnormal gene will actually result in disease, most often associated with dominant disorders. A highly penetrant disorder is one in which the majority of individuals who have the mutated gene will develop the disorder. A less than fully penetrant disorder might be one in which only a percentage of mutation carriers will ever develop the disorder. Contrast with Expressivity.

Percutaneous Umbilical Blood Sampling (PUBS): A prenatal diagnosis method that uses the blood from the umbilical cord for a rapid chromosome analysis or other testing, often within 48 hours after blood is drawn. It involves the insertion of a needle through the pregnant mother’s abdomen, like amniocentesis, to draw a blood sample from a vein in the umbilical cord. PUBS is performed at about 18 weeks gestation or later and may be recommended if a physician feels that an ultrasound, amniocentesis, or chorionic villus sampling do not provide enough information about the fetus. It might also be performed to test for certain infections and blood disorders. If a baby is suspected to be anemic, PUBS is the only way to confirm this, and it allows for transfusion while the needle is in place. The procedure is also known as umbilical vein sampling, fetal blood sampling, and cordocentesis. The miscarriage risk is about 2%. For other types of prenatal diagnosis, see Amniocentesis, Chorionic Villus Sampling, Ultrasound, and Preimplantation Genetic Diagnosis

Pericarditis: Inflammation of the pericardium, or the lining around the heart.

Peritonitis: Inflammation of the peritoneum, or the lining of the abdomen.

Phenotype: Observable characteristics of an organism produced by the organism’s genotype (the specific genes possessed by an organism) interacting with the environment.

Physical Maps: DNA maps showing the location of identifiable landmarks, either genes or distinctive short sequences of DNA. The lowest resolution physical map shows the banding pattern on the 24 different chromosomes; the highest resolution map depicts the complete nucleotide sequence of the chromosomes. (See Contig Map).

Polygenic: When multiple genes are involved. This often refers to the phenotype that occurs due to the interaction of multiple genes, such as height or heart disease.

Polyp: A growth, most often benign, that develops on the inside wall of a hollow organ, such as in the colon.

Population Screening: Testing individuals in a specific population, such as the Ashkenazi Jewish population, who are at greater risk for being carriers for genes that could cause specific genetic disorders.

Postnatal: After a baby is born.

Precancerous Polyps: Protruding growths in the colon or large intestine that are benign but have the potential to become cancerous. The classic polyp has a stalk with a bulb-shape on the end of it.

Predispositional Gene Tests: Tests to identify gene abnormalities that may make a person susceptible to certain diseases or disorders. An example of this is BRCA1 and BRCA2 testing for susceptibility to breast and ovarian cancer. See Overview of BRCA1 and BRCA2 for more details.

Predisposition gene: A gene that increases the likelihood for an individual to develop a disease when it is mutated. Genetic testing for a predisposition gene only changes the likelihood for an individual to develop for a disorder. For example:

    • – If a gene mutation is found, an individual is more likely but not guaranteed to develop the disease
    – If a gene mutation is not found that is known to cause the disease in a family, the individual with the negative test is not guaranteed to not develop the disorder but is instead only less likely to do so.

Examples of predisposition genes are BRCA1 and BRCA2.

Predictive gene: A gene that when they are mutated are known to cause disease. An example of this type of gene is the Huntington gene associated with Huntington’s disease.

Preimplantation Genetic Diagnosis: A prenatal diagnosis method that involves testing the embryo for a genetic disorder at a very early stage of development. This involves in vitro fertilization, where the early embryo undergoes genetic testing prior to implantation. First, eggs and sperm are harvested and mixed together in a sterile environment. After a sperm fertilizes an egg, this cell divides to become 2 identical cells. These cells divide to become 4 identical cells and these 4 cells divide to become 8 identical cells. This 8-cell stage occurs approximately 3 days after the egg is fertilized, and at this point, it is possible to remove a single cell without damaging the future development of the embryo. In other words, the 7 remaining cells are still capable of the same development seen in an embryo fertilized in the more traditional manner. The DNA of the removed cell is then used to do genetic analysis for specific disorders.

For other types of prenatal diagnosis, see Amniocentesis, Chorionic Villus Sampling, Ultrasound, and Percutaneous Umbilical Blood Sampling.

Prenatal: Before a baby is born.

Prenatal Screening: Examining the genetic makeup of the embryo or fetus to detect birth defects or genetic disorders before a baby is born. Testing methods include Chorionic Villus Sampling, Amniocentesis, Percutaneous Umbilical Blood Sampling, and Ultrasound. Ultrasound only detects visible signs of birth defects. It is the only screening test listed here that has no associated miscarriage risk. The other screening methods listed look directly at the fetus’s DNA, gene products, or metabolites and have a small miscarriage risk due to the invasiveness of the procedure. Any of these tests might be recommended if there is history of genetic disorders in the family, if the mother is over age 34, or if there are any problems detected on an earlier ultrasound.

Prenatal Diagnosis: Examining fetal cells taken from the amniotic fluid, the chorion (embryonic form of the placenta) or umbilical cord blood for biochemical, chromosomal, or gene alterations.

Primary Tumor: Either the first tumor or a tumor at the original cancer site. This is in contrast to a Secondary Tumor, which is a tumor arising at a different location from where it originated due to metastasis.

Prophylactic: Describing a preventive measure, something done to prevent something else from occurring. For example, a prophylactic mastectomy is the removal of breast tissue before the formation of a cancerous tumor in order to try to prevent breast cancer from occurring

Prophylactic Surgery: Surgery to remove tissue that is in danger of becoming cancerous, before cancer has developed. An example is a prophylactic mastectomy, which is removal of the breasts of women at high risk of developing breast cancer.

Protein: A large, complex molecule composed of amino acids. The sequence of the amino acids (and the function of the protein) is determined by the sequence of the base pairs in the gene that encodes it. Proteins are essential to the structure, function, and regulation of the body. Examples include hormones, enzymes, and antibodies.

Proteome: All of the proteins expressed by the genome.

Proteomic marker profile: A group of protein markers that may be used for clinical diagnostic, prognostic, or treatment purposes. This approach is currently being used for ovarian cancer research. By looking at changes in the proteomic marker profile, researchers may be able to provide information about whose cancer will stay in remission and whose will recur.

Proteomics: A field of research involving the study of the proteins produced by the human body (the proteome). One of the research topics of proteomics is the association of a protein product with a disease. Researchers do this by looking at the altered levels of protein between individuals with and without the same disease. It enables correlations to be drawn between the range of proteins produced by a cell or tissue and the initiation or progression of a disease state. This may further allow for the discovery of new protein markers for diagnostic purposes and drug treatments.



Recessive Allele: An allele that is expressed only if both copies are mutated or if it is the only copy present. For more information, see Autosomal Recessive Inheritance. For contrast, see Dominant Allele.

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

Risk Factor: A characteristic that increases an individual’s chance of developing a disease. For example, smoking is a risk factor for lung and other cancers.

RNA: A chemical similar to DNA. The several classes of RNA molecules play important roles in protein synthesis and other cell activities.


Salpingo-oophorectomy: Surgical removal of the Fallopian tube and the ovary, usually done bilaterally (on both sides) if it is a prophylactic surgery

Screening: Looking for evidence of a particular disease, such as cancer, in persons with no symptoms of disease. For more information, see Predictive Screening, Prenatal Screening, Carrier Screening, and Population Screening.

Secondary Tumor: A tumor arising at a distant site from its origin, due to metastasis. See Primary Tumor for a contrast.

Selective Estrogen-Receptor Modulator (SERM): A class of drugs that have some of the same biochemical activity as estrogen and are often used to treat breast cancer. An example of a SERM is raloxifene. Like estrogen, raloxifene prevents bone loss and lowers serum cholesterol. However, unlike estrogen, it does not stimulate the division of cells in the lining of the endometrium and thus does not increase the risk of endometrial cancer.

Sephardic Jew: A Jew who has ancestors from the Mediterranean region, usually Spain, Portugal, and North Africa. Any Jew who is not of Ashkenazi, Yemenite, or Iranian descent.

Sigmoidoscopy: A procedure for examining the lower one third of the colon. It is less invasive than a colonoscopy, but does not visualize the entire colon. A flexible, lighted tube is inserted into the rectum and colon, permitting the examiner to see the inside of the rectum and sigmoid colon. Physicians may use this procedure to find the cause of diarrhea, abdominal pain, or constipation. When it is used to look for early signs of cancer in the colon and rectum, it is often combined with additional tests so as to not miss signs of cancer existing only in the upper parts of the colon where it does not reach. The discovery of a polyp or other signs of cancer by sigmoidoscopy (or other tests) indicates the need for a complete colonoscopy. See Colonoscopy.

Single base-pair mismatch: The pairing of DNA bases in which a mismatch has occurred. DNA bases ordinarily pair up with an adenosine (A) base binding to a thymidine (T) base and a cytosine (C) base binding to a guanine (G) base. See below.

Spina Bifida: See Neural Tube Defect.

Stem Cells: Cells that are unspecialized and have the ability to grow into any type of cell found in the body. These cells have the ability to divide throughout life and produce highly specialized cells to take the place of cells that die. This allows for the repair and replacement of damaged tissue.

Sensitivity: A measure for assessing the results of a diagnostic or screening test. It can be thought of as the probability that a person with a disease will have a positive test result, or as the percentage of people who test positive for a disease that truly have that disease.

Sex Chromosomes: Chromosomes that determine the sex of an organism. In humans, the chromosomes that designate the sex of an individual are the 23rd pair of chromosomes: females usually have two X chromosomes and males have one X and one Y chromosome.

Sickle-Cell Anemia: A serious, inherited disease in which a defect in hemoglobin, the oxygen-carrying pigment in the blood, causes distortion (sickling) of the red blood cell and anemia (not enough red blood cells), which can produce damage to organs throughout the body.

Splenectomy: Surgery to remove the spleen.

Somatic Cells: All of the cells of the body except the reproductive cells.

Somatic Mutations: See Acquired mutations.

Specificity: A measure for assessing the results of a diagnostic or screening test. It can be thought of as the probability that a test will be negative in a person free of a disease, or as the percentage of people who test negative for a disease who are truly negative for that disease.

Synchronous tumors: This term describes tumors in the same tissue diagnosed within 6 months of each other.

Synovitis: Inflammation of the synovial membrane, or the lining of the joints.


Tay-Sachs Disease: A disorder in which infants have normal development until approximately 6 months of age, at which time they present with an excessive startle response, a cherry red spot on the retina, and mild motor weakness. There is progressive neurodegeneration and death usually occurs by 3-5 years of age. This disorder has a higher incidence in the Ashkenazi Jewish population than in the general population. For more information, see Tay-Sachs Disease.

Tay-Sachs disease, late-onset: A form of Tay-Sachs disease that does not show onset until 10 years of age or later. It is often less severe and has a slower progression than classical Tay-Sachs disease. It is not clear if late-onset Tay-Sachs disease has a higher incidence in the Ashkenazi Jewish population than in the general population. For more information, see Tay-Sachs Disease.

Translocation: The relocation of a chromosomal segment to a different position within the genome. A chromosomal configuration in which part of a chromosome becomes attached to a different chromosome.

Transvaginal ultrasound: This is a method for imaging the genital tract in women to look for any abnormalities of the reproductive tract. A hand-held probe is inserted into the vagina and high-frequency soundwaves are bounced off the internal body structures to create a picture. This type of imaging is used in endometrial and ovarian cancer screening.

Triple Marker Screening: A screening test offered to pregnant women between 15-18 weeks gestation (length of pregnancy) that can detect an increased risk for certain abnormalities in the fetus such as Down Syndrome, Trisomy 18, and neural tube defects. It involves analyzing a blood sample from a pregnant woman and presents no risks to the fetus. It measures three substances: MSAFP (maternal serum alpha-fetoprotein), hCG (human chorionic gonadotropin), and uE3 (unconjugate estriol).

Trisomy: The presence of three copies of any specific chromosome instead of the normal two copies. An example is Trisomy 21, or Down Syndrome, in which there are three copies of chromosome 21 instead of two.

Trisomy 18: The presence of three copies of part or all of chromosome 18. This results in profound mental retardation, distinct facial features, feet malformations, heart malformations, and kidney malformations. This condition rarely allows for survival to birth and of those who survive to be born, greater than 95% do not live past their first birthday. This condition occurs in approximately 1 in 5000 liveborn babies.

Trisomy 21: See Down Syndrome.

Tumor Suppressor: A gene that normally limits the growth of cells. When a tumor suppressor gene is mutated, it may allow cancer to develop. Examples of tumor suppressor genes are BRCA1 and BRCA2.


uE3 (uncongugated estriol): The hormone estriol, which is produced by the ovaries and in large amounts by the placenta during pregnancy. Levels typically increase during the second trimester. uE3 is one of three markers, in addition to MSAFP and hCG, which is used to calculate the likelihood of a fetal abnormality such as Down Syndrome, Trisomy 18, or a neural tube defect. However, measurements of uE3 levels during pregnancy are NOT diagnostic of fetal abnormality.

Ulcer: A site-specific area of tissue damage or erosion produced by the sloughing away of inflammatory necrotic (dead) tissue.

Ulcerative Colitis: This disease is a form of inflammatory bowel disease and it causes inflammation of the large intestine (colon). Symptoms are intermittent rectal bleeding, abdominal pain (cramps), and diarrhea. It is diagnosed by tissue biopsy done after the colon is visualized by a sigmoidoscopy or colonoscopy. The cause of ulcerative colitis is unclear but patients with long-standing cases are known to be at an increased risk for colon cancer. Treatment consists of medications and possibly surgery.

Ultrasound: A prenatal screening method that uses high frequency sound waves bounced off tissues to create images on a video monitor. This non-invasive technique is used to examine tissues and is widely used during pregnancy to screen for structural defects in the fetus. There is no evidence of any danger from ultrasound during pregnancy and it is regarded as a highly effective and useful tool for screening procedures. For other types of prenatal diagnosis, see Amniocentesis, Chorionic Villus Sampling, Percutaneous Umbilical Blood Sampling, and Preimplantation Genetic Diagnosis.


Variable Expressivity: See Expressivity.


Wild Type: A genetic term referring to the normal, non-mutated version of a gene.


X Chromosome: A sex chromosome; normal human females carry two X chromosomes, and normal human males have one X chromosome.

X-Linked: Genes found on the X chromosome. A male who inherits a mutation on an X chromosome will manifest the disease because he has no second copy of the gene to produce functioning protein product. For more information, see X-Linked Disorders.


Y Chromosome: A sex chromosome found only in males; normal human males carry one Y and one X chromosome.


Currently, there are no definitions beginning with Z.