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Health Library : Medical Genetics


Glossary - Medical Genetics

A| B| C| D| E| F| G| H| I| J| K| L| M| N
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abnormality-a health problem or feature not normally present in a healthy individual.

amplification-the production of multiple copies of a region of DNA.

Angelman syndrome-  a combination of birth defects caused by inheriting both copies of a section of the #15 chromosome from the father.

anomaly-a health problem or feature not normally present in a healthy individual; a deviation from the normal.

autosomal dominant inheritance-a gene on one of the first 22 pairs of chromosomes; when present in one copy, causes a trait or disease to be expressed.

autosomal recessive inheritance-a gene on one of the first 22 pairs of chromosomes; when present in two copies, causes a trait or disease to be expressed.

autosome-one of the first 22 pairs of chromosomes; any other than the sex chromosomes.


biochemical genetic testing-a test to study specific enzymes in the body.

birth defect-a health problem present at birth.


carrier testing-testing performed to determine whether a person carries one copy of an altered gene for a particular recessive disease.

centromere-the center part of a chromosome that appears "pinched" between the p and q arms; required for normal segregation in mitosis and meiosis.

chromosome-a structure in the nucleus of cells which contains genetic information (DNA).

chromosomal microarray analysis-a new laboratory test used to detect chromosomal imbalance at a higher resolution than current standard chromosome or FISH techniques.

codon-a triplet of three letters (base pairs) of the DNA alphabet.

congenital-present at birth.

congenital anomaly-a health problem present at birth (not necessarily genetic).

congenital varicella syndrome-a combination of birth defects caused by a maternal chickenpox infection during the first 20 weeks of pregnancy.

Cri du Chat syndrome-a rare combination of birth defects caused by a deletion of chromosome 5p.

cystic fibrosis-an inherited autosomal recessive condition that causes the secretion of abnormal mucus in the lungs and problems with pancreas function and food absorption.

cytogenetics-the study of chromosomal material.


de novo-new, not present previously (not inherited).

deletion-when a part of a chromosome is missing, or part of the DNA code is missing.

diagnostic testing-used to identify or confirm the diagnosis of a disease or a condition in a person or a family.

direct DNA studies-studies which look directly at the gene in question for an error (mutation).

DNA-deoxyribosenucleic acid, the chemical which makes up our genes.

Down syndrome-A combination of birth defects caused by the presence of an extra #21 chromosome, trisomy 21.

duplication-when a part of a chromosome is present in two copies.


enzyme replacement therapy (ERT)-a treatment that replaces the enzyme which is missing or defective in a genetic disease.

extended banding chromosome study-when the chromosomes are studied at a higher resolution than a standard chromosome study, allowing you to see smaller pieces of the chromosome material.


fetal alcohol syndrome-a combination of birth defects caused by the mother's consumption of alcohol during pregnancy.

fetal hydantoin syndrome-a combination of birth defects caused by the mother's use of phenytoin, an anti-seizure medication, during the first trimester of pregnancy.

first-degree relative-a relative that you share 1/2 of your genes with - such as your parents, children, and siblings.

fluorescence in situ hybridization (FISH)-a laboratory technique used to determine how many copies of a specific segment of DNA are present or absent in a cell.

folic acid-a nutrient found in some green leafy vegetables, nuts, beans, citrus fruits, fortified breakfast cereals, and some vitamin supplements. Folic acid can help reduce the risk of birth defects of the brain and spinal cord.

fragile-X syndrome-a combination of health problems caused by an abnormally high number of trinucleotide repeats in the FMR1 gene.


galactosemia-an autosomal recessive metabolic disorder tested for by newborn screening. Dietary treatment is available for this disorder.

gene-a segment of DNA that produces a protein product; genes determine traits.

gene therapy-inserting a normal copy of a gene into a person, to replace a non-working or missing gene.

genetic-determined by genes or chromosomes.

genetic counseling-providing an assessment of heritable risk factors and information to patients and their relatives concerning the consequences of a disorder, the probability of developing or transmitting it, and ways in which it can be prevented, treated, and managed.


hemophilia A-a blood clotting disorder caused by a mutation in a gene on the X chromosome.

Human Genome Project-a government-funded project to sequence and map all of the human genes (25,000) on the 46 chromosomes (23 pairs).

hypothyroidism-a treatable disorder tested for by many newborn screening programs. Low thyroid levels in a newborn can cause major health problems if left untreated.


inborn error of metabolism-a biochemical disorder caused by an inherited defect in an enzyme pathway which affects the body's metabolism.

incontinentia pigmenti (IP)-an inherited X-linked dominant condition that causes skin abnormalities and many other health problems.

indirect DNA studies-studies that look at markers around the gene in question, due to an inability to study the gene itself; also called "linkage studies."

inheritance-used to describe a trait or gene passed from one generation to the next.

inversion-a chromosomal rearrangement when a chromosome breaks and the piece turns upside down and reattaches itself. Inversions may or may not cause birth defects depending upon their exact structure.

isolated case  -refers to an individual who is the only affected member of his/her family, either by chance or through a new mutation, of a birth defect.



karyotype-a picture of the 46 chromosomes, lined up into 23 pairs.


Lebers Hereditary Optic Neuropathy (LHON)-a disorder of central vision loss caused by an altered gene inherited from the mother's mitochondria.


Marfan syndrome-a connective tissue disorder that has autosomal dominant inheritance.

markers-known DNA sequences used to track a gene in a family.

meiosis-the cell division process that eggs and sperm undergo, which halves the chromosome number from 46 to 23.

Mendel-an Austrian monk who performed experiments on garden peas to understand inheritance patterns.

metabolism-a term used to describe how the body converts food to energy, and then gets rid of waste products.

mitochondrial inheritance-when a gene from the mitochondria is passed through a family (always from the mother) that results in a trait or disease.

mitosis-the process of ordinary cell division that other cells, besides eggs and sperm, undergo. Results in two cells genetically identical to to the parent cell (with 46 chromosomes).

monosomy-having a single copy of a chromosome, rather than the usual pair.

mosaicism-the presence of two or more chromosome patterns in the cells of a person, resulting in two or more cell lines (i.e., some with 46 chromosomes, others with 47).

multifactorial-an inheritance pattern involving both genetic and environmental factors.

mutations-changes in the DNA that codes for a gene, which may or may not cause a normal working gene to become a non-working gene.


newborn screening-a test done on every baby born in the United States within the first few days of life, to look for inherited, treatable, metabolic disorders.

nondisjunction-an error in cell division where the chromosomes fail to disjoin, so that both pass to the same daughter cell.


open neural tube defects (ONTDs)-defects in the primitive spine, called the neural tube, such as spina bifida (open spine) and anencephaly (open skull).


"p" arm-the top half, or shorter arm of a chromosome.

Pallister Killian-a combination of birth defects caused by a duplication of the #12 chromosome material.

pedigree-a diagram of a family tree indicating the family members and their relationship to the person with an inherited disorder.

phenylketonuria (PKU)-an autosomal recessive metabolic disorder tested for by newborn screening and often treatable by dietary changes.

Prader-Willi syndrome-a combination of birth defects caused by inheriting both copies of a section on the #15 chromosome from the mother or by inheriting a deletion of a region of chromosome #15 from the father.

preconception visit-a pre-pregnancy examination performed by your physician before you become pregnant to assess overall health and identify potential risk factors that may complicate a pregnancy.

predictive genetic testing-determines the chances that a healthy individual with or without a family history of a certain disease might develop that disease.

preimplantation diagnosis-used following in vitro fertilization to diagnose a genetic disease or condition in an embryo before it is implanted into the mother's uterus.

prenatal diagnosis-used to diagnose a genetic disease or condition in the developing fetus.

presymptomatic genetic testing-used to determine whether persons who have a family history of a disease, but no current symptoms, carry the gene alteration associated with the disease.


"q" arm-the bottom half, or longer arm, of a chromosome.


red-green color blindness-an altered gene on the X chromosome which causes difficulty with distinguishing shades of red and green.

ring chromosome-a chromosome whose ends stick together to form a circle or ring.


second-degree relative-a relative that you share 1/4 of your genes with such as your aunts, uncles, nieces, nephews, and grandparents.

sex chromosomes-the 23rd pair of human chromosomes which determine gender; females have two X chromosomes; males have one X and one Y chromosome.

sibling-brother or sister.

sickle cell anemia-an inherited autosomal recessive condition that causes abnormal hemoglobin in blood cells, leading to infections and organ damage.

syndrome-a collection of traits, health problems, and/or birth defects in an individual which usually has a single underlying cause.


Tay Sachs disease-an inherited autosomal recessive condition that causes a progressive degeneration of the central nervous system, which is fatal (usually by age 5).

teratogen-an agent that has the potential to cause birth defects if exposure to a fetus occurs at a critical time in pregnancy.

third-degree relative-a relative that you share 1/8 of your genes with such as your first cousins.

threshold-a term used to describe the level of liability genes and environmental triggers needed to cause expression of a disorder in multifactorial inheritance; the level may differ between males and females.

translocation-when the location of specific chromosome material moves to another chromosome.

trinucleotide repeats-several triplets of the DNA alphabet in a row.

trisomy-having three copies of one chromosome, rather than two copies.

trisomy 13-the presence of three #13 chromosomes, also known as Patau syndrome.

trisomy 18-the presence of three #18 chromosomes, also known as Edwards syndrome.

trisomy 21-the presence of three #21 chromosomes, also known as Down syndrome.

Turner syndrome-A combination of health problems caused by the absence of one sex chromosome, leaving a single X chromosome, or 45 total.


uniparental disomy-when two copies of a chromosome come from one parent, rather than one copy from the mother, and one copy from the father.


varicella-the virus which causes chickenpox.

vitamin therapy-using nutrition to decrease the incidence of disease or symptoms.

VZIG-varicella-zoster immune globulin, a vaccine for chickenpox.



X-linked inheritance-a gene on the X chromosome passed through a family, resulting in a specific trait or disease seen more commonly in males than females.



 Sources & References




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