Genetics Northern California


Abdominal Wall DefectsBabies with these defects have abnormal openings along the abdomen (stomach), so that intestines and other organs grow outside the baby’s body. An opening where the umbilical cord is attached is called omphalocele. This occurs in about 1 out of 4,000 births. An opening beside the umbilical cord is called gastroschisis. This occurs in about 1 out of 10,000 births. These openings may be present along with other birth defects. Surgery after birth can often repair an abdominal wall defect.

Not normal; contrary to the usual structure, position, condition, behavior, or rule.

Advanced maternal ageA term used to describe the maternal age at which the risk to have a child with a chromosomal abnormality is great enough that prenatal diagnosis is offered based on age alone. In the United States, this is generally 35 and older.
Aerobic exerciseActive exercise involving elevation of heart rate and breathing for the rapid exchange of oxygen in and out of the body.

Individual(s) in a pedigree that exhibit the specific condition being studied.


One of the alternative versions of a gene that may occupy a given locus (gene site).

Allelic Heterogeneity

The situation in which there are different mutant alleles at the same locus (gene site), each capable of producing an abnormal phenotype.

Alpha (a) chainOne of the two different polypeptide chains of hemoglobin. There are 2 alpha chains in each hemoglobin molecule. An alpha chain is made up of 141 amino acids.
Alpha-fetoproteinAlpha-fetoprotein (AFP) is a protein produced mainly in the developing baby's liver and released into the amniotic fluid. A small amount crosses the placenta and can be measured in a pregnant woman's blood towards the end of the first trimester. Levels rise at a steady rate throughout the second trimester. In most babies affected with open spina bifida, anencephaly, or an abdominal wall defect, more AFP is in the amniotic fluid and that leads to a higher amount of AFP in the maternal blood. In Down syndrome, trisomy 18, and SLOS, AFP levels may be lower than average.  
Amino Acid DisordersA group of conditions that cause amino acid(s) to build up to toxic levels in the body. Usually this build-up is caused by an enzyme that is missing or not working properly. The defective enzyme stops the person from being able to metabolize (break down) protein. Protein is made up of amino acids and, if it cannot be broken down properly, the amino acid(s) will build up to toxic levels. Amino Acid Disorders include Phenylketonuria (PKU), Maple Syrup Urine Disease (MSUD), and Homocystinuria (HCU), among others.
The way amino acid disorders are inherited is called autosomal recessive. Please visit our metabolic autosomal recessive inheritance section, Metabolic Clinic page and Metabolic Clinic links section for more information.
Amino AcidsThe building blocks of proteins. These are coded for within the DNA.
AmniocentesisA procedure used in prenatal diagnosis to obtain amniotic fluid. Amniotic fluid is withdrawn from the amniotic sac by syringe after insertion of a hollow needle into the sac through the abdominal and uterine wall.

Cells obtained by amniocentesis.

Amniotic fluidFluid that surrounds and protects the fetus in the amniotic sac, made up mostly of fetal urine.

In molecular biology, the production of multiple copies of a sequence of DNA.

AnencephalyA condition in which the skull cap and brain do not develop.
AnomaliesMarked deviations from the normal standards. Unusual features, characteristics, or constitutions (paraphrased from Dorland's).

The term used to denote the progressively earlier appearance and increased severity of a disease in successive generations.

AnticoagulantA drug that helps prevent the clotting (coagulation) of blood. These drugs tend to prevent new clots from forming or an existing clot from enlarging. They don't dissolve a blood clot.
Anti-oxidantsA synthetic or natural substance added to prevent or delay deterioration by the action of oxygen in the air (i.e., vitamin A, vitamin C, and vitamin E.)
ApneaAbsence of breathing. For example, sleep apnea, which is often associated with frequent awakening during sleep, and daytime sleepiness.
ArachnodactylyDescribes very long and slender fingers and toes (descriptive of individuals with Marfan syndrome).

The occurrence together in a population of two or more phenotypic characteristics more often than expected by chance.

AsymptomaticShowing no evidence of disease.
AtherosclerosisDeposition of fats and lipids on the walls of arteries, also known as hardening of the arteries due to these plaques of fat being laid down on the artery wall.
AtresiaAbsence or narrowing of a normal body opening or tubular organ - i.e., anus or esophagus.
Atypical hyperplasiaIn breast cancer, atypical hyperplasia refers to an abnormal pattern of growth is seen in the breast lobule and/or breast duct, which is associated with an increased risk of developing breast cancer.
AudiologyThe study of hearing with specific emphasis on treatment and diagnosis of impaired hearing or deafness.
AutoimmuneDirected against the body's own tissue - i.e., rheumatoid arthritis.
Autosomal dominantRefers to one of several inheritance patterns for single gene disorders, equally likely in both males and females. Autosomal means that both males and females can manifest symptoms of the disorder. Dominant means that of the two copies of each gene we carry, only one needs to be mutated in order to develop symptoms of the condition. Most lipid problems are inherited in this manner.
Autosomal recessiveRefers to one of several inheritance patterns for single gene disorders, equally likely in males and females. Autosomal means that both males and females can manifest symptoms of the disorder. Recessive means that of the two copies of each gene we carry, both must be mutated in order to develop symptoms of the condition.

Any chromosome found in the cell nucleus other than the sex chromosomes. Humans typically have 22 pairs.