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Alpha-fetoprotein --- The predominant protein present in fetal blood.  Small amounts are present in the maternal circulation during pregnancy, and the values change as the fetus grows.  Abnormally high or low values are associated with increased risks of spina bifida and Down syndrome, respectively.
Amniocentesis --- A procedure for prenatal diagnosis in which about one tablespoon of amniotic fluid surrounding the fetus is withdrawn from the amniotic sac using a very fine needle.  Continuous ultrasound guidance during the amniocentesis makes the 60 to 90 second procedure as safe and painless as possible.  The risk for miscarriage or other procedure related complications is very low (<0.5%).  Genetic amniocentesis is typically performed as early as 15-17 weeks gestation.
Autosomal recessive / dominant --- Patterns of inheritance in which females are as likely to be affected as males.  In recessive disorders, one gene (of the pair) needs to function properly to prevent the disorder.  For a dominant disorder, one gene functioning abnormally is sufficient to cause the disorder.
BRCA1 and BRCA2 --- Two genes that are important in familial breast cancer.  Deleterious changes in either BRCA1 or BRCA2 result in an increased risk of developing breast and/or ovarian cancer.  These two genes account for the vast majority of inherited susceptibility to these two cancers.  However, only 5-10% of breast or ovarian cancer seems to be due to inherited susceptibility factors.
Carrier --- An individual who has one non-functioning gene for a recessive disorder can transmit that gene to his or her offspring.  Carriers most often have no outward manifestations of their condition.
Chromosome microarray analysis --- A technique using DNA in which very small deletions or duplications of chromosomes can be identified.  This is an important test for children with developmental disabilities or autism, with or without additional physical or developmental abnormalities.
Chromosomes --- Microscopic structures, present in the nucleus of each cell, which contain all of our genes.  Abnormal size or number of chromosomes is a cause of some birth defects and mental retardation.
Chorionic villus sampling (CVS) --- A prenatal diagnostic technique to obtain fetal cells from the placenta.  This can be performed several weeks earlier than amniocentesis, albeit with a somewhat higher risk (~1%) of procedure-related complications.
Cytogenetics --- The study of chromosomes.
DNA --- The molecule which contains each individual cell's genetic code.  In humans, DNA is organized into 23 pairs of chromosomes.
Down syndrome --- A condition involving mental retardation and distinctive facial features which is due to an extra chromosome 21.  This can be detected prenatally by chromosome analysis on fetal cells obtained by amniocentesis or CVS.
Familial --- Occurring in more than one family member.  A familial condition may be due to an inherited (genetic) factor, more than one genetic factor, an environmental factor, or some combination of these.
First trimester screening --- A blood test taken at 10-13 weeks gestation to determine whether there is an increased risk for a fetus having Down syndrome or trisomy 18.  This is usually performed in conjunction with an ultrasound to measure the fetal nuchal translucency (a structure visible on ultrasound at the back of the neck).  Substances measured in the blood are beta-hCG and PAPP-A.
Gene --- A portion of a chromosome, too small to be seen under a microscope, which encodes a particular function.  Humans are estimated to have approximately 20,000 different genes.  Abnormalities of genes can cause various problems, depending on the gene involved and specific changes within that gene.
Genetic consultation --- An opportunity to meet with a geneticist to deal with genetic issues.  The visit usually entails a complete medical history, family history, sometimes a physical examination, and review of previous laboratory testing.  Any indicated testing is ordered, and when all testing has been completed, the geneticist provides appropriate genetic counseling and a written report to the referring physician.
Genetic counseling --- Performed by a geneticist or a genetic counselor supervised by a geneticist, counseling is the process of conveying genetic information specific to the family and medical situation.
Geneticist --- A physician or Ph.D. with special training in human genetics.  Medical Genetics is a specialty recognized by the American Board of Medical Specialties.  The geneticists and genetic counselor at The Genetics Center are board-certified.
Human chorionic gonadotropin (hCG) --- A hormone produced by the placenta during a pregnancy.  Measurement of this hormone is the basis of pregnancy tests.  An unusually high level of this hormone in the second trimester is one reason that further testing for Down syndrome may be recommended.  An unusually low level may indicate a rare chromosome abnormality called trisomy 18.
Maternal serum screening --- A blood test taken at 14-20 weeks gestation to determine whether the fetus is at increased risk for Down syndrome, trisomy 18, or spina bifida.  The test is also known as the alpha-fetoprotein or AFP test, triple screen, or quad screen, depending on which combination of substances are measures on the blood sample.  When combined with results of first trimester screening, the testing is known as sequential or integrated screening.
Multifactorial inheritance --- A pattern of inheritance exhibited by a number of conditions, such as heart disease, diabetes, and certain birth defects.  Multiple genetic or non-genetic factors--not just one--are involved in causing a Multifactorial condition.
Mutation --- A permanent, heritable change in the DNA of a particular gene.
Non-invasive prenatal testing --- A blood test taken after 10 weeks gestation that identifies pregnant women who are carrying a fetus with Down syndrome or trisomy 18 by analysis of cell-free fetal DNA in the maternal circulation.  Presently, the testing is only appropriate for women at increased risk.
Presymptomatic testing --- Testing to determine whether a particular condition will develop in the future.  Presymptomatic testing for breast cancer involves analyzing the BRCA1 and BRCA2 genes.
Risk assessment --- Determining the chance of a particular outcome by using a combination of family history and laboratory testing.  An example of this is predicting breast cancer risk for a woman with a family history of breast cancer who has had BRCA gene testing.
Spina bifida --- A birth defect caused when the neural tube, the structure in the very early embryo which develops into the brain and spina cord, fails to close properly.  Spina bifida due to a neural tube defect typically results in paralysis as well as hydrocephalus ("water on the brain").  Anencephaly is a severe form of a neural tube defect in which no normal brain tissue develops.  Spina bifida occulta is a minor defect of the spine which is not related to spina bifida from an open neural tube defect.
Translocation --- A chromosome rearrangement in which the chromosomes are not arranged in the typical set of 23 pairs.  Even if the correct amount of genetic material is present, an individual with a chromosome translocation is at increased risk for having problems such as difficulty conceiving or maintaining a pregnancy.  Chromosome translocations are often inherited.
Trisomy --- Having three of a given chromosome instead of the normal two.  Trisomy for chromosome 21 is the cause of Down syndrome.  Trisomy for chromosome 18 and trisomy for chromosome 13 cause very serious birth defects.
Ultrasonography --- The use of sound waves higher than humans can hear to examine internal body structures, such as a fetus contained in the uterus.  An ultrasound examination is also known as a sonogram.
X-linked --- Hemophilia, Duchenne muscular dystrophy, and color blindness are examples of diseases whose genes are located on the X chromosome.  Males are most often affected with X-linked disorders.  This is the same as the formerly used term, sex-linked.
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Revised February 8, 2013