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Td (tetanus, diphtheria) vaccine

A booster to the DTaP vaccine to help prevent tetanus and diphtheria. It's given to children age 11 years or older and every 10 years throughout life.

Tetralogy of Fallot

A type of heart defect that is present at birth and causes low oxygen levels in the blood. This leads to cyanosis (a bluish-purple color to the skin). The classic form includes four defects of the heart and its major blood vessels:

  • Ventricular septal defect (hole between the right and left ventricles)
  • Narrowing of the pulmonary outflow tract (the valve and artery that connect the heart with the lungs)
  • Overriding aorta (the artery that carries oxygen-rich blood to the body) that is shifted over the right ventricle and ventricular septal defect, instead of coming out only from the left ventricle
  • Thickened wall of the right ventricle (right ventricular hypertrophy)


The Genetic Information Nondiscrimination Act

Also known as GINA, federal legislation that makes it unlawful to discriminate against individuals on the basis of their genetic profiles in regard to health insurance and employment. These protections are intended to encourage Americans to take advantage of genetic testing as part of their medical care. President George W. Bush signed GINA into law on May 22, 2008.


Third Trimester

Weeks 29-40 of a pregnancy



Amino acid found in most proteins and essential for nutrition. Restricted amino acid for patients with propionic acidemia.

Traditional cytogenetic testing

See Karyotype


The addition of an amine group.


A type of chromosomal abnormality in which a chromosome breaks and a portion of it reattaches to a different chromosome. Chromosomal translocations can be detected by analyzing karyotypes of the affected cells.



To carry from one area to another in the body or within a cell.


Trembling or shaking and lack of control of voluntary muscles.

Trichorrhexis Nodosa

A node-like appearance of fragile hair.

Triggering Event

An episode that causes a reaction or illness.


A state of having three complete sets of chromosomes, rather than the normal two sets. Triploidy in humans can occur in different ways, described as 69,XXX; 69,XXY; 69,XYY.


Trisomy 13

(also called Patau syndrome) is a genetic disorder in which a person has three copies of genetic material from chromosome 13, instead of the usual two copies. The extra material interferes with normal development. Trisomy 13 occurs in about 1 out of every 10,000 newborns. Most cases are not passed down through families (inherited). Instead, the events that lead to trisomy 13 occur in either the sperm or the egg that forms the fetus. Symptoms include:

  • Cleft lip or palate
  • Clenched hands (with outer fingers on top of the inner fingers)
  • Close-set eyes -- eyes may actually fuse together into one
  • Decreased muscle tone
  • Extra fingers or toes (polydactyly)
  • Hernias: umbilical hernia, inguinal hernia
  • Hole, split, or cleft in the iris (coloboma)
  • Low-set ears
  • Intellectual disability, severe
  • Scalp defects (missing skin)
  • Seizures
  • Single palmar crease
  • Skeletal (limb) abnormalities
  • Small eyes
  • Small head (microcephaly)
  • Small lower jaw (micrognathia)
  • Undescended testicle (cryptorchidism)

More than 80% of children with trisomy 13 die in the first year.


Trisomy 18

A genetic disorder in which a person has a third copy of material from chromosome 18, instead of the usual two copies.

Trisomy 18 is a somewhat common syndrome. It is three times more common in girls than boys.

The syndrome occurs when there is extra material from chromosome 18. The extra material affects normal development. Symptoms include:

  • Clenched hands
  • Crossed legs
  • Feet with a rounded bottom (rocker-bottom feet)
  • Low birth weight
  • Low-set ears
  • Mental delay
  • Poorly developed fingernails
  • Small head (microcephaly)
  • Small jaw (micrognathia)
  • Undescended testicle
  • Unusual shaped chest (pectus carinatum)

Half of infants with this condition do not survive beyond the first week of life. Some children have survived to the teenage years, but with serious medical and developmental problems.


Trisomy 21

See Down syndrome

Turner Syndrome

Turner syndrome is a genetic disorder that affects a girl's development. The cause is a missing or incomplete X chromosome. Girls who have it are short, and their ovaries don't work properly. Other physical features typical of Turner syndrome are:

  • Short, "webbed" neck with folds of skin from tops of shoulders to sides of neck
  • Low hairline in the back
  • Low-set ears
  • Swollen hands and feet

Most women with Turner syndrome are infertile. They are at risk for health difficulties such as high blood pressure, kidney problems, diabetes, cataracts, osteoporosis, and thyroid problems.

Doctors diagnose Turner syndrome based on symptoms and a genetic test. Sometimes it is found in prenatal testing. There is no cure for Turner syndrome, but there are some treatments for the symptoms. Growth hormone often helps girls reach heights that are close to average. Hormone replacement can help start sexual development. Assisted reproduction techniques can help some women with Turner syndrome get pregnant.


Unclassfied variant

See Variant of Uncertain Significance


Found in blood and lymph, urea is the primary nitrogenous end-product of protein metabolism, formed in the liver from amino acids and from ammonia compounds. Because urea contains ammonia which is toxic to the body, it must be quickly filtered from the blood by the kidneys, where it is excreted in the urine.

Urea Cycle

A metabolic process in which waste (nitrogen) from the breakdown of proteins in the body is changed by the urea cycle enzymes in the liver into urea in the bloodstream. The blood is then cleansed by the kidneys and ammonia is excreted in the urine.

Urea cycle disorder

A disorder caused by a genetic mutation resulting in deficiency of one of the six enzymes in the urea cycle, located in the liver, responsible for detoxifying ammonia.


The transformation of proteins to a urea compound that can be excreted in the urine.


Amino acid found in most proteins and essential for nutrition. Restricted amino acid for patients with propionic acidemia.

Variant of Uncertain Significance

A variation in a genetic sequence whose association with disease risk is unknown. Also called unclassified variant and VUS.



A retrovirus (retroviruses are a class of RNA viruses that are capable of converting their RNA into DNA, and integrating into the host genome) or adenovirus (carrier of common viruses) that has had its own genetic material modified or removed and replaced by other genes (like those that contain instructions to make OTC enzyme). The vector can then be introduced into a patient who suffers from an enzyme deficiency, carrying the new needed genetic material into that individual.


See Variant of Uncertain Significance


Unusable or excess material, lost by breaking down of the body


A sex chromosome in both males and females (males have one X-chromosome and one Y chromosome, XY=male, and females have two X-chromosomes, XX=female).

X-linked disorder

A disorder occurring when an individual receives an affected X chromosome (carrying the disorder) from the mother.


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