Glossary

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21-hydroxylase (21-OH)
This is an enzyme made by the body. It helps make cortisol and aldosterone. The enzyme is found in the adrenal glands and is essential in making the amounts of cortisol and aldosterone needed by the body.

21-hydroxylase deficiency (21-OHD)
This is the most common type of inherited enzyme deficiency causing congenital adrenal hyperplasia (CAH). Children who are completely missing this enzyme, or have very little working enzyme, usually have the ‘salt-wasting’ form of classic CAH. Children who have a reduced level of 21-OH but still make some of this enzyme usually have the ‘simple-virilizing’ form of classic CAH.

17-hydroxyprogesterone (17-OHP)
This is the chemical that cortisol is made from.  Children and adults with congenital adrenal hyperplasia (CAH) have much higher levels of 17-OHP in their blood because they cannot turn it into cortisol.  Detecting increased amounts of 17-OHP in the blood is one of the tests that confirm the diagnosis of CAH. 

Acylcarnitine
Our bodies break down the fat that we eat into substances called fatty acids. Fatty acids are then broken down even further into substances called acylcarnitines. Acylcarnitines are used to create energy for the body.

Acylcarnitine Profile
This is a blood test that can be used to diagnose certain fatty acid oxidation and organic acid disorders. Fatty acids get broken down into substances called acylcarnitines. These substances are then used to create energy for the body. Certain fatty acid oxidation disorders cause changes in the usual pattern of acylcarnitines.

Adrenal crisis
This is a life-threatening state in which the body is not making enough cortisol.  Babies with classic CAH may have frequent adrenal crises if they are not treated. They are also at risk for adrenal crises during illness or times of stress if their medication is not increased.  Symptoms of adrenal crisis include: dizziness, paleness and sweating (if blood sugar is low), abdominal pain, vomiting, low blood pressure, dehydration, confusion, rapid heart beat, and, if not treated, coma and possibly death. 

Adrenal glands
The adrenals are a pair of glands near the kidneys that make three types of hormones important in helping the body function normally.  The main hormones made in the adrenal glands are cortisol, aldosterone and sex-hormones such as estrogen, progesterone, and male-like androgens (including testosterone).  When the adrenal glands make more or less of one or more of these hormone, health problems occur.

Affected family member
A family member who has the disease or medical condition in question.

Aldosterone
This is a hormone made in the adrenal glands.  It helps the body keep a normal amount of salt and water in the blood.  This keeps the blood pressure at a normal level. High levels of aldosterone cause too much salt and water to be kept in the blood and causes high blood pressure.  Low levels of aldosterone cause the kidney to remove too much salt and water from the blood and excrete it in the urine.  This causes dehydration and low blood pressure.  If the blood pressure drops too low, it can cause adrenal crisis and lead to a coma and possibly death.

Allele
Name for different forms of a gene. Different alleles of a gene may produce different forms of a protein. Some alleles of a gene may form altered proteins that do not function properly. These can often lead to disease.

Amino acid
Amino acids are small molecules that make up proteins. There are over 100 different amino acids, but our body uses only 20 amino acids to make all of its proteins. Our genes determine the sequence of amino acids in a protein. This sequence determines what shape the protein takes, and what function that protein serves in the body.

Amino acid disorders (AA)
These are a group of rare inherited conditions. People with amino acid disorders cannot digest particular amino acids - the building blocks of protein. These amino acids, along with other toxic substances, build up in the body. This can cause serious effects on health, growth, and learning.

Ammonia (NH3)
This is a waste product made when protein is broken down for the body to use. Ammonia is harmful to the body. It is usually changed to a harmless substance called "urea." Urea is then removed from the body in the urine. People with urea cycle disorders cannot get rid of ammonia. If these conditions are not treated, ammonia can build up and cause serious health problems.

Amniocentesis
This is a test done during pregnancy. A needle is used to remove a small sample of fluid from the sac around the fetus. The sample can be used to test for certain genetic disorders in the fetus. Amniocentesis to test for genetic conditions is usually done between 13 and 20 weeks of pregnancy.

Androgens
This is a group of sex hormones that help create male sex characteristics and masculine traits.  Androgens help with normal development of the penis and testicles in male fetuses.  At puberty, androgen levels go up and cause secondary male sex characteristics such as facial hair, pubic and underarm hair, growth of the penis, sperm formation, muscle development and growth in height. Girls make androgens, too, but in much smaller amounts than boys. 

Anemia
This is a condition in which the blood has too few red blood cells or too little hemoglobin, the oxygen carrying molecule in the blood. People with anemia are often pale, tired, weak, dizzy and short of breath.

Arginine (arg)
This is one of 20 amino acids that make up protein. It is also known as "Arg." Most of the arginine needed is made by the body. In most cases, it does not need to be eaten in the diet. Arginine can also be bought in pill form. Children with certain urea cycle disorders are often given arginine supplements. These supplements can help clear the body of ammonia.  

Aspartame
This is one type of sugar substitute. It is made up of the amino acids phenylalanine (Phe) and aspartic acid (Asp). It is sold under the brand names "Equal," "Nutrasweet," "Sweetmate" and "Canderal." Aspartame is harmful for people with PKU because it causes Phe to build up in the blood. High levels of Phe can cause health problems and brain damage. It is important for people with PKU to avoid any foods or drinks that contain aspartame.

Asymptomatic
Not showing any of the signs of the condition or disease.

Ataxia
This is the lack of ability to coordinate muscle movements. It causes balance problems and difficulty walking. People with ataxia often walk with jerky, unsteady movements. Ataxia has many different causes.

Autosomal dominant
Genes come in pairs. If one gene of a pair causes, by itself, a particular trait or disorder to be present, it is called ‘dominant.’ If the gene is found on one of the first 22 pairs of chromosomes, it is called autosomal dominant. (chromosomes 1 through 22 are called ‘autosomes’). Although genes are always in pairs, in autosomal dominant inheritance, you only need to inherit one copy of a dominant gene in order to show a particular trait.

Autosomal dominant conditions can be inherited from one parent, who also has that condition or the gene for that condition, or can occur in one person in the family for the first time (called a ‘new mutation’). If neither parent has the autosomal dominant gene change that the child has, it is a ‘new mutation’ just present in that child, and there is a low risk for it to affect other siblings. However, the person with the autosomal dominant gene change has a 50% chance to pass it on to each of his or her children.

Autosomal recessive
Most of the metabolic disorders that can be detected by newborn screenings are inherited in an "autosomal recessive" pattern. Autosomal recessive conditions affect both boys and girls equally.

How autosomal recessive inheritance works:
Everyone has a pair of genes for each enzyme in the body. A separate pair of genes is responsible for making each enzyme. A person with a metabolic disorder has one enzyme that is either missing or not working properly. The problem is caused by a pair of "recessive" genes that are not working correctly. They do not make the needed enzyme.

A person has to have two non-working "recessive" genes in order to have an autosomal recessive metabolic disorder. A person with an autosomal recessive disorder inherits one non-working gene from his or her mother and the other from his or her father. The parents are called carriers for that condition.

Parents of children with a metabolic disorder rarely have the disorder themselves. Instead, for that pair of genes, each parent has one that is working correctly and one that is not working (called the "recessive" gene). People with a single non-working gene are called carriers. If one gene of the pair is working correctly, it makes up for the recessive non-working gene. Therefore, carriers usually will not have the condition.

Autosome
One of the two types of chromosomes found in most animals. Autosomes contain many thousands of genes, but do not contain genes that determine the sex of an organism. Sex chromosomes, on the other hand, do contain sex-determining genes. Humans have a total of 46 chromosomes; 44 autosomes (chromosome pairs 1-22) and two sex chromosomes (X and Y).

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Base pair
Bases are the components that make up DNA. There are 4 bases: Adenine (A), Cytosine (C), Guanine (G), and Thymine (T). In a DNA molecule, the bases pair with each other to hold together the two strands in a double strand of DNA. A always pairs with T, and G always pairs with C. Base pairs are also used as a unit of measure to indicate a length of DNA. A piece of DNA that is 10bp long has 10 base pairs in it. Likewise, a gene that is 2Kb long has 2,000 base pairs. The human genome has around 3 billion base pairs.

Benign
Something that is not dangerous to one's health, or can be cured.

Betaine
This is a vitamin-like substance found in grains and other foods. It can also be bought in pill form. It is sometimes used as a treatment for homocystinuria. It can help lower the amount of homocystine in the blood. It may also help lessen the risk of blood clots in people with this condition.

Bicarbonate
This is a substance that lowers the amount of acid in the blood. It is sometimes used as part of the treatment for children with certain organic acid disorders.

Biotin
This is one type of B vitamin. It helps break down fat, protein, and carbohydrates from food for use by the body. Biotin is found in chicken, salmon, cheese, eggs, and some vegetables. It can also be bought in pill form. Children with certain metabolic disorders may be treated with biotin supplements.

Biopsy
This is a procedure that involves taking a small piece of tissue (like skin) from a person to help diagnose a condition.

Blood relative
Everyone in your family who is genetically related to you. Blood relatives DO NOT include step children, adopted individuals, and people who have married into the family.

Bone age
This is an estimate of the age, or degree of maturity, of a child’s bones.  It is done by taking an X-ray of the wrists and hands and comparing the bone’s development to the average for the child’s chronological age.  The test can tell a doctor a child’s remaining growth potential and whether or not the growth plates are closing too early. 

Bone marrow
Bone marrow is the tissue that is located around the hollow center of bones. Bone marrow produces new red blood cells.

Bone marrow transplant
When a person does not have enough working red blood cells, they may have a bone marrow transplant. In this surgical procedure, bone marrow is removed from another person (donor) and implanted into the patient. The patient and bone marrow donor must be a genetic match. There are many risks associated with bone marrow transplants.

Branched-chain amino acids (BCAA)
These are amino acids that have a chemical structure that looks like branches on a tree. Leucine (leu), valine (val), and isoleucine (ile) are three of the BCAAs. They are found in all foods that contain protein. Large amounts are found in meat, eggs, milk, and other dairy foods. Smaller amounts are found in flour, cereal, and in some vegetables and fruits.

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Carbohydrate
This is one of the three types of nutrients in food (carbohydrates, fats, proteins). Sugars and starches are the most important types of carbohydrates. They are broken down by the body into glucose, the main source of energy for the body. Carbohydrates are found in foods such as: bread, cereal, pasta, grains, fruits and vegetables. Milk and dairy products, fruit drinks, sugar, candy, and sweet desserts also contain carbohydrates.

Cardiomyopathy
This is a condition that causes the heart muscle to become weak and enlarged. These changes make it more difficult for the heart to pump blood. If not treated, it can be life-threatening. Some of the causes include certain metabolic disorders, viruses, alcohol, or a heart attack.

L-Carnitine
This is a natural substance found in muscle cells. It helps the body make energy from the fat in food and the fat stored in the body. It also helps the cells get rid of harmful wastes made during the breakdown of fat. Carnitine supplements may be used as a treatment for some metabolic disorders.

Carnitine uptake test
This is a special test done on a skin sample. It is used to diagnose carnitine deficiency disorders. It is sometimes used to test for carnitine transporter deficiency (CTD).

Carrier
A person who has one copy of a gene mutation for a particular autosomal recessive disorder (remember genes come in pairs). Carriers are not affected by the disorder. However, they can pass on the gene mutation to their children. Children who inherit two such gene mutations will be affected by the disorder.

Cataracts
This is when the lens of the eye becomes cloudy and hard to see through.  In a normal eye, the lens is almost transparent.  Its job is to focus the things we are looking at onto the retina at the back of the eye so we can see objects at different distances clearly.  Cloudiness of the lens causes dimming of the eyesight and fuzzy images.  Cataracts are usually caused by exposure to too much sunlight or by aging.  In young infants with galactosemia, cataracts may be caused by the buildup of galactose and other substances in the blood. 

Cell
The smallest living unit. Cells make up all organs and tissues in multi-cellular organisms, like humans. They can also live independently, as in bacteria and other microorganisms. At a minimum, a cell is surrounded by a membrane, contains DNA at some stage in its life, and is able to replicate itself into two equal parts.

Cerebral palsy (CP)
This is a life-long condition that causes uncoordinated movements and spasticity (tight muscles of the limbs). It is caused by lack of oxygen to the brain. This damages the part of the brain that controls movement.

Choreoathetosis
This describes episodes of involuntary jerking movements combined with movements that are often irregular and twisting. These movements can flow from one body part to another. They commonly affect the arms, legs, trunk, and the muscles of the face.

Chorionic Villus Sampling (CVS)
This is a special test done early in pregnancy. During CVS, a small sample of the placenta is removed for testing. This sample can be used to test for certain genetic disorders in the fetus. CVS is usually done between 10 and 12 weeks of pregnancy.

Chromosome
A strand of DNA contained within a cell. Each chromosome contains many thousands of genes. In humans, there are a total of 46 chromosomes, half of which come from each parent. The combined total of all chromosomes in a cell is the genome.

Chronic
Any condition that lasts for a long period of time or occurs frequently. For example, diabetes and hemochromatosis are both chronic conditions.

Chemotherapy
This describes treating a disease by using chemicals or drugs.

Cirrhosis
This is scarring of the liver. It can result from serious liver problems. It has many causes. Certain metabolic disorders, alcoholism, some medications and some viruses can lead to cirrhosis.

Citrulline
This is an amino acid made by the body. It does not usually need to be eaten in the diet. It helps the body get rid of the ammonia, a harmful substance. People with citrullinemia have trouble clearing this amino acid from the body.  

Clinical trial
Clinical trials test the effectiveness of new drugs or treatments. These studies determine how well the treatments work, what risks are associated with them, and whether they work better than the current treatment.

Clitoris
This is a small organ located under a fold of skin at the front of the labia in females. It is made of a small amount of erectile tissue that allows sexual pleasure during stimulation.

Close relative
A parent, sibling, child, aunt, uncle, niece, nephew, grandparent, or grandchild are all considered close relatives.

Coma
This is a sleep-like condition from which a person cannot be awakened. People in a coma are unconscious. A coma can be caused by many things. Two causes are untreated low blood sugar (hypoglycemia) and a metabolic crisis.

Computed tomography (CT)
This is a computerized x-ray machine used to take a series of pictures of areas inside the body. It allows doctors to get a detailed view of certain organs and bones of the body.

Congenital
This means ‘present at or before birth.’ It usually refers to health conditions or birth defects that are present in a baby at or prior to birth.

Cortisol
This is the hormone made in the adrenal glands that helps keep the amount of glucose (sugar) in the blood at a normal level.  It also helps protect the body from chronic stress by reducing inflammation and immune system reactions.

Cortisone
This is an artificial steroid medication similar to cortisol. It is used to treat children with congenital adrenal hyperplasia (CAH). It replaces the cortisol their own bodies are not making. 

Cushing syndrome
This is a condition that occurs when your body is exposed to high levels of the hormone cortisol for a prolonged period of time. Signs of Cushing syndrome include: red or purple stretch marks on the skin, rounded face, fatty hump on the back, high blood pressure and bone loss.  Most people with Cushing syndrome develop the symptoms from taking too much cortisone medication.  The symptoms go away once the medication dosage is corrected. 

Cyanocobalamin (CN-cbl)
This is a form of vitamin B12 that can be given orally or by injection. This treatment increases the amount of vitamin B12 in the body. This is an important treatment for children with some forms of methylmalonic acidemia.

Cystine (cys) (or L-cystine)
This is one of 20 amino acids that make up protein. It is also called "cys." It is made by the body and does not need to be eaten in the diet. It may be part of a special medical formula given to people with homocystinuria or certain other metabolic disorders. It can also be given as a supplement in pill form.

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Dehydration
Dehydration occurs when a person loses more water than they take in. Symptoms of dehydration are vague and include thirst, lightheadedness and diarrhea. Extreme dehydration requires hospitalization. In people with sickle cell disease, dehydration can trigger a crisis. Dehydration can be prevented by drinking plenty of water, especially when the individual is ill, in hot temperatures or participating in physical activity.

Deoxyribonucleic acid
Deoxyribonucleic acid (DNA) is a molecule found in the chromosomes that carries genetic information. DNA is composed of four units (called bases) that are designated A, T, G, and C. The sequence of the bases spell out instructions for making all of the proteins needed by an organism. A gene is a section of DNA that holds the instructions for a specific protein. A change in one or more of the DNA bases making up a gene is called a mutation. Some mutations change the protein instructions and can lead to particular health problems or disorders. Each parent passes half of their chromosomes, and thus half of their DNA instructions, onto their children. It is these instructions that cause certain traits, such as eye or hair color, to be inherited.

Dexamethasone
This is a synthetic type of steroid hormone similar to cortisol but more potent. One of its uses is for prenatal treatment of congenital adrenal hyperplasia in female fetuses. It may be given to a mother during pregnancy if her fetus is female and affected with congenital adrenal hyperplasia (CAH).  It may reduce some of the physical changes to the genitals that occur with CAH, but may have other risks; thus, it is important to discuss potential treatment with this drug with your physician.   

Dialysis
This is a way to clean the blood when the kidneys aren't working properly. Dialysis is done with a machine that acts as an "artificial kidney." It filters the blood and removes the wastes.

Dietician
This is a person with special training in food and nutrition. A registered dietician (RD) has met the necessary educational requirements and has passed a national examination. If your child has a metabolic disorder, it is important for you to work with a registered dietician. They help create healthy food plans for people with metabolic disorders or other special needs.

Distant Relative
A relative who is not closely related to you. Distant relatives do not include parents, siblings, children, aunts, uncles, nieces, nephews, grandparents, and grandchildren. Great-aunts, cousins, and second cousins are examples of distant relatives.

DNA
Deoxyribonucleic acid (DNA) is a molecule found in the chromosomes that carries genetic information. DNA is composed of four units (called bases) that are designated A, T, G, and C. The sequence of the bases spell out instructions for making all of the proteins needed by an organism. A gene is a section of DNA that holds the instructions for a specific protein. A change in one or more of the DNA bases making up a gene is called a mutation. Some mutations change the protein instructions and can lead to particular health problems or disorders. Each parent passes half of their chromosomes, and thus half of their DNA instructions, onto their children. It is these instructions that cause certain traits, such as eye or hair color, to be inherited.

Docosahexanoic acid (DHA)
This is one type of "omega-3" fatty acid. DHA can be bought in capsule form as a supplement. These supplements may help prevent or lessen the loss of vision caused by LCHADD. DHA is found in small amounts in foods such as fish (tuna, sardines, salmon), red meat, eggs and some vegetable oils.  

Dystonia
This is a condition that causes muscle spasms. These spasms can force the body into awkward and sometimes painful positions. It commonly affects the arms and the legs but can occur in any part of the body. People with dystonia also have episodes of involuntary jerking movements.

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Eczema
This is a skin condition that causes an itchy red rash. It also leads to rough, scaly skin or blisters. It is usually located on the face, elbows, knees and arms. Two of the many causes are allergies or certain inherited metabolic disorders.   

Endocrinologist:
This is a doctor with special training in endocrine conditions –disorders caused by problems with one or more endocrine glands and their hormones.  Some of the endocrine glands in the body include the adrenal glands above the kidneys, the pituitary glands, the pancreatic islets, and the thyroid glands.  Some endocrine conditions include diabetes, hypothyroidism and congenital adrenal hyperplasia. 

Enzyme
A molecule that helps chemical reactions take place. For example, enzymes in the stomach speed up the process of breaking down food. Each enzyme can participate in many chemical reactions without changing or being used up.

Enzyme Replacement Therapy
A type of therapy that can be used in some metabolic disorders. In people who have certain enzymes either missing or not working correctly, an IV infusion that contains the enzyme can be given. Enzyme replacement therapy is not a cure, but can help with the health problems and symptoms of the disorder.

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False negative
A test result indicating that a person does not have a mutation or change predisposing them to a disease when, in fact, they do have this mutation or disease.

False positive
A test result indicating that a person has a mutation or a disease when, in fact, they do not have a mutation or disease.

Family medical history
Information about the medical conditions that affect your family members. Doctors can use your family medical history to determine what disease you may be at higher risk for developing.

Fat
Fat is one of the three main nutrients in food (carbohydrates, fat, protein). Some foods that contain fat are butter, margarine, oils, nuts, meats, poultry, fish and some dairy products. Extra calories from food are stored as body fat. Stored body fat provides the body with a reserve supply of energy.  

Fatigue
Fatigue is a feeling of weariness, tiredness, or lack of energy. Fatigue can be a normal and important response to physical exertion, emotional stress, or lack of sleep. However, it can also be a sign of a more serious health disorder.

Fatty acids
These are the building blocks of fat. Fat from food is broken down by enzymes into fatty acids. These are changed by other enzymes and are used as energy or stored as body fat. Some types of fatty acids are: short-chain, medium-chain, long-chain, and very-long chain.  

Fatty acid oxidation
This is the process used by the body to change fatty acids, the building blocks of fat, into energy. It happens in the mitochondria - the small energy-producing parts found in each cell of the body. 

Fatty acid oxidation disorders (FAODs)
This is a group of rare inherited conditions. FAODs are caused by missing or non-working enzymes. Normally, many enzymes are used by the body to break down fat into energy. In people with FAODs, one of these enzymes is not working and fat cannot be used for energy. If these conditions are not treated, they can lead to serious health problems.

Fatty acid oxidation probe
This is a special test done on a skin sample. It can be used to diagnose certain fatty acid oxidation disorders (FAODs). It is sometimes used to test for long-chain fatty acid disorders such as LCHADD and VLCADD. It can also be used to test for these disorders during pregnancy.

Ferritin
Ferritin is a protein that is normally found inside the cells of the body. The ferritin test measures how much iron is stored within the cells. The ferritin test shows whether a person has too much or too little iron in his or her cells.

Fetal blood sampling
This is a special test done during pregnancy. A needle is used to remove a small amount of blood from the umbilical cord of the fetus. This blood sample can be used to test for certain genetic disorders in the fetus. Fetal blood sampling, also called PUBS, is usually done between 18 and 23 weeks of pregnancy.  

First degree relative
A relative who is a person's biological mother, father, full sibling, or child. Step parents, step children, or half siblings are not first degree relatives.

Folic Acid
Folic acid is a form of vitamin B9. Folic acid is required to make healthy red blood cells and to prevent anemia. Folic acid can be found in certain foods and it can also be bought in pill form. It is recommended that people with Hemoglobin H take folic acid.

Fontanel
These are the areas on the top middle of a baby’s skull that are covered by membranes and skin but not yet by bone. They are also called the ‘soft spots.’ The anterior fontanel is located in the front of the skull just above the forehead. It is the largest soft spot and should be closed by about 18 months. Babies with untreated congenital hypothyroidism may have very large fontanels that are delayed in closing.

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Galactose
This is one of the types of sugar molecules that make up lactose, the main type of sugar in milk and all products that contain milk.  Galactose is found in milk and all milk products such as cottage cheese, yogurt, ice cream, buttermilk, and cheese.  Galactose is also in sugar beets and a few other fruits and vegetables.  It is also found in fillers used in some packaged foods and some medications. 

The milk sugar lactose is broken down in the digestive tract into two smaller sugars, galactose and glucose.  Galactose is then changed into glucose so it can be used as fuel for the body.  Galactose is also made by the body. 

Gallstones
Gallstones are hardened balls of digestive fluid that form in the gallbladder. The gallbladder is the organ that releases digestive fluid during digestion. Gallstones can be as small as a grain of sand or as large as a golf ball. They cause no symptoms in some people and many complications in others. Signs of gallstones include yellowing of the eyes and pain in the upper right abdomen.

Gene
A segment of DNA that contains the instructions to make a specific protein (or part of a protein). Genes are contained on chromosomes. Chromosomes, and the genes on those chromosomes, are passed on from parent to child. Errors in the DNA that make up a gene are called mutations and can lead to diseases.

Gene therapy
Gene therapy is an experimental technique that treats diseases using genes. There are many ways that gene therapy can be done. Sometimes a gene with a disease-causing mutation can be replaced with a healthy gene. Sometimes a gene with a disease-causing mutation can be inactivated or “silenced.” And sometimes a new gene can be put into the body to help fight a disease. Gene therapy is still experimental.

Genetic
Relating to (or due to) genes and heredity or the field of studying genes and heredity.

Genetic code
The genetic code determines how the sequence of bases in a gene code for the sequence of amino acids in a protein. A gene is made of bases that are designated A, T, G, and C. Each series of three bases is essentially a word that codes for one of the 20 amino acids that make up all proteins. For example, the sequence AAG within a gene tells a cell to insert the amino acid lysine into a growing protein.

Genetic counseling
Genetic counseling gives patients and their families' education and information about genetic-related conditions. It can help families make informed decisions. It is often provided by Genetic counselors or Medical geneticists who have special training in inherited disorders.

Genetic counselor
These are health care providers who have special training in genetic conditions. They help families understand genetic disorders and how they are passed down. Genetic counselors offer information and support to people who have genetic conditions in their families or are concerned that they may have a child with an inherited disorder.

Genetic map
A map of where genes are located relative to each other on the chromosome. Genetic maps are also called linkage maps.

Genetic marker
An identifiable region of DNA that is at a known location on a chromosome. Genetic markers are used to help scientists locate where a gene is on a chromosome. Scientists who are looking for genes that cause a specific disease or trait can look at the DNA of family members with the disease and compare it with the DNA of family members without the disease. If they find that certain genetic markers are present in the group with the disease, then the scientists can narrow their search to the area surrounding that marker.

Geneticist
A person who specializes in genetics.

Genitals
This is a term use to describe the external sexual organs in both males and females.  Male genitals include the penis and scrotum; female genitals include the vagina, clitoris and labia. 

Genome
All of the genetic material (DNA) contained in a full set of chromosomes in an organism. In humans, about three billion base pairs make up our genome.

Genomic DNA
The DNA that is found in the nucleus of a cell. Most of our genes are contained in the genomic DNA. However, some genes are contained within tiny organs called mitochondria that are within cells. This is called mitochondrial DNA.

Glaucoma
This is a serious eye condition. It causes increased pressure of the eyeball. If it is not treated, it can damage the nerve to the eye and cause blindness. If detected and treated in time, blindness can be prevented. Glaucoma is the second leading cause of blindness in the United States.

Growth Plates
The areas at the end of the long bones of the body that stay open during childhood to allow further growth. The cells of the growth plate divide when stimulated by growth hormone. After the growth plates close at the end of puberty, the bones cannot grow any longer. A person whose growth plates have closed has reached his or her final adult height.

Glucose
This is a type of sugar made from the carbohydrates in food. Glucose is found in the blood. It is the main source of energy for the body and brain.  

Glutaric acid
This is a harmful waste product made when certain amino acids (lysine, tryptophan and hydroxylysine) are broken down in the body.

Glycine (gly)
This is one of 20 amino acids that make up protein. It has many functions, one of which is helping cells create energy for the body. It is made by the body and does not need to be eaten in the diet. It is also available as a supplement. People with isovaleric acidemia (IVA) are often given glycine to help prevent health problems.

Glycogen
Glycogen is the storage form of glucose. When the body does not need to use all of its available glucose, it stores it mainly in the liver and muscles as glycogen. Glycogen is made up of many connected molecules of glucose.

Glycogen Storage Disorder
These are a group of rare inherited disorders. People with glycogen storage disorders have problems either with the buildup or breakdown of a sugar called glycogen. When the enzymes involved in these processes are missing or not working properly, glycogen can build up in the body and cause a variety of health problems.

Goiter
This is an enlarged thyroid gland. It shows up as a swelling in the front of the neck. It can occur with different types of thyroid disease. Both hypothyroidism (low thyroid hormone) and hyperthyroidism (excess thyroid hormone) can cause goiters. They are also common in people with iodine deficiency. The enlarged thyroid will return to normal size once the underlying thyroid condition is treated.

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Hematologist
A hematologist is a doctor who specializes in treating patients with blood disorders. Children with inherited blood conditions may have frequent check-ups with a hematologist.

Hemoglobin
Hemoglobin is the protein in red blood cells that carries oxygen from the lungs throughout the body. In each red blood cell, there are hundreds of hemoglobin molecules. Hemoglobin molecules are made of two subunits, alpha and beta. Both the alpha and beta subunits are needed for hemoglobin to function correctly. When there is a change in the way hemoglobin is made, the red blood cells cannot deliver oxygen as efficiently.

Hereditary
Something that is passed on from a parent to the child, usually through the genes.

Homocystine (hcy)
This is a type of amino acid made by the body. High levels can occur in people with homocystinuria or certain other metabolic disorders.

Hormone
Hormones include many different types of chemicals that act as messengers around the body.  Hormones are made by specific endocrine glands and are secreted into the blood when needed by other parts of the body.  Hormones travel to other organs and tissues in the body to signal them to do something.  For example, insulin is a hormone that signals muscle and fat cells to remove glucose from the blood and enter the cells. 

Hydrops fetalis
Hydrops fetalis is the accumulation of fluid in two or more body parts of a developing fetus. Hydrops fetalis can be detected on ultrasound. Developing fetuses with hydrops fetalis are at risk for serious complications.

Hydrocortisone
This is the synthetic version of cortisol.  It can be taken in pill form or by injection.  It is a medication needed by people with CAH throughout life.  It is also called ‘cortisone.’ 

Hydroxocobalamin
This is a form of vitamin B12 that is given by injection. These injections increase the amount of vitamin B12 in the body. This is an important treatment for children with some forms of methylmalonic acidemia.

Hydroxylysine
This is an amino acid made by the body. When it is broken down, it makes glutaric acid.  

Hyperactivity
This refers to children who are unusually overactive. These children have serious problems sitting still. They tend to fidget and run around much more than usual. Some children with hyperactivity need treatment. They may be helped by behavior treatment, medication or both of these therapies.

Hyperplasia
This is an abnormal increase in the number of cells in a particular organ or tissue. It causes the organ or tissue to become larger than normal. 

Hypertonia
This is an increase in muscle tone that leads to tight muscles. It also causes stiff, tight joints. It can be caused by injury, certain diseases or genetic disorders.  

Hypoglycemia
This happens when there is too little sugar (glucose) in the blood. It may cause a person to feel nervous, shaky, weak, or sweaty. It may also cause headache, blurred vision, and hunger. If left untreated, it can cause unconsciousness (coma) and sometimes even death.

Hypotonia
This is a decrease in muscle tone that leads to loose or weak muscles and joints. Babies with hypotonia may have limbs and joints that feel "floppy" like a rag-doll. It can be caused by brain damage, muscle disorders or genetic conditions.  

Hypothyroidism
This is a condition in which the person does not make enough thyroid hormone. It is most common in adult women and causes symptoms such as cold, pale, dry skin, brittle hair and nails, fatigue, weight gain and sluggishness. When the thyroid is not making enough hormone at birth, it is called congenital hypothyroidism. Hypothyroidism is treated by a medication that replaces the natural thyroid hormone the person cannot make on his or her own.

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Informed consent
Voluntarily agreeing to do something after receiving and understanding all of the relevant information. Participation in all medical trials requires informed consent.

Inherit
Acquiring a trait from one's parents. Most traits, such as eye color or hair color, are inherited from a parent through genes.

Insulin
This is a hormone made in the pancreas. It controls the level of glucose (sugar) in the blood.  

Iodine
This is an element needed in order for the body to make thyroid hormone. It is found in certain foods such as shellfish, seafood and seaweed. Without enough iodine in the diet, a person will develop hypothyroidism and possibly a goiter. In the United States, iodine deficiency is rare because iodine is added to table salt (iodized salt).

Iron
Iron is a natural element that is needed for the body to function properly. Iron can be found in many foods. Having a high amount of iron can cause damage to organs. Talk with your doctor about the risks and benefits if your child takes iron supplements or has blood transfusions.

Isoleucine (ile)
This is one of 20 amino acids that make up protein. It is not made by the body and must be eaten in the diet. It is found in all foods that contain protein.  

IV (Intravenous)
This is a small tube placed into a vein, usually in the hand or arm. Medications and nutrients can be given through this tube directly into the blood. IV's are often used in the hospital to give fluids, glucose and medication over a long period of time.

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Jaundice
This is a yellow color to the skin and whites of the eyes. It is often a sign of liver damage.

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Karyotype
An arranged picture of the chromosomes from a single cell. The chromosomes are arranged in order (by numbered pair, size and shape) from a picture of chromosomes taken under a microscope. A karyotype allows cytogeneticists (scientists who study chromosomes) to see whether an individual has any extra or missing genetic material, or any rearrangements that are large enough to be seen.

Ketones
These are substances made when the body breaks down fat (fatty acids) for energy. In people with certain metabolic disorders or diabetes, ketones can build up in the blood and spill over into the urine. If ketones build up in the blood, they can cause acidosis which can lead to serious health problems.

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L-thyroxine (levothyroxine)
This is the most common synthetic form of thyroxine (thyroid hormone). It is given in pill form to children or adults who have hypothyroidism. It is important that the pills be taken on a daily basis. Stopping this medication causes the symptoms of hypothyroidism to return.

Labia
These are the two pair of skin folds that surround the vagina in females.  The outer pair of folds is called the labia majora.  The inner pair of folds is called the labia minora.   

Lens dislocation
This is a condition in which the lens of the eye becomes loose and moves out of place. If it is not treated, it can lead to a serious eye condition called glaucoma. This can lead to blindness if left untreated.  

Leucine (leu)
This is one of 20 amino acids that make up protein. It is not made by the body and must be eaten in the diet. It is found in all foods that contain protein.

Long-chain fatty acids (LCFA)
This is one of many types of fatty acids, the building blocks of fat. Long-chain fatty acids are made up of chains of 12 or more carbon atoms.  

Lysine (lys)
This is one of 20 amino acids that make up protein. It is not made by the body and must be eaten in the diet. It is found in all foods that contain protein.

Lysosomal Storage Disorder
These are a group of rare inherited disorders. People with lysosomal storage disorders have problems with breaking down large molecules within the lysosome. When the enzymes inside lysosomes are missing or not working properly, there can be a buildup of these molecules which causes a variety of health problems.

Lysosome
Lysosomes are like recycling centers for cells. They are small sacs filled with many enzymes that help break down large molecules into smaller molecules that the body can re-use.

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Magnetic Resonance Imaging (MRI)
This test uses magnetic waves to make pictures of the body. It can sometimes be used instead of x-rays to look at areas inside the body. It is often used to look at the brain or other inner organs.  

Maternal
Having to do with the mother.

Maternal PKU syndrome
This is a group of birth defects found in some babies born to women who have PKU. Women with PKU who are not treated before and throughout pregnancy have a high chance to have babies with one or more of these health problems. Symptoms of maternal PKU can include microcephaly (heads and brains that are too small), intellectual disabilities, heart defects, and low birth weight.  

Medical geneticist
A doctor who treats and diagnoses genetic conditions.

Medical release form
A form that gives permission to get a person's medical records. Medical release forms have to be signed by the person whose records you wish to access or by a close relative if that person is deceased.

Medium-chain fatty acids (MCFA)
This is one of many types of fatty acids, the building blocks of fat. Medium-chain fatty acids are made up of chains of 4 to 12 carbon atoms.  

Medium chain triglyceride (MCT) oil
This is a special type of oil made up of medium chain triglycerides - a type of fatty acid. MCT oil can be used for energy by people with long chain fatty acid oxidation disorders (LCHADD and VLCADD). A doctor's prescription is needed to get MCT oil. It should be used only under the guidance of a registered dietician.

Metabolic acidosis
This happens when acidic substances build up in the blood. It can cause rapid breathing, confusion, and extreme sleepiness. If not treated, it may lead to shock, coma or death.  

Metabolic crisis
This is a serious health condition caused by low blood sugar and the build-up of toxic substances in the blood. Symptoms of a metabolic crisis are poor appetite, nausea, vomiting, diarrhea, extreme sleepiness, irritable mood and behavior changes. If not treated, breathing problems, seizures, coma, and sometimes even death can occur.

Metabolic crises happen more often in people with certain metabolic disorders (some fatty acid oxidation disorders, amino acid disorders, and organic acid disorders). They are often triggered by things like illness or infection, going without food for a long time, and, in some cases, heavy exercise.  

Metabolic disorder
People with metabolic disorders are not able to break down certain substances in food. Too much or too little of a certain substance (like fat, carbohydrate or protein) can cause serious health problems. Fatty acid oxidation disorders (FAODs), amino acid disorders (AAs) and organic acid disorders (OAs) are three types of metabolic disorders. 

Metabolic doctor
This is a doctor or medical geneticist who has special training in diagnosing and treating metabolic disorders. They often work in university hospitals or large medical centers.  

Metabolic Genetic Specialist
In the info sheet, link this to entry for Metabolic Doctor

Metabolism
This is the process of using food substances for energy and for growth and repair of the body.  

Methionine (met)
This is one of 20
amino acids that make up protein. It is not made by the body and must be eaten in the diet. It is found in all foods that contain protein.

Methylmalonic acid
This is a substance made when the body digests food containing protein and fat. Normally, it is quickly changed by the body into other substances. If the body is missing an enzyme needed to break down methylmalonic acid, it builds up in the blood and causes illness. Excess methylmalonic acid is found in the blood of people with methylmalonic acidemia.

Microcephaly
This is a head size that is too small for a child's age. It is usually caused by lack of brain growth. It can happen for many reasons. Many children with microcephaly have intellectual disabilities.  

Mitochondria
These are the parts within each cell that make energy for the body.

Muscle tone
In order to hold our posture and control our movements, a certain amount of tension is needed in our muscles. This tension is called "muscle tone." People with hypotonia have too little tension in their muscles and appear "floppy." People with hypertonia have too much tension in their muscles and have problems with tight, rigid muscles and joints.

Mutation
A change or alteration that occurs in the DNA. Mutations can be caused by the environment (sun, radiation, or chemicals), aging, or chance. Often the causes of mutations are never known. Some mutations do not affect the information contained in the DNA. Other mutations have serious consequences on how that gene functions.

Myxedema
This is a condition seen in people with hypothyroidism. It causes swelling of the skin and an enlarged tongue. People with myxedema develop thick and puffy eyes, cheeks, lips and nose. Dry skin also occurs. Myxedema symptoms go away once the hypothyroidism is treated.

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Neurologic crises
These are episodes of weakness, pain, or numbness caused by nerves that are not working correctly. They often affect the arms and legs but can happen in any part of the body. Breathing problems and rapid heartbeat sometimes also occur. These episodes are more common in people who have tyrosinemia or certain other metabolic disorders. Medication can often stop or prevent these crises.

Newborn Metabolic Screening
A screening test that looks for different metabolic disorders using a small sample of blood taken from a newborn’s heel. A positive or abnormal newborn metabolic screening result means that slight differences were found in the baby’s blood, and further testing is needed to figure out if the baby has a metabolic disorder.

Newborn Screening
A screening test that looks for different disorders using a small sample of blood taken from a newborn’s heel. A positive or abnormal newborn screening result means that there are slight differences that were found in the baby’s blood, and further testing is needed to figure out if the baby has a metabolic disorder.

Nitisinone
This is the medication used to treat tyrosinemia. The brand name of this drug is Orfadin. It was previously called NTBC. This medication helps improve liver and kidney function in people with tyrosinemia.  

Nitrogen
This is an element that makes up part of protein. It is released when protein is broken down in the body. The body changes it into ammonia (one nitrogen molecule combined with three hydrogens). Since ammonia is harmful to the body it is usually changed to urea. Urea is a non-toxic substance that is removed from the body in the urine.  

Nodular cirrhosis
This is a rare type of scarring of the liver. It can occur in people with tyrosinemia. It often gets worse over time and can lead to serious liver problems. If not treated, it can cause liver failure, liver cancer, and death.

Nonsymptomatic
Not showing any of the signs of the condition or disease.

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Organic acid disorders (OA)
This is a group of rare inherited conditions. OA disorders are caused by one or more enzymes that do not work properly. People with these conditions cannot digest certain parts of protein from the food they eat. This causes harmful substances to build up in their blood and urine. This can cause serious effects on health, growth, and learning.

Organic acids
These are substances in the cells of the body that help break down fat, sugar and protein from the food we eat. Babies with organic acid disorders often have excess amounts of specific organic acids in their blood and urine. This can cause serious effects on health, growth, and learning.

Osteoporosis
This is a condition that causes the bones to become thinner over time. People with this condition have a higher chance for bone fractures.

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Pancreatitis
This happens when the pancreas, an organ near the stomach, becomes inflamed. It causes severe abdominal pain. It can be caused by some metabolic disorders, gallstones or too much alcohol.

Pedigree
A medical drawing that includes all of a person's close relatives, the relationship between family members, and health information. A pedigree is used by health care professionals to analyze a family for genetic diseases.

Penicillin
Penicillin is a prescription medication that works to prevent infections. Children with sickle cell disease take penicillin daily. Talk with your doctor about the risks and benefits of taking penicillin.

Phenylalanine (Phe)
This is one of 20 amino acids that make up protein. It is also called "Phe." It is not made by the body and must be eaten in the diet. It is found in all foods that contain protein.  

Platelets
This is a type of blood cell that helps to clot the blood. Platelets help prevent or control bleeding.

Polycystic ovary syndrome (PCOS)
This is a condition that occurs in women who make too much of the androgen (male-like) hormones.  It occurs in about 5 to 10% of all women.  Some of the features of PCOS include: multiple cysts on the ovaries, excess body hair, severe acne, abdominal obesity, abnormal or lack of periods, and infertility.  Women with PCOS are also at increased risk for type 2 diabetes.   

Polymorphism
A change in the DNA sequence (mutation) that is present in at least 1 percent of the population and is not considered harmful. One example of a polymorphism is in the hair color gene. Slight changes in the DNA sequences make the hair blond or brown.

Premature ovarian failure (POF)
This is a condition in which the ovaries stop working too early (earlier than age 40).  This causes the menstrual periods to stop.  It is found only in about 1% of all women.  It is sometimes caused by diabetes, other hormone disorders, autoimmune disorders, and can be a result of cancer treatments or other medications.  It often occurs in women with galactosemia.  Symptoms include hot flashes, mood swings, night sweats, vaginal dryness and infertility. 

Propionic acid
This is one type of fatty acid. Large amounts build up in the blood of people with a metabolic disorder called propionic acidemia. If not treated, this condition can cause serious health problems.

Protein
A molecule that makes up many parts of every cell in the body. Examples of proteins include hormones, enzymes, hair, and antibodies. Proteins are made up of 20 different types of individual units called amino acids. It is the order of these amino acids in a protein that determines what form and function a protein has. Each gene holds the instructions for making a single protein.

Pschosis
This is a mental disorder in which the person is not in contact with reality. It can include sensing things that aren't really there (hallucinations) or having beliefs that aren't based on reality (delusions).

Pseudodeficiency
A pseudodeficiency mutation causes less of an enzyme to be produced, but does not actually cause symptoms of the corresponding disease itself. Pseudodeficiency mutations can cause false positive screening results.

Psychiatric disorder
This is another term for mental illness. It can cause severe problems in a person's thinking and emotions.

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Red blood cells
Red blood cells are the major component in blood that delivers oxygen from the lungs to the body tissues.

Riboflavin
This is a one type of B vitamin (vitamin B2). It helps change carbohydrates, protein, and fat into energy for the body. Some foods high in riboflavin are dairy products, yogurt, cheese, meats, poultry, whole and enriched grains, and green vegetables. Some children with metabolic disorders may be helped by taking riboflavin supplements.

Rickets
This is a condition that causes soft and weak bones. It is often caused by a lack of vitamin D. People with liver or kidney disease have a higher chance of developing rickets.

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Screening
The process of testing for disease in a person who does not show signs of having the disease (nonsymptomatic or asymptomatic person). The goal of screening is to catch the disease in its early stages.

Shock
This is a type of blood cell that helps to clot the blood. Platelets help prevent or control bleeding.

Scrotum
This is the loose pouch of thin wrinkled skin that holds the testicles in males.  It is located just below the penis.

Sequencing
The process of determining the order of the bases in a region of DNA.

Short-chain fatty acids (SCFA)
This is one of many types of fatty acids, the building blocks of fat. Short chain fatty acids are made up of chains of two to six carbon atoms.

Sibling
A person who shares the same mother or father. A brother or sister who shares both parents is called a full sibling. A brother or sister who only shares one parent is called a half sibling.

Sickle cell crisis
A sickle cell crisis is an episode of pain that occurs in people with sickle cell disease when they do not have enough oxygen in their tissues.

Sickle cell trait
People with sickle cell trait have one working copy of the sickle cell gene and one non-working copy. Having sickle cell trait does not cause a person to be sick. A person with sickle cell trait can pass it on to his/her children and may be at risk to have a child with sickle cell disease. About 1/12 African Americans has sickle cell trait. Talk with your doctor about testing for sickle cell trait.

Spasticity
This is rigidity of the muscles and increased reflexes. It is caused by increased muscle tone. It results in abnormal tightness or stiffness of the muscles and joints.

Spleen
This is an organ located behind the stomach. It gets rid of old blood cells and helps fight infection. The spleen can enlarge in people with cirrhosis or with certain metabolic disorders.

Sporadic
Occurring occasionally or randomly. In medical terms, a sporadic disease is one in which the disease occurs in people with no family history and no inherited cause.

Stroke
This is when the blood or oxygen supply to part of the brain is stopped. It can be caused either by a blood clot or a leak in a blood vessel. A stroke may cause loss of speech and language and the ability to move certain body parts. If severe, it can cause death.  

Succinylacetone
This is a harmful substance made from an amino acid called tyrosine. People with tyrosinemia can develop very high levels of this substance in their blood. If not treated, it can cause liver and kidney damage. It can also cause serious nerve problems.

Surveillance
The process of monitoring or checking for signs or symptoms of a disease.

Susceptibility
The likelihood that a person will develop a condition or disease.

Syndrome
A group of symptoms and clinical findings that, when found together, make up a particular condition or disease.

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Tandem Mass Spectrometry
A technology that may be used in newborn metabolic screening to test newborns for many different metabolic disorders. To do this, an instrument called a tandem mass spectrometer measures the level of certain compounds in the newborn’s blood. Too much or too little of these compounds could mean that the newborn has an inherited metabolic disorder.

Thalassemias
Thalassemias are a group of inherited blood conditions caused by decreased production of hemoglobin.

Thiamine
This is one type of B vitamin. Thiamine is found in grains, pork, beans, seeds and nuts. It plays a key role in changing food into energy.    

Threonine (thr)
This is one of 20 amino acids that make up protein. It is not made by the body and must be eaten in the diet. It is found in all foods that contain protein.

Truncated
To be cut short or terminated early. In the case of a truncated protein, only the first part of the protein is made.

Tryptophan (trp)
This is one of 20 amino acids that make up protein. It is not made by the body and must be eaten in the diet. It is found in all foods that contain protein.

Thyroid stimulating hormone (TSH)
This is a hormone made by the pituitary gland located at the base of the brain. It is released when there is too little thyroid hormone (thyroxine) in the blood. Its job is to stimulate the thyroid gland to release more thyroid hormone. People with hypothyroidism do not make enough thyroid hormone so the pituitary releases more TSH in order to try to increase the amount of thyroid hormone released. Testing for the level of both thyroxine (T4) which is low in people with hypothyroidism, and the level of TSH, which is high in people with hypothyroidism, helps doctors make the diagnosis.

Tyrosine (tyr)
This is one of 20 amino acids that make up protein. It is made by the body and does not need to be eaten in the diet.

Thyroid uptake and scan
This is an imaging test that looks at the structure and function of the thyroid gland. It can show whether the thyroid is in the normal place, has a normal size and whether it is working correctly. The scan is done by injecting a small amount of radioactive material into a vein or having the patient swallow it by mouth. The material collects in the thyroid gland so an imaging machine can create a picture of the thyroid.

Thyroxine
Thyroxine, also called T4, is the major thyroid hormone made by the thyroid gland. One of the essential components of thyroxine is iodine. This is why iodine deficiency results in hypothyroidism and goiter.

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Ulcer
An ulcer is an open break in the skin that does not heal. Some children with sickle cell disease develop ulcers on the lower leg.

Umbilical hernia
This is an out-pouching of skin over the navel. Babies with untreated congenital hypothyroidism often have umbilical hernias. It can also be the result of a birth defect or may occur as a result of one of a number of other medical conditions.

Urea
This is a harmless waste product made by the body. Its job is to get rid of ammonia, a harmful substance made when protein is broken down for use by the body. The kidneys flush urea from the body in the urine.

Urea cycle disorder (UCD)
These are inherited conditions that cause ammonia to build up in the body. Ammonia is a harmful waste product made during the breakdown of protein and its amino acid building blocks.

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Valine (val)
This is one of 20 amino acids that make up protein. It is not made by the body and must be eaten in the diet. It is found in all foods that contain protein.

Very long-chain fatty acids
This is one of many types of fatty acids, the building blocks of fat. Very long-chain fatty acids are made up of chains of 21 to 32 carbon atoms.  

Virilization
This is a process in which male sex characteristics occur in a female or at an earlier age than normal in a boy.  Some traits that occur with virilization include: increased body hair, facial hair, deepening of the voice, and male-pattern baldness (hair loss at the temples).  Girls with virilization may have enlargement of the clitoris and fusion of the folds of the labia that make the labia resemble a scrotum. 

Vitamin B6
This is a type of B vitamin. It is also called pyridoxine. It helps in the breakdown and use of food for energy. It is found in beans, meat, fortified cereal, and some fruits and vegetables. It can also be bought in pill form. Vitamin B6 supplements are sometimes given to people with homocystinuria.

Vitamin B12
This is a type of B vitamin. It is also called cobalamin. It helps in the breakdown and use of food for energy. It also helps to make red blood cells and keep the nervous system healthy. It is found in meat, dairy products, and eggs. It can also be given by injection to people who have low levels or who have certain metabolic disorders.  

Vitamin D
This vitamin helps make strong bones and teeth. It also helps prevent a bone condition called "rickets." Your skin makes vitamin D when you go out in the sun. It is also found in fortified milk and cereals, egg yolks, and fatty fish.

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Wild type
The version of the gene that is found most common in nature. The typical version of a gene.

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X chromosome
One of the two chromosomes that are responsible for determining the sex of an organism. The other sex chromosome is called the Y chromosome . Both the X and the Y chromosome contain several genes, only some of which are involved in determining sex. In humans, the X chromosome is the larger of the two sex chromosomes. Females usually have two X chromosomes and males usually have one X chromosome and one Y chromosome.

X-Linked
A mode of inheritance. X-linked genes are found on the X chromosome. They have a different inheritance pattern than other genes because women have two X chromosomes while men only have one. Any mutation on the X chromosome may not cause a disease in women if the gene on the other chromosome is normal. However, that same mutation on one X chromosome in men will cause the disease because they have no second copy of the gene to compensate.

X-linked recessive
This is a pattern of inheritance in which a gene for a particular trait or disorder is located on the X chromosome. Genes that are ‘recessive’ cause traits or conditions only when they are paired with a dominant gene. Genes usually come in pairs, except on the sex chromosomes in males. Males have one X chromosome that they inherit from their mothers and one Y chromosome that they inherit from their fathers. If a gene on the X chromosome causes a particular trait or disorder, a male will always show that trait or condition as they do not have another gene to cover up its effects. Females usually have a normal copy of that gene on their other X chromosome which covers up the effects of the recessive gene. Some common X-linked recessive disorders include hemophilia, Duchenne muscular dystrophy and color blindness.

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Y chromosome
One of the two sex chromosomes that is responsible for determining the sex of an organism. The other sex chromosome is called the X chromosome. Both the X and the Y chromosome contain several genes, only some of which are involved in determining sex. In humans, the Y chromosome is the smaller of the two sex chromosomes. Most females have two X chromosomes and most males have one X chromosome and one Y chromosome.

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