Spina Bifida
This is a birth defect of the backbone and sometimes the spinal cord. It is one of a group of birth defects called neural tube defects. The neural tube is the embryonic structure that develops into the brain and spine. Spina bifida is among the most common severe birth defects in the country. Approximately 2,000 babies (one in every 2,000 live births ) are born with this condition each year. Spina bifida occurs more frequently in certain racial/ethnic groups, particularly whites of European extraction. It is less common among Jews, Asians, and blacks. Causes of spina bifida are not well understood. Scientists believe that genetic and environmental factors act together to cause this and other neural tube defects. Although spina bifida appears to run in certain families, it does not follow any particular law of inheritance. If parents have one child with spina bifida, the risk of recurrence in any subsequent pregnancy is about one in 40. Studies show that the development of spina bifida and other neural tube defects may be influenced by the mother’s diet, especially the amount of the B-vitamin folic acid she consumes. The most important time to take folic acid is at least one month before conception and during the first month of pregnancy, since this is when neural tube defects occur. Spina bifida can often be detected before birth. The AFP screening test detects pregnancies at higher-than-average risk of spina bifida and other neural tube defects. A detailed ultrasound examination to inspect the fetal spine may follow high AFP test results. It has been suggested that caesarean delivery prior to the onset of labor may reduce the severity of paralysis in babies with spina bifida, although, this is still controversial.
Down Syndrome
This is a combination of birth defects including some degree of mental retardation and characteristic facial features. About 30 to 50 percent of babies with Down syndrome also have congenital heart defects, and many have visual and hearing impairment and other health problems. The severity of these problems varies greatly. Down syndrome is one of the most common genetic birth defects. It affects all races and economic levels equally. Approximately one in 800 to one in 1,000 babies are born with the disorder. In this country, there are about 250,000 individuals with Down syndrome. The major cause of Down syndrome is an extra chromosome number 21. The features of Down syndrome result from having this extra chromosome 21 in each of the body’s cells. This is called trisomy 21, because of the presence of three number 21 chromosomes. Translocation Down syndrome occurs when the extra chromosome 21 is attached to another chromosome in the egg or sperm. This condition, in either parent, increases the chances of having another child with Down syndrome. The risk of Down syndrome increases with age (1 in 106 at age 40 ), however, 70 percent of babies with Down syndrome are born to women who are under age 35. Prenatal testing using amniocentesis or CVS can diagnose Down syndrome.
Tay-Sachs Disease This disease is an inherited birth defect. The symptoms first appear at about 6 months, when an apparently normal baby gradually stops smiling, crawling, or turning over, and eventually becomes blind, paralyzed, and unaware of his surroundings. Death occurs by age five. Babies with Tay-Sachs disease lack an enzyme called hex A that is necessary for breaking down certain fatty substances in brain and nerve cells. These fatty substances build up and gradually destroy brain and nerve cells, until the entire central nervous system stops working. Other forms of the disease are referred to as juvenile, chronic and adult-onset forms of hex A deficiency. Both infantile and adult forms of Tay-Sachs disease occur most frequently in descendants of Central and Eastern European Jews. Non-Jewish individuals of French-Canadian ancestry are at similarly increased risk. Both groups have about 100 times the rate of occurrence of other ethnic groups. A Tay-Sachs carrier has one normal gene for hex A and one Tay-Sachs gene. When two carriers become parents there is a one-in-four chance that any child they have will inherit a Tay-Sachs gene from each parent and have the disease. Prenatal amniocentesis and CVS can diagnose Tay-Sachs disease before birth. Generally, if prenatal testing shows that hex A is present, the baby will not have Tay-Sachs disease. If it is missing, the baby will be affected.
Sickle Cell Disease
This is an inherited blood disease which can cause bouts of pain, damage to vital organs, and for some, death in childhood or early adulthood. Sickle cell disease affects a protein inside the red blood cells. It occurs when a person inherits two sickle cell genes or a combination of one sickle cell gene plus another abnormal genes that affect the red blood cells. Red blood cells are normally round and flexible. But when oxygen is released by the red blood cells in people with sickle cell disease, the cells become distorted, forming a rigid banana or sickle shape that can clog blood vessels. Sickle cells tend to become trapped and destroyed in the liver and spleen. This results in anemia, a shortage of red blood cells. The effects of sickle cell disease vary greatly from one person to the next. Some affected people rarely see their doctors; others may be hospitalized frequently. To inherit the disease, a child must receive two sickle cell genes- one from each parent who carries the sickle cell gene. In the United States, most cases of sickle cell disease occur among blacks and Hispanics. About one in every 400 to 600 blacks inherits the sickle cell disease. A blood test can identify people who have either sickle cell trait or the disease. There is also a prenatal test to determine whether the fetus will have sickle cell disease, carry the trait, or be unaffected. Infants have an increased chance for fatal infection unless treated with penicillin.
PKU
Phenylketonuria is an inherited disorder of body chemistry that, if untreated, causes mental retardation. Most affected newborns are now diagnosed and treated early, allowing them to grow up free of mental retardation. About one baby in 12,000 is born with PKU in the United States. The disorder occurs in all ethnic groups, but it is most common in individuals of Northern European ancestry. PKU is a disease that affects the way the body processes food. Children with PKU cannot process phenylalanine, a constituent of protein. As a result, phenylalanine builds up in the bloodstream and causes brain damage and mental retardation. PKU is inherited when both parents have the PKU gene and pass it on to their baby. When both parents are carriers, there is a one-in-four chance that each will pass the PKU gene on to a child, causing it to be born with the disease. Using a test developed in the 1960s, babies are now tested for PKU before they leave the hospital. The test is highly accurate when performed between 24 hours of age and less than 7 days of age. A second testing at two weeks is recommended in some cases. Mental retardation can be prevented, in affected newborns, if they are treated with a special diet that is low in phenylalanine beginning before the fourth week of life. Individuals with PKU should remain on a restricted diet throughout childhood, and most likely for life.
Marfan Syndrome
This syndrome, which affects about one in 10,000 Americans, is one of the most common inherited disorders of connective tissue. It affects males and females from all racial and ethnic groups. Symptoms of Marfan syndrome may be mild or severe, and may be present at birth or appear in adult life. The disorder sometimes causes sudden death in adults who were unaware that they had it. It is one of more than 100 inherited disorders of connective tissue with abnormalities that may affect the heart, blood vessels, lungs, eyes, bones and ligaments. Affected individuals are often tall, slender, and loose-jointed. Arms and legs may be unusually long. The spine may have a curve, and the breastbone may protrude or look caved in. The face may be long and narrow, with a high roof of the mouth and crowded teeth. Heart and blood vessels nearly always are affected. Sudden large splits in the aorta can result in death. Persons with Marfan syndrome are also prone to sudden lung collapse. Individuals are plagued with various eye problems as well. Marfan syndrome is caused by a defective gene on chromosome 15. A whole variety of mutations in this gene can cause the syndrome. Normally, this gene tells the body to produce a protein called fibrillin. Individuals with Marfan syndrome have scant or faulty fibrillin in their affected tissues. The abnormal gene is usually inherited from one parent who has the disorder. It is a dominant gene; each child has a 50-50 chance of inheriting it. There is currently no single, conclusive test to diagnose Marfan syndrome.
Congenital Heart Defects
More than 25,000 infants (one out of every 125-150) are born with heart defects each year in the United States. Heart defects are among the most common birth defects, and are the leading cause of birth defect-related deaths. A condition is called congenital when it is present at birth. Heart defects begin in the early part of pregnancy when the heart is forming. Congenital heart defects can affect any of the different parts or functions of the heart. In most cases, scientists do not know what makes a baby’s heart develop abnormally. Both genetic and environmental factors appear to play some role. Women who contract rubella during the first three months of pregnancy have a high risk of having a baby with a heart defect. Other viruses also may play a role in heart defects. Certain medications also increase the risk. These include the acne medication Accutane, lithium, and possibly certain anti-seizure medications, as well as other problems. Drinking alcohol in pregnancy also can increase the risk of heart defects. Studies also suggest that use of cocaine in pregnancy increases the risk of these birth defects. A special form of ultrasound called echo cardiography can accurately detect many heart defects. If certain heart problems are diagnosed before birth, medications often help the problem before the fetal heart starts to fail. In other cases, knowing that a problem exists, enables doctors to be ready to give the baby the treatment it needs as soon as it is born.
Achondroplasia
This is a genetic disorder of bone growth that is evident at birth. It affects about one in every 26,000 births and it occurs in all races and in both sexes. It is one of the oldest recorded birth defects. During fetal development and childhood cartilage normally develops into bone. In individuals with achondroplasia, something goes wrong during this process, especially in the long bones. Only a small amount of cartilage in the growth plates of the long bones turns into bone, leading to short bones and reduced height. Generally, the head is large, and the nose is flat at the bridge. Teeth may be crowded and poorly aligned. Because of the large head, short arms and legs and loose joints, a baby with achondroplasia is slow to sit, stand and walk alone. Most children with Achondroplasia have normal intelligence. Psychological problems may arise because of the difficulties in adjusting to a world geared to normal-sized people. Achondroplasia is caused by an abnormal gene located on one of the chromosome 4 pair. In some cases, a child inherits achondroplasia from a parent who has the condition. In most cases (over 80 percent) achondroplasia is not inherited but results from a new mutation that occurred in the egg or sperm cell that formed the embryo. The parents of children with achondroplasia resulting from new mutations are usually average-sized. Geneticists have observed that older-than-average fathers (aged 50 and older) are somewhat more likely to have children with achondroplasia and certain other autosomal dominant conditions caused by new mutations. Because researchers have identified the gene that causes achondroplasia, highly accurate prenatal tests are available which can diagnose or rule out achondroplasia.
Thalassemia
This disorder consists of a group of inherited diseases of the blood. About 100,000 babies worldwide are born with severe forms of the disease each year. It occurs most frequently in people of Italian, Greek, Middle Eastern, Southern Asian and African ancestry. Thalassemia includes a number of different forms of anemia. The most severe form results in fetal or newborn death. Most individuals have milder forms of the disease, with varying degrees of anemia. Many children appear healthy at birth, but become ill during the first year or two of life. They grow slowly and often develop jaundice. Without treatment, the spleen, liver, and heart soon become greatly enlarged. Bones become thin and brittle; face bones become distorted, and children with thalassemia often look alike. Frequent blood transfusions and antibiotics are used to treat the disease when needed. All forms of thalassemia are transmitted only through heredity. The disease is passed on through parents who carry the thalassemia gene in their cells. Blood tests and family genetic studies can show whether an individual has thalassemia or is a carrier. Prenatal testing using CVS or amniocentesis can detect or rule out thalassemia in the fetus. Early diagnosis is important so that treatment can prevent as many complications as possible.
Cleft Lip and Palate
A cleft is an opening in the lip, the roof of the mouth, or the soft tissue in the back of the mouth. These openings are normally present in early fetal development, and usually close by the tenth to twelfth week of pregnancy. They fail to close in approximately one in every 700 babies born. Clefts occur more often among Asians and certain groups of American Indians than among whites. They occur less frequently among blacks. The causes of cleft lip and palate are not well understood. Information suggests that they may be the result of a combination of genetic elements with environmental factors, such as drugs, infections, maternal illnesses, and possibly deficiency of folic acid. Children with clefts have special problems, particularly with feeding, ear diseases and speech development, as well as dental problems. Little is known about how to prevent clefts. Studies have shown that fetuses with a certain predisposing gene may be at increased risk of developing cleft palate if their mothers smoke. Other factors, such as maternal alcohol abuse, and maternal diabetes have been linked to increased risk of clefts.
Clubfoot
This is one of the most common of all birth defects, affecting some 9,000 babies (about one in 400) born in the United States each year. Boys are affected twice as often as girls. Clubfoot is a term used for several kinds of ankle and foot deformities usually present at birth. The defect can be mild or severe, and it can happen to one foot or both. The exact cause of clubfoot still is not clear in many cases. Many scientists think the defect starts early in pregnancy, before the baby is large enough to stay in one position very long. Clubfoot is probably caused by a combination of heredity and other factors that may affect prenatal growth. Although the effects of clubfoot often may be prevented through early treatment, there is no method of preventing the defect at this time. Genetic counseling can help parents understand the odds with each pregnancy for having a child with clubfoot.
Cystic Fibrosis
This is one of the most common inherited diseases, affecting about one in 2500 Caucasians in the United States. The condition is less common in people of other ethnic or racial backgrounds. Cystic fibrosis results from having two CF genes. Individuals with cystic fibrosis have inherited one CF gene from each parent. A parent who has one CF gene and one normal gene is a carrier. A carrier is not affected with the disease, but may pass on either the CF gene or the normal gene to his offspring. With each pregnancy, there is a one in four chance that the child will inherit two CF genes and will have cystic fibrosis. There is a 75% chance that the child will not have the disease. The offspring could be a carrier (50%), or could inherit two normal genes (25%). These chances are the same for each pregnancy. Cystic fibrosis causes the body to produce large amounts of abnormally thick mucus. This collects in the lungs, causing illness. The pancreas is also often damaged by the disease. Deficiency in pancreas enzymes may cause diarrhea and poor growth Cystic fibrosis does not affect intellect. Currently there is no known cure, however, individuals with the disease are living longer than in the past, often into their late 20’s or 30’s. Genetic testing is available to determine whether an individual carries the CF gene. This test can find approximately 90% of Caucasian carriers. The rate of accuracy is higher for non-caucasians. In every population some CF carriers will be missed by the test. Prenatal diagnosis involves testing cell samples obtained through CVS or amniocentesis. The prenatal test can tell with great accuracy whether the unborn baby does or does not have genes which result in cystic fibrosis. However, in some instances the results are inconclusive.
Muscular Dystrophy
There are several forms of muscular dystrophy, the most common and severe type is called Duchenne Muscular Dystrophy. This is a muscle-wasting disorder which affects boys almost exclusively. The onset is usually between the ages of two and four and progresses rapidly. Few individuals survive their early twenties. There is no known cure at this time. Flaws in muscle protein genes cause muscular dystrophies. These disorders are generally inherited, but in some cases no family history of the disease exists. DNA testing can be done to diagnose or rule out Duchenne or Becker muscular dystrophies. Scientists have discovered the gene that, when defective, is responsible for these dystrophies. Carriers of Duchenne and Becker muscular dystrophies may be identified through protein and DNA-based detection tests.
Hemophilia
This is a hereditary disorder in which one of the plasma proteins needed to form a clot is missing or reduced. The most common type of hemophilia is factor VIII deficiency, or hemophilia A. The second most common type is factor IX deficiency or hemophilia B. When a person with hemophilia is injured he will have prolonged bleeding because he cannot make a firm clot. Some bleeding episodes occur as a result of injury, but many occur seemingly without cause. Approximately one in 10,000 males born in the United States has hemophilia. All races are affected equally. Hemophilia is a sex-linked hereditary bleeding disorder transmitted on a gene of the X chromosome. If all of a person’s X chromosomes have the hemophilia gene, then that person will have hemophilia. In some cases hemophilia is dormant for many generations if no affected male children are born. The gene for hemophilia in such cases is carried through several generations of females who, because they have a second X chromosome that is normal, do not suffer from the disease themselves. Other cases may have no family history, meaning that the change in the X chromosome is a new one. This would be a mutation of the gene. Very rarely, a female with hemophilia is born if her mother is a carrier and her father has hemophilia. DNA testing can establish that a female is a carrier of the disorder. In the event of pregnancy, she may then have tests, such as CVS or amniocentesis, to determine the sex of the baby. A male fetus would have a 50-50 chance of of inheriting hemophilia.
Fragile X Syndrome
This is an inherited abnormality of the X chromosome which causes intellectual problems ranging from mild learning disabilities to severe mental retardation. It is the leading known cause of genetically linked mental retardation. Most individuals with Fragile X have physical characteristics which can be linked to connective tissue disorder. The three most common physical features are large testicles, large ears, and a long narrow face. Approximately 80% of adult Fragile X males have one or more of these features. Severe speech and language delays are seen in Fragile X individuals, along with unique behavioral, and attentional characteristics. It is believed that these characteristics are the result of sensory integration dysfunction. The brain in affected individuals does not process or organize the flow of sensory impulses correctly. The Fragile X syndrome is called an X-linked disorder because its gene is located on an X chromosome. It is estimated that one in 1000 individuals are affected and about one in 700 females are carriers. Many cases of Fragile X are not the result of a new mutation and may be passed from generation to generation. Female are not affected as often or usually as severely if they carry a Fragile X since their normal X acts as a compensator. Conversely, since males have one X and one Y, they are almost always affected. Presently there is no known cure for Fragile X, however, individuals do benefit from various forms of special education. The test for Fragile X is different than the normal chromosome testing. Both the Fragile X test and the regular chromosome test need to be done as sometimes the individual does not have Fragile X but another chromosomal abnormality. A molecular DNA test is 790% accurate in detecting Fragile X.