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The Genetic Code |
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Genetic Disorders |
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Dominant Gene Disorders |
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Recessive Gene Disorders. Batten Disease/JNCL Fall's into this category |
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X-Linked Gene Disorders |
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Chromosomal Disorders |
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Multifactorial Disorders |
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Other Defects |
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Genetic Counseling |
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Finally |
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Glossary |
Genetics is the study of how traits are passed on, or inherited, from parent to child. Many physical traits, such a height and eye colour, are inherited. Unfortunately, some disorders can be passed on the same way. Some genetic problems can be detected during pregnancy or, in some cases before you become pregnant. Through prenatal counselling and testing, you may be able to learn whether your foetus is at risk for having certain genetic disorders. About 3% of babies born in the United States have some type of major birth defect. The cause is not always known, but about 20% may be genetic. Some birth defects may be caused by other factors, such as the foetus being exposed to alcohol or certain drugs. These birth defects are not genetic, so it can be harder to detect them during pregnancy. Still other birth defects have no known cause. Birth defects can affect the baby's health or ability to function or the way an infant looks. Some can be treated or corrected with surgery or medicine. A baby may be born with more than one birth defect. Sometimes these different birth defects may have a single cause.
Most of the cells in the human body contain 46 chromosomes that are arranged in pairs. Each chromosome carries many genes. Genes determine the traits a person inherits from his or her parents. Normally, each sperm from the father and each egg from the mother contains 23 chromosomes, or one member of each pair found in all other cells in the body. When an egg joins with a sperm [fertilisation], the 23 chromosomes from each come together as the 46 chromosomes of the foetus. One pair of the chromosomes - one each from the sperm and the egg – are the sex chromosomes. There are two types of sex chromosomes, called X and Y chromosomes. A normal sperm carries either an X or a Y. A normal egg always carries and X chromosome. The sex chromosome from the man's sperm Determines the sex of his child. If a sperm with a Y chromosome fertilises the egg, the child will be male [XY]; if a sperm with an X chromosome fertilises the egg, the child will be female [XX].
Genetic disorders may be caused by problems with either genes or chromosomes. A person may have a genetic disorder that may not be visible or may carry a disorder without knowing it. Some disorders can be inherited if only one parent carries the gene. Some occur at random. Even if a couple has had a normal baby, if they are at risk for having a baby with a genetic disorder, they are at risk with each pregnancy.
A dominant genetic disorder can be caused by a single abnormal gene from either parent. Even though one member of the gene pair is normal, the abnormal gene overrules the normal gene, or is dominant. Some of these disorders are: ·
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| If one parent has a dominant gene disorder, there is a 50% chance that it will be passed to each child. |
| If both parents are carriers of a recessive disorder, there is a 25% chance that a boy or girl will be affected, a 50% chance that a child will be carrier, and a 25% chance that the child will not carry the gene. |
| If a woman is a carrier of an x-linked disorder, there is a 50 % chance that she will pass the disorder to her son and a 50% chance that her daughter will be carrier. If the father has an x-linked disorder, all the daughters will be carries and none of the sons will be affected. |
Polydactyly – A fairly common problem in which the child has extra fingers or toes. It can be corrected by surgery. Prenatal testing cannot detect this problem in most cases.
Achondroplasia – A very rare abnormality of the skeleton in which the arms and legs are shorter than normal. This can be detected by ultrasound.
Huntington disease – A problem with the nervous system that causes loss of control over movements and mental function, usually starting in midlife. Prenatal testing is possible.
Recessive Gene Disorders – Batten Disease / JNCL
A recessive genetic disorder can occur when both members of the gene pair – one from each parent – are abnormal. Everyone carries a few abnormal genes. In most people, no defect appears because the abnormal gene is overruled by a normal gene. A person who has one abnormal gene of a particular recessive disorder is considered a carrier. A carrier shows no signs of the disorder but could pass the gene on to his or her children. In some groups, it is more likely that both parents will carry the same abnormal recessive gene. These disorders are more common in certain ethnic groups and in children whose parents are close blood relatives. Common recessive disorders include:
Cystic fibrosis – A condition that causes very thick mucus to be produced in the lungs, clogging them and causing lung disease. It also causes problems with the pancreas. It occurs most often in whites.
Sickle cell disease – A disease that causes the red blood cells to take on a crescent, or "sickle" shape. These abnormal cells tend to get stuck in the blood vessels. This cuts off oxygen to tissues and causes pain. Anaemia [blood that is low in red blood cells] often occurs. These disease affects mostly African Americans. ·
Tay-Sachs disease – A disease that first appears at about 6 months of age and causes mental retardation. Blindness, seizures , and death within a few years. It is found mostly in persons of eastern European Jewish descent [Ashkenazi Jews]. ·
Thalassemia – A condition that causes anaemia. One form [beta-thalassemia] occurs most often in persons of Mediterranean descent, such as Italians and Greeks. Another form [alpha-thalassemia] occurs more often in Asians.
Both carrier testing of the parents and prenatal testing of the foetus are available for these recessive disorders.
X-linked disorders are also call sex-linked disorders because they are caused by an abnormal gene on the X chromosome. A woman may carry the abnormal gene that causes this disorder. If the gene on her other X Prevents her from having the disorder. In such cases, because a male child will get only one X chromosome from his mother, there is a 50% risk that he may have the disorder. Examples of X-linked disorders include:
Haemophilia – A condition in which the blood lacks a substance needed for clotting. Internal bleeding can be life-threatening to people with haemophilia, because they are very slow to stop bleeding:
Duchenne muscular dystrophy – A disease in which the muscles become weak.
Prenatal testing of the foetus is available for these X-linked disorders.
Genetic disorders may also be caused by problems with the foetus's chromosomes. Some of these problems are inherited, but most are caused by an error that occurred with the egg or sperm was being formed. There are many chromosomal disorders. Some are caused by an extra or missing chromosome. Sometimes, just a piece of a chromosome is missing or extra. Although these problems can occur in any pregnancy, the risk increases with age. The most common chromosomal disorder in new-borns is Down syndrome. Down syndrome causes varying degrees of mental retardation and sometimes birth defects such as heart defects. Affected people have certain facial features: a flat face, slanting eyes, and low-set ears. Instead of two number 21 chromosomes, persons with Down syndrome have three. This is called trisomy. Trisomy can also occur with other chromosomes. Another common disorder is fragile X syndrome, in which a certain gene on the X chromosome is abnormal. It causes mental retardation. Some people with fragile X syndrome are autistic, which makes it had for them to interact with other people. They may repeat certain actions and have language problems. Other disorders can occur when a person has too many or too few sex chromosomes. For example, some boys have Klinefelter syndrome. They have an extra X chromosome – that is, they have two X chromosomes and one Y chromosome. They have testicles that are smaller than normal and may be infertile. Some are Some. This is called Turner syndrome. These girls tend to be short, have puffy hands and feet, and have webbing at the back of their necks. They may not go through puberty normally, but early hormone treatment can help. They have normal intelligence but are almost always infertile.
Many disorders are thought to be the result of a mix of genetic and environmental factors. This is known as multifactorial inheritance. Two common ones are con-genital hear defects and open neural tube defects [ONTD's]. When a baby is born with a heart defect, it is usually the result of multifactorial inheritance. Other causes include infection with syphilis or rubella virus [German Measles], alcohol abuse, or a specific chromosomal problem. A heart defect can sometimes be identified before birth by ultrasound. With an ONTD, the foetus's brain, spinal cord, or their coverings do not form normally. Spina bifida [sometimes called "open spine"] is one type of ONTD. When spina bifida is open [not covered by skin], it may be detected with testing. The effects of this defect vary. Some people with spina bifida have only mild problems. In others, it may cause leg paralysis, loss of feeling, lack of bladder and bowel control, scoliosis [curved spine], hydrocephalus [water on the brain], mental retardation, and even death. Folic acid may help prevent neural tube defects if it is taken before pregnancy. Another type of ONTD, anencephaly, occurs when the brain and head do not develop normally. Babies with anencephaly are either stillborn or die soon after birth. Other disorders also can be caused by multifactorial inheritance: ·
Pyloric stenosis – The opening from the stomach to the intestine is blocked. ·
Clubfoot – The affected foot is twisted at the ankle. · Cleft lip and palate A gap or space occurs in the lip or a hole in the palate [roof of the mouth]. ·
Hip dislocations – The ball and socket of the hip joint do not fit together well. This occurs more often in girls than in boys.
Some defects can occur that are sometimes, but no always, linked to a chromosome problem. These defects do not have a patter of being inherited. An example is abdominal wall defects. With one type of such defect, the skin does not completely form over the abdomen, leaving it open. This problem can sometimes be detected by testing during pregnancy.
Genetic counselling can help a couple find out their chances of having a child with a genetic disorder. Anyone can have genetic counselling. It is strongly advised for some couples. This includes those who may have an increased risk of having children with birth defects, such as those with a family history of the disorder or certain ethnic background. Genetic counselling is often helpful to assist couples in making choices about whether to become pregnant or have prenatal testing during pregnancy. If you have special concerns, consider genetic counselling before you get pregnant. During counselling, the couple may be asked about their ethnic backgrounds and family history of diseases. Lab testing and physical exams may also be performed. If a family member has had a problem, his or her medical records may be requested. All this is done to find out whether any specific types of prenatal testing should be offered. Some of the genetic tests that might be helpful, as well as the couple's concerns, are discussed. It is your decision whether to have genetic testing. Some women find it to be reassuring. Others would rather not know. The results of genetic tests can help some women make decisions about their options. If you learn genetic information that might affect other family members as well, think about sharing it with them.
Testing is now available for many genetic disorders. It is your decision whether to have a test. If you know you have risk factors for giving birth to a baby with a genetic disorder, think about seeking counselling before you get pregnant. Still, even couples in a high-risk group usually have normal, healthy children. Most children are born healthy, with no major birth defects. Your risk of passing on a disorder exists with each pregnancy even though you may already have normal children.
Alpha-fetoprotein [AFP]: A protein produced by a growing foetus; it is present in amniotic fluid and, in smaller amounts, in the mother's blood.
Amniocentesis: A procedure in which a small amount of amniotic fluid is taken from the sac surrounding the foetus and tested.
Carrier: A person who shows no signs of a particular disorder but could pass the gene on to his or her Children.
Chorionic villus sampling [CVS]: A procedure in which a small sample of cells is taken from the placenta and Tested.
Chromosomes: Structures that are located inside each cell in the body and contain the genes that determines a Person's physical makeup.
Down Syndrome: A genetic disorder caused by the presence of an extra chromosome and characterised by mental Retardation, abnormal facial features, and medical problems such as heart defects.
Foetal Blood Sampling: A procedure in which a sample of blood is taken from the umbilical cord and tested.
Gene: A DNA "blueprint" that codes for specific traits, such as hair and eye colour.
Miscarriage: The spontaneous loss of a pregnancy before the foetus can survive outside the uterus.
Puberty: The stage of life at which a child turns into a young woman or young man, when the reproductive organs become functional and secondary sex characteristics develop.
Ultrasound: A test in which sound waves are used to examine the foetus or view the internal organs.