Guide To Genetics
|Genetic Disorders - An Explanation
The majority of women have normal pregnancies and go on to give birth to healthy babies. However, in a certain percentage of cases babies are born with birth defects. Examples of defects include Down syndrome, cleft palate, Spina Bifida, heart problems and clubfoot. Potential defects can sometimes be spotted through routine prenatal tests, and checked further by diagnostic tests like an amniocentesis test or chorionic villus sampling test. Other defects (known as functional birth defects) may however only appear later in a child's life in the form of a disability such as autism or a metabolic disorder. There are literally thousands of types of birth defects and the primary cause of most is a genetic disorder. A genetic disorder is a disease which is caused by an abnormality in an individual's DNA. Abnormalities can range from a small mutation in a single gene to there being too many or two few of an entire chromosome or set of chromosomes. Most conditions are inherited; some are aggravated by environmental factors (such as the pregnant woman smoking, drinking or taking drugs) - and many others, scientists still do not understand.
Single Gene Mutations
Also known as monogenetic disorders, these are abnormalities that result from a mutation in the protein of one single gene, where the protein is altered or missing. Singe gene mutations are relatively rare in comparison to multifactorial disorders. Examples of monogenetic disorders include Huntington's disease, Maple Syrup urine disease (MSUD), Neurofibromatosis 1 (NFI), Galactosemia, Cystic Fibrosis, Polycystic Kidney Disease, Tay-Sachs disease, Alpha- 1 Antitrypsin deficiency, Adenosine Deaminase (ADA) deficiency, Sickle Cell disease, Smith-Lemli-Opitz syndrome (SLOS), Pachyonychia Congenita, Phenylketonuria and Neurofibromatosis 1 (NF1). One major distinction between the various monogenic disorders is whether they are 'dominant' or 'recessive'. Dominant diseases are caused when just one parent has the disease gene. The chance of a child inheriting the disease is therefore 50 percent. Dominant diseases include Marfan syndrome and Huntington's disease. Recessive diseases on the other hand require the disease gene to be present on both parents side. In this case, the chance of a child inheriting a recessive disease is 25 percent. Examples of recessive diseases include Tay-Sachs disease and Cystic Fibrosis.
In these disorders, entire chromosomes or large parts of them are missing, altered or duplicated. For example, Down syndrome is caused by a duplicate copy of chromosome 21 (called trisomy 21) - so no individual gene on the chromosome is abnormal or missing. Prader-Willi syndrome, on the other hand, is caused by the absence of a group of genes on chromosome 15. Chronic Myeloid Leukemia (CMLA), a specific type of blood cancer may be caused by a chromosomal mix-up in which parts of two chromosomes (chromosomes 9 and 22) are exchanged. No chromosomal material is lost or gained but a new, abnormal gene is formed that leads to the formation of cancer. Other abnormalities which can result from chromosome disorders include: Cri-du-Chat syndrome, Klinefelter syndrome, Williams syndrome and Turner syndrome.
These are by far the most common form of disorders. Functional disorders result from mutations in multiple genes, often coupled with environmental factors. Examples of this type of disorder include hypothyroidism, colon cancer, diabetes, breast /ovarian cancer (see genetic testing for breast cancer) and Alzheimer's disease. Behaviors –the way in which disorders manifest themselves- are still relatively unknown. They are affected by a multitude of conditions and factors. This is an area of research that a lot of money is being spent on. Indications are that multifactorial disorders may also contribute to behavioral disorders such as obesity, alcoholism and mental illness.
Genetic disorders are also one of the leading causes of pregnancy complications like early miscarriages. This may in fact be a form of natural Darwinism, where the body expels what it considers an unhealthy embryo.
As far as scientists know, many genetic disorders are not preventable - it literally is in the genes. Sometimes a disorder is inherited, sometimes it's just the way the dice falls, a random mix-up during the cell formation stage. However, where environmental factors are involved, experts advise pregnant women to be careful. Stay away from toxic chemicals, prescription drugs (unless advised by a doctor), smoking and excessive alcohol. Read also about taking aspirin during pregnancy. These are known factors to cause genetic mutations where there may be no heredity history. There are several prenatal care steps you can take before and during pregnancy to help prevent birth defects. One of the most important things you can do is start preparing for pregnancy before becoming pregnant. Have a medical checkup before you conceive, to ensure any pre-existing conditions are under control and that any illnesses are sorted. Taking at least 400 micrograms of folic acid every day is essential, particularly during pregnancy. Most birth defects happen in the first 3 months for first trimester of pregnancy, so it is important to follow prenatal guidelines.
Every year, research into genetic disorders is opening the way for more genetic testing, which can be used to diagnosis genetic conditions. Genetic tests are available to adults and children, to determine their likelihood of developing a disease. It is also available to pregnant women to test the fetus. Three diagnostic procedures are common in prenatal testing: pregnancy ultrasound scans, amniocentesis and chorionic villus sampling (CVS). These tests are generally offered if you are in a risk factor group, for example if there is a history of birth defects in the family or if you a woman planning a pregnancy after 35. If this applies to you, ask your doctor to recommend a genetic counselor. Genetic counselors are medical experts who advise couples on their odds of having a healthy baby based on their genetic profiles, for example if both parents come from an ethic background prone to certain conditions. They may also ally a pregnant woman’s fear over common disorders such as Down syndrome. See genetic testing in pregnancy and genetic tests before pregnancy.
A genetic counselor can advise you on which tests, if any, are recommended.
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For more conditions related to birth defects and prenatal care, see the following:
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