Tuesday, September 8, 2009

Med Info : Genetic Disorders

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Genetic Diseases

One of the most important threats for human’s health is, undoubtedly, the genetic diseases. What a genetic disease is? It is a disorder caused by genetic factors and especially abnormalities in the human genetic material (genome). There are four main types of genetic disorders. Of course, some of these changes in genome can cause interesting advantages in specific environments (Darwinian Fitness). But there is no doubt that all these abnormalities (disorders) bring destructive results to a living being in the present environment.


Types of Genetic Diseases

There are several types of genetic disorders. The way in which the disorder is inherited can help determine the risks it will have on a pregnancy and the risk it will recur in future children. Risks for having a baby with a birth defect from a genetic abnormality may be increased when:

* the parents have another child with a genetic disorder.
* there is a family history of a genetic disorder.
* one parent has a chromosomal abnormality.
* the fetus has abnormalities seen on ultrasound


What are the types of genetic disease?

The following are the different types of genetic diseases:

* chromosomal abnormalities
* single gene defects
* multifactorial problems
* teratogenic problems



Chromosomal Genetic Diseases:

Chromosomes are big DNA molecules composed from genes. The chromosomes are locatedDown Syndrome 1 in the cell nucleus. Abnormalities in the structure, number (and not only) Down Syndromeof the chromosomes can cause some of the most dangerous genetic disorders. This type of disorders seem to be much easier to observe because they are, sometimes, detected by examination with microscope. Down Syndrome is the most well known disease caused by chromosomal abnormalities. In this disorder there is a third copy of chromosome 21 (there are two copies of each chromosome in the cells of healthy humans). Chromosomal diseases can be also caused by segments and joins of parts of chromosomes.

Chromosomal abnormalities in the baby may be inherited from the parent or may occur with no family history. The following chromosomal problems are the most common:

* aneuploidy - more or fewer chromosomes than the normal number, including:
o Down syndrome (trisomy 21) - cells contain three #21 chromosomes.
o Turner syndrome - one of the two sex chromosomes is not transferred, leaving a single X chromosome, or 45 total.
* deletion - part of a chromosome is missing, or part of the DNA code is missing.
* inversion - when a chromosome breaks and the piece of the chromosome turns upside down and reattaches itself. Inversions may or may not cause birth defects depending upon their exact structure.
* translocation - a rearrangement of a chromosome segment from one location to another, either within the same chromosome or to another.
o balanced translocation - the DNA is equally exchanged between chromosomes and none is lost or added. A parent with a balanced translocation is healthy, but he/she may be at risk for passing unbalanced chromosomes in a pregnancy.
o Robertsonian translocation - a balanced translocation in which one chromosome joins the end of another.
* mosaicism - the presence of two or more chromosome patterns in the cells of a person, resulting in two or more cell lines (i.e., some with 46 chromosomes, others with 47).



Single-Gene/monogenic Genetic Diseases:

In this category the starting point is a mutation/change in one gene. The next question is how a change in the sequence of a single gene can cause severe disorders. Genes code for proteins which are some of the most important tools for theCystic Fibrosis living beings, and also take place in the structures of the cells. The results of a mutation that happens in a part of gene that codes for a functional part of a protein are unwelcome. The protein is no more functional and as a result, many severe consequences take place. Almost 6000 single gene disorders are known and it is estimated that 1 of 200 newborns face a single gene genetic disorder. Some of these are sickle cell anemia, cystic fibrosis, Aicardi Syndrome, Huntington’s disease.

These are also known as Mendelian inheritance disorders, from the first genetic work of Gregor Mendel. In these disorders, a single gene is responsible for a defect or abnormality. Single gene disorders usually have greater risks of inheritance. Single gene disorders can be:

* dominant - an abnormality occurs when only one of the genes from one parent is abnormal. If the parent has the disorder, the baby has a 50 percent chance of inheriting it. Examples include the following:
o achondroplasia - imperfect bone development causing dwarfism.
o Marfan syndrome - a connective tissue disorder causing long limbs and heart defects.
* recessive - an abnormality only occurs when both parents have abnormal genes. If both parents are carriers, a baby has a 25 percent chance of having the disorder. Examples include the following:
o cystic fibrosis - a disorder of the glands causing excess mucus in the lungs and problems with pancreas function and food absorption.
o sickle cell disease - a condition causing abnormal red blood cells.
o Tay Sachs disease - an inherited autosomal recessive condition that causes a progressive degeneration of the central nervous system which is fatal (usually by age 5).
* X-linked - the disorder is determined by genes on the X chromosome. Males are mainly affected and have the disorder. Daughters of men with the disorder are carriers of the trait and have a one in two chance of passing it to their children. Sons of women who are carriers each have a one in two chance of having the disorder. Examples include the following:
o Duchenne muscular dystrophy - a disease of muscle wasting
o hemophilia - a bleeding disorder caused by low levels, or absence of, a blood protein that is essential for clotting.


Multifactorial/Polygonic Genetic Diseases:

The second type of human genetic diseases is caused by mutations in more than one genes. The environment combines with these mutations in order these diseases to appear. We can easily conclude that polygenic disorders are more complicated than the previous type (single gene diseases). These abnormalities are also difficult to analyze, because there are many factors that researchers should take into consideration in order to reach to some useful conclusions. Many well known chronic diseases are Multifactorial Genetic Diseases. Everybody knows Alzheimer, diabetes, obesity and arthritis. Besides many cancer types are caused by multi mutations.

Some birth defects do not follow a single gene or chromosomal abnormality pattern. They may be due to several problems, or a combined effect of genes and the environment. It is difficult to predict inheritance of abnormalities caused by multiple factors. Examples include heart defects, cleft lip or cleft palate, and neural tube defects (defects in the spine or brain).


Teratogenic Problems Dieases:

Certain substances are known to cause abnormalities in babies. Many birth defects occur when the fetus is exposed to teratogens (substances that cause abnormalities) during the first trimester of pregnancy when organs are forming. Some known teratogens include the following:

* certain medications (always consult your physician before taking any medications during pregnancy)
* alcohol
* high level radiation exposure
* lead
* certain infections (such as rubella)


How are genetic problems diagnosed?

Families at risk for genetic diseases may want to consult a specialized genetics counselor. A careful family pedigree (chart of members of the family) and history may help determine risks for certain problems. Counseling also helps parents understand the effects of a disorder and ways it may be prevented or treated.

It may be necessary to check each parent's DNA to learn about some genetic inheritance patterns. Prenatal testing is also available to check the fetus for problems. Testing may include ultrasound (using sound waves to look at internal structures), chorionic villus sampling (testing the tissues around the fetus), or amniocentesis (withdrawing a sample of the amniotic fluid).


Some Genetic Disorders :

Genetic Diseases are caused due to mutation inside one of the genes, deletion of any specific genes, chromosomal abberrations, abnormal extension of gene in its length. Unfortunately we don’t know ways to stop genetic disorders before giving birth to a child. Here is the list of the most common genetic disorders.


Angelman syndrome

Angelman Syndrome is a rare disorder, having neuro-genetic causes. The syndrom was first described in 1965 by Dr. Angelman. A syndrome is a based on group characteristics and manifest a specific condition. This syndrom can be described by an intellectual and evolutional delay, difficulty to speak, sleep problems, hand, laughter or just smiling and a happy appeareance.


Canavan disease

Canavan disease is a genetic disorder which produces gradual damage to nerve cells of the brain. This disease belongs to the category of genetic disorders named leukodystrophies. Leukodystrophies’ feature is myelin degeneration that is the phospholipid layer which protects nerve fibers.


Celiac disease

Celiac disease is a disorder which is autoimmune affecting the small bowel, which appears in the case of genetically predisposed persons belonging to all age categories, after first infancy stages. Its symptoms can be diarrhoea or fatigue, although these can be symptoms of other diseases too.
Celiac disease is produced by a reaction to gliadin, which is a gluten protein from wheat The lining of the small intestine will become flat, which prevents the absorption of nutrients. The unique efficient treatment is a permanent gluten-free diet.


Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease is a heterogeneous genetic disorder of nerves which is defined by touch sensation and loss of muscle tissue, especially in the legs and feet but in the arms and hands as well, in the advanced phases of disease. Even if for now it can’t be cured, this disorder is one of the most popular genetic neurological disorders. According to statistics, 36 in 100,000 are affected.


Color blindness

Color blindness, or deficiency to perceive colors can be of genetic nature, but can as well appear because of brain, eye, or nerve damage, or because of contact with some chemicals products. In 1798, the English chemist John Dalton studied for the first time this aspect, that is why it is sometimes called daltonism.


Cri du Chat

Cri du chat syndrome is a rare genetic disorder which affects an approximately 1 in 20,000 to 50,000 live births. The disease does not depend on ethnic backgrounds, but is most common in the case of women.

The disorder gets its name from the typical cry of babies born with this syndrom. The baby sounds like a kitten, because of problems with the nervous system and larynx. The good news is that about 1/3 of kids recover by the age of 2. Negative aspects of this disease may be:

1. Feeding problems because they can’t suck and swallow well
2. Low weight at birth and poor evolution,
3. Motor, cognitive, and speech delays,
4. Behavioral problems such as aggression, hyperactivity, and repetitive movements,
5. Uncommon facial traits that can change in time.


Cystic fibrosis

Cystic fibrosis is an inherited disorder which can affect the entire body, leading to gradual disability and death.
The most common symptoms are difficulty breathing and not enough enzyme production in the pancreas. Low immune system and dense mucous production lead to frequent lung infections, that are treated, but not always cured, sometimes by intravenous and oral antibiotics. Many other symptoms, like sinus infections, poor evolution, diarrhea can be effects of this disorder in the case of other parts of the body. Sometimes, recurrent lung infections during infancy or childhood may be a sign of cystic fibrosis.


Down syndrome

Down syndrome is a hereditary disorder which features problems related to cognitive ability, physical evolution, and facial traits. It is called after John Langdon Down, the doctor who described it in 1866, in Britain. Most of the times it is detected at birth.


Duchenne muscular dystrophy

Duchenne muscular dystrophy is a mortal disorder which is defined by rapidly gradual muscle weakness and damaged muscular tissue beginning in the pelvis and legs and then affecting the entire.


Hemophilia

Haemophilia is the name of some genetic disorders which mean the body's inability to control bleeding. The bleeding might be exterior, if the skin is broken by a cut, scrape, or abrasion, or it can be interior, into joints, muscles, or organs which are hollow. The result can be visible on the skin or subtle (e.g., brain bleeding).


Klinefelter syndrome

Klinefelter's syndrome has as main effect abnormal testicular evolution and decreased fertility.


Neurofibromatosis

Neurofibromatosis includes a more different genetic disorder which makes tumors develop along different nerves and, besides, may influence the evolution of tissues which are not nervous, like skin and bones. The tumors may develop anywhere in or on the body.

The list may continue with other genetic disorders like: phenylketonuria, Prader-Willi syndrome, Sickle-cell disease, Spina bifida, Tay-Sachs disease or Turner syndrome.
Unfortunately the list does not finishes here and researchers in the domain of medicine must work a lot in order to discover remedies for them.

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