Genetic Diseases


Down Syndrome

A few words about Down Syndrome

Down Syndrome is one of the most serious chromosomal genetic diseases. It happens due to errors during the cell division. When meiosis happens there is an important possibility of undesirable events to happen. More specifically, during meiosis, the pairs of chromosomes split up and go to different spots. These spots will be the starting point for the creation of the nucleus of the daughter cells. In some abnormal situations, one pair of chromosomes doesn’t split. As a result, the resulting nucleuses carry 22 and 24 chromosomes but not 23. This is what we call nondisjunction and can also happens during mitosis (duplication of somatic cells).

In the case of Down Syndrome the nondisjunction happens in chromosome 21 (Trisomy 21). This is the cause of Down Syndrome. To be more specific, the reproductive cells created by this abnormal meiosis carry 22 or 24 chromosomes. When a sperm or egg carries two copies of chromosome 21, we can easily conclude that the fertilized egg will have three 21 chromosomes. The results of recent research show that in most of the cases (90%) the abnormality happens in the female reproductive cells. Unfortunately, the reasons for this abnormalities are not found, but there is a connection between Trisomy 21 and maternal age.

Why this abnormality causes the destructive results of Down Syndrome? The answer is simple. Chromosomes carry genes, which code for proteins. When chromosome 21 is in a double quantity, it is obvious that the cell carries two copies of every gene found in this chromosome. The presence of two genes leads to over expression for the 21 chromosome’s genes which results to increased production of some proteins. This is the starting point for the symptoms of Down Syndrome.

There are three types of Down Syndrome:

a)Trisomy 21: In most cases (95%), all the body cells of the patient have three chromosomes 21, which happens due to the meiosis non-disjunction.

b)Mosaic Down Syndrome: Only some body cells have an extra chromosome 21 (non disjunction in mitosis).

c)Translocation Down Syndrome: In this case, a part of chromosome 21 is attached to another chromosome. So the patients have 2 chromosomes 21 plus a part of a third chromosome 21.

Symptoms of Down Syndrome

Though the symptoms of Down syndrome are very dangerous, it is possible that the patients won’t experience all of them. Most of them are morphological abnormalities like asymmetrical, small skull, abnormal round head with a flat side at the back. Besides, a child with Down syndrome can have slanting eyes, short hands (sometimes broad) and fingers. The nose is usually flattened, the mouth is small with an enlarged tongue (which causes serious problems). The children with Down Syndrome also have low muscle tone (which cause feedinb problems) and loose joints. Besides the rate of weight increase at the DS newborns is slower than the normal newborns. Unfortunately DS affects also the mental abilities of the child. DS patients suffer from moderate mental retardation. Besides, these newborns face problems in developing some skills like feeding, toilet teaching etc. Besides there is a positive connection between Down Syndrome and Alzheimer and Leukemia.

Down Syndrome Inheritance

Most cases of Down syndrome are not inherited. The first and second type of Down happens due to random accidents during meiosis or mitosis (in fetal development). But in the third type, there is a possibility of inheritance. It seems that when a parent has a translocated part of chromosome 21 attached in another chromosome, (balanced translocation: a rearrangement of genetic material between chromosome 21 and another chromosome) he may be unaffected, but there is a high risk that his/her children will suffer from Down.

Down Syndrome Prenatal Diagnosis

There are some tests that can be carried out in order to identify women at increased risk of giving birth to a baby with Down Syndrome. Screening tests can say with certainty if an embryo is affected but has no possibilities of miscarriages. On the other side, Diagnostic tests are more accurate, but have risk of miscarriages. The best choice is to combine screening and diagnostic tests.

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