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Down Syndrome - The Most Common Congenital AnomalyBY: Zandro Cabaral | Category: Genetics | Submitted: 2011-02-05 10:26:57
Down syndrome, also known as Trisomy 21, is the most common congenital anomaly in the world. It is also the most common cause of congenital mental retardation. Down syndrome is also the most famous and the most known of all the anomalies. What is really Down syndrome? What is the cause of this congenital anomaly? What are the maternal factors that have high risk for Down syndrome? All this questions will be answered as you read further in this article.
Down syndrome is a congenital abnormality or anomaly that is due to a defect in one of the genetic processes in cell division during the developmental stages of the fetus leading to the formation of a defective chromosome. In this case, the chromosome 21 is the defective component. In a human's developmental stage, there is what is called Meiosis. It is the process in which our cells divide and replicate. During normal meiosis, each pair of chromosomes divides or splits into 2 identical chromosomes and goes into the different spots of the dividing cell. This is called the process of Dysjunction. In Down syndrome however, one pair of chromosome, which is chromosome 21, does not divide and the whole pair goes into the same spot in the dividing cell. As a result, instead of a pair, it becomes 3 pieces of chromosome 21 in the spot thus, termed Trisomy 21 and the meiotic accident is called the Non-Disjunction.
Physical characteristics of children with Down syndrome are:
• The palms have a distinctive simian crease line
• an unusually round face
• up slanting palpebral fissures
• macroglossia or a lips and tongue larger than normal
• an epicanthic fold of the eyelids
Sadly, children with Down syndrome have MR and a very low IQ. Life expectancies of these individuals are up to 35 years. About 50 - 60% of the cases of Down syndrome also suffer from a congenital defect known as Endocardial Cushion Defect of the heart that makes them susceptible to Cardiac Failure in the later ages. Another congenital defect commonly present in Down syndrome is Doudenal Atresia. This is an abnormality in the duodenum or the small intestines characterized by the narrowing of the second part of the duodenum distal to the Ligament of Trietz and are diagnosed by an abdominal x-ray showing a "double bubble" sign. It is manifested by malabsorption, vomiting and constipation.
Down syndrome is associated with Alzheimer's disease because they have the same chromosome affected which is chromosome 21. Due to this fact, when patients with Down syndrome unusually live beyond the age of 40, they will develop Alzheimer's disease and would manifest the symptoms of the disease such as the decline in cognition accompanied with a slow-progressing dementia, and the presence of Neurofibrillary tangles and Tau protein in the brain tissue. However, with the other systemic anomalies that escort Down syndrome, it is uncommon for these individuals to live beyond 40.
The occurrence of Down syndrome is linked primarily to the maternal factors during the fetal life of the child. A number of these factors specifically points to the anomaly. Such as:
• Maternal Age during Conception. A high maternal age when conceiving the child specifically the pre-menopausal, menopausal and the post-menopausal maternal age group.
• Birth Rank of the Child. Birth rank is defined as the baby's birth order by age among his or her siblings. There's a higher risk of Down syndrome in babies with high birth rank.
• Incest. There are many cases of Down syndrome in cultures which allow incest or the mating of two directly related individuals.
• Asian, Southeast Asian bloodlines. These countries have the most cases of Down syndrome.
There are ongoing studies today about using stem cells to increase the quality of life in patients with Down syndrome but so far no result has been published. The specific prevention of Down syndrome is not known just like the other congenital anomalies such as Trisomy 18 or Edward Syndrome, trisomy 22 and Marfan's syndrome which is the defect in fibrillin. "Genetic Accidents" as they are described, happen randomly and are in the genetic level which makes these genetic defects very difficult to study and treat. The only thing sure about these congenital defects is that they are all associated to maternal exposure to mutagens and risk factors during pregnancy.
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