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People suffering from Down syndrome are prone to show a specific behavioral phenotype – a pattern of anomalies across different domains of development. Some scientists feel that developmental research towards Down syndrome effects on phenotype, including diagnosis and examination of the disorder, will help science help members of the affected population.
Down syndrome is an abnormal mental and physical condition, which can vary from mild to severe. People with Down syndrome usually show slower mental and physical development than those without it.
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Chemistry of Down Syndrome
Chromosomes are present in every cell of the human body and carry genetic information necessary for cells to develop. A chromosome is a threadlike structure that consists of DNA and proteins. In a human being, there are 46 chromosomes in every cell, which are arranged in 23 pairs. Out of these, 22 pairs are alike in both males and females – these are called autosomes. The 23rd pair is called the sex chromosome (XX in females and XY in males).
Human cells divide in two different ways: the first one is mitosis. In this kind of cell division, one cell divides into two and the newly formed cells possess the same chromosome number as the parent cell. The second kind of cell division, called meiosis, occurs in reproductive cells such as those in the ovaries and testicles. In this kind of cell division, the daughter cell formed contains half the number of chromosomes of the parent cell.
Errors can occur during cell division. In meiotic division, the chromosome pair splits and goes to the opposite poles in the dividing cell. This event is called ‘disjunction.’ However, in some rare cases, the pair does not divide and the complete pair goes to one pole. As a result, in the resulting cells, one will contain 24 pairs and the other will contain 22 pairs of chromosomes. This event is called ‘nondisjunction.’ During the mating process, if a sperm or an egg containing an abnormal number of chromosomes (24 pairs) merges with a normal mate (23 pairs of chromosomes), the fertilized egg will have abnormal chromosome numbers. In Down’s syndrome, almost 95 percent of the cases occur due to this reason. One cell contains two 21st chromosomes instead of one and, therefore, the resulting fertilized eggs have three 21st chromosomes (also called Trisomy 21). Researchers have found that approximately 90 percent of the cases occur due to abnormal eggs.
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Phenotypic Features of Children with Down Syndrome
Trisomy 21 occurs in one out of 750 live births. The clinical presentation of Down syndrome is very complex and variable. However, a few features, which are quite common, occur to some degree in every individual. They include facial dysmorphology, a small and hypocellular brain and the histopathology of Alzheimer's disease. Hyponia (a disorder that causes low muscle tone and often involves reduced muscle length) occurs frequently in newborns.
Children with Down syndrome are often smaller and their physical and mental development is slower than normal children. Physical features are important for doctors to diagnose clinically. Though not all children with Down syndrome have all the characteristics in common, they may posses features such as flattening of the back of the head, slanting of eyelids, small skin folds at inner corner of the eye, small ears, small mouth, loose ligaments, and small hands and feet, among others.
In addition, cognitive development and language delays have been found to be characteristic of Down syndrome. This adversely affects children’s academic achievement during their learning period in schools and colleges. Research into early interventions may provide support for children with Down syndrome and their families.
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(Web): NCBI: Understanding the Basis for Down Syndrome Phenotypes -- http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1420680/
(Web): Down syndrome phenotypes: the consequences of chromosomal imbalance -- http://www.pnas.org/content/91/11/4997.abstract