What It Is
Down syndrome is a chromosomal (genetic) disorder that generally leads to an intellectual disability. This disorder occurs when there is a full or partial extra chromosome 21. This additional genetic material alters the course of development.
History
For centuries individuals with Down syndrome have been displayed in art, science, and literature. However,it wasn't until the late 19th century when John Langdon Down, an English physician, published an accurate description of an individual with Down Syndrome. Although other people had previously realized characteristics of the disorder Down was the first to publish the combined characteristics as a disorder itself. He published his work in 1866 and earned the title of the "father" of the syndrome, hence the name " Down Syndrome". In 1959, French physician Jérôme Lejeune identified it as a chromosomal disorder. He noticed that there were 47 chromosomes instead of 46. In the year 2000 a team of scientist successfully identified and catalogued approximately 329 genes on chromosome 21.
Different Types of Down SyndromeThere are three different types of Down syndrome:
Trisomy 21 (Nondisjunction) Nondisjunction is the most common form,95% of cases. Nondisjunction occurs in the embryo when there are three copies of chromosome 21 rather than the normal two. Prior to or at conception, the pair of 21st chromosomes in either the egg or sperm fails to separate. As the embryo develops, the extra chromosome is replicated in every cell of the body. Mosaicism Mosaicism occurs when nondisjunction of chromosome 21 occurs in some but not all of the initial cell divisions after fertilization. Here there is a mixture of cells, some with the normal 46 chromosomes and some with 47 chromosomes. Cells with 47 chromosomes contain an extra copy of chromosome 21. This form of Down syndrome only accounts for about 1% of all cases. Research has hinted that individuals with mosaic Down syndrome have fewer characteristics of Down syndrome. However, large generalizations like this are not possible because of the wide range of abilities that people with Down syndrome possess. Translocation During Translocation a part of chromosome 21 detaches itself from the main body of the chromosome and reattaches itself to another chromosome. In general the piece of chromosome 21 attaches itself to chromosome 14. Here the total number of chromosomes stays the same (46), but the presence of an extra part of chromosome 21is accountable for the present characteristics of Down Syndrome. About 4% of cases are due to translocation. Causes of Down Syndrome
Any individual with Down syndrome ,regardless of the type, all have an extra critical portion of chromosome 21 in all or some of their cells. The additional genetic material alters the course of development which causes the characteristics of Down syndrome. The cause of nondisjunction is currently unknown, but research has shown that the likelihood of having a child with Down syndrome increases with the aging of women. Due to higher birth rates in younger women, 80% of children with Down syndrome are born to women under the the age of 35. No definitive research has been found that indicates Down syndrome is caused by environmental factors or the parents' activities before or during the pregnancy. The additional whole or piece of chromosome 21 can originate from the father or mother. 5% of cases have been traced to the father. Likelihood of Having a Child with Down Syndrome Down syndrome occurs in all races and economic levels. Each year a mother ages the risk of having a child with Down Syndrome increases. However, the age of the mother does not affect the risk of translocation. Due to multiple couples postponing parenting until later in life, risks of having a child with Down syndrome is expected to increase. Because of this, genetic counseling is becoming increasingly important.
Does Down Syndrome Run in Families? Each type of Down syndrome is a genetic condition. Only 1% of all cases have a hereditary component. In nondisjunction and mosaicism heredity is not a factor; but translocation does have a heredity factor accounting for about 1% of cases. Likelihood of Having Multiple Children with Down Syndrome Once a female has given birth to a child with nondisjunction or translocation, her chances of having another baby with nondisjunction goes up to 1 inn 100 until she turns 40. Risk of recurrence of translocation is approximately 3% if the father is the carrier and about 10-15% if the mother is the carrier. Genetic counseling can determine the origin of translocation. How is the Diagnosis Made? Prenatally The two types of tests that can be performed for Down syndrome before a baby is born are: screening and diagnostic tests. Prenatal screens estimate the probability of the child having Down syndrome rather than a definitive diagnosis. Screening tests generally consist of a blood test (serum screening test) and an ultrasound (sonogram). The blood tests measure the quantities of various substances in the mother's blood. Blood test and age are used to estimate the child's risk of having Down syndrome. Chorionic villus sampling (CVS) is a diagnostic procedure available for prenatal diagnosis. CVS is generally performed in the first trimester between 9 and 11 weeks. Amniocentesis is then usually performed in the second trimester after 15 weeks of gestation. Although these procedures are practically 100% accurate there is a 1% risk of causing a spontaneous termination (miscarriage). At Birth When diagnosing Down syndrome at birth it is usually identified by certain physical traits such as: low muscle tone, a single deep crease across the palm of the hand, a slightly flattened facial profile, and an upward slant towards the eyes. Although physical features are helpful towards diagnosis in babies the diagnosis is not solely base on physical features because these traits may be present in babies without Down syndrome. Chromosomal analysis known as karyotyping is done to confirm the diagnosis. When karyotyping the first thing a doctor does is take a blood sample from the baby and examine the cells. Next they use special instruments to photograph the chromosomes and then they group them by size,number, and shape. After examining the karyotype the doctor can diagnose Down syndrome. FISH [(Fluorescence In Situ Hybridization) another genetic test] applies similar principals and confirms a diagnosis in a shorter period of time. Impact of Down Syndrome On Society
Organizations such as schools, health care systems, work forces, social, and recreational activities are becoming more accepting and helping integrate people with Down syndrome. Individual with Down syndrome generally have mild to moderate cognitive delays, but mild to severe cognitive delays can occur. Recent medical advancements have increased the life expectancy of people with Down syndrome greatly. In 1910, individuals with Down syndrome were expected to live till age 9. With the discovery of antibiotics the age increased to 19-20. Now with recent advancements in clinical treatments, especially corrective heart surgeries, as many as 80% of adults with Down syndrome live till 60 or older. The need for widespread public education and expectancy is continuing to increase as more and more Americans are interacting and being accepting of individuals who have Down syndrome. |
Bibliography:
York & Chapel - National Down Syndrome Society - ndss - http://www.ndss.org/
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York & Chapel - National Down Syndrome Society - ndss - http://www.ndss.org/
"google images"