Down Syndrome Report Essayby: SCHOOL SUXThe chromosomal abnormality involved in most cases of Down syndrome istrisomy-21, or the presence of three copies of the 21st chromosome. As a result,the affected person has 47 chromosomes in all body cells instead of the normal46, although how this causes the condition’s symptoms is not yet known. Scientists assume that the reason for the abnormal chromosomal assortment is thefertilization of an ovum having 24 chromosomes by a sperm with a normalassortment of 23, but they have also found that the sperm can carry the extrachromosome as well. The abnormal ovum or sperm is derived from a germ cell inwhich the pair of 21st chromosomes holds together and passes into the same spermor ovum instead of separating. In the type of Down syndrome calledtranslocation, the extra chromosome 21 material is attached to one of the otherchromosomes; when some, but not all, of the body’s cells carry an extrachromosome 21, the condition is a type of Down syndrome called mosaicism.
Because of the extra chromosome 21, children with Down syndrome oftenhave some characteristic physical features, such as a small head, a flat face,slightly upward slanted eyelids, skin folds at the inner corners of the eyes,small nose and mouth, and small hands and feet. Most of these characteristic donot interfere with the child’s functioning, a doctor primarily uses thecharacteristics for diagnostic purposes. These physical features are variable,and children with Down syndrome are usually more like other children than theyare different. Individuals with Down syndrome also often have certain medicalconditions such as weak muscles, neurological impairments, heart disease,intestinal abnormalities, poorly functioning thyroid gland, eye abnormalities,hearing problems, and skeletal problems. Almost all children with Down syndromeare mentally retarded, in the mild to moderate range.
The degree of mentalretardation varies considerably. Through medical treatment the disorders andinfections accompanying Down syndrome have no effect on an almost normal lifespan. The overall incidence of Down syndrome is approximately one in 700births, but the risk varies with the age of the mother. The incidence of Downsyndrome in children born to 25-year-old mothers is approximately 1 in 1200; therisk increases to approximately 1 in 350 for 35-year-olds and approximately 1 in120 for women older than 40 years. Prenatal tests can be used to detectchromosome abnormality causing Down syndrome.
SOURCES1) “Down Syndrome,” Microsoft(R) Encarta(R) 96 Encyclopedia. (c) 1993-1995Microsoft Corporation. All rights reserved. (c) Funk & Wagnalls Corporation. All rights reserved. 2) “Down Syndrome,” Encyclopedia of Sleep and Dreaming.
Science .
