Down Syndrome 4 Essay, Research Paper

Introduction

Down syndrome is a chromosomal abnormality. It is the most frequent

form of mental retardation and is characterised by well-defined, distinctive

phenotypic features and natural history. An error in cell development results

in forty-seven chromosomes rather than the usual forty-six. There are three

major types of Down syndrome. About ninety-five percent of individuals with

Down syndrome have Trisomy 21. There is also Translocation and

Mosaicism. Down syndrome can also be referred to as Mongolism, the

condition is named for John Langdon Down. Down was the superintendent of

an asylum for children with mental retardation in Surrey, England when he

made the first distinction between children who were cretins and what he

referred to as “Mongoloids.” Down based this unfortunate name on his notion

that these children looked like people from Mongolia, who were thought then

to have an arrested development. This ethnic insult came under fire in the

1960s from Asian genetic researchers, and the term was dropped from

scientific use. Instead, the condition became “Down’s syndrome.” In the

1970s, an American revision of scientific terms changed it simply to “Down

syndrome,” while it still is called “Down’s” in Europe.

In the first part of the twentieth century, there was much speculation of

the cause of Down syndrome. The first people to speculate that it might be

due to chromosomal abnormalities were Waardenberg and Bleyer in the

1930s. But it wasn’t until 1959 that Jerome Lejeune and Patricia Jacobs,

working independently, first determined the cause to be trisomy of the 21st

chromosome. Cases of Down syndrome due to translocation and mosaicism

were described over the next three years.

The estimated incidence of Down syndrome is between 1 in 1,000 to 1

in 1,100 live births. Each year approximately 3,000 to 5,000 children are born

with this chromosome disorder. It is believed there are about 250,000 families

in the United States who are affected by Down syndrome.

Mode of Inheritance

Although many theories have been developed, it is not known what

actually causes Down syndrome. Some professionals believe that hormonal

abnormalities (X-rays, viral infections, immunologic problems, or genetic

predisposition) may be the cause of the improper cell division resulting in

Down syndrome.

It has been known for some time that the risk of having a child with

Down syndrome increases with advancing age of the mother. For example

the older the mother, the greater the possibility that she may have a child with

Down syndrome. However, most babies with Down syndrome (more than 85

percent) are born to mothers younger than 35 years. Some investigators

reported that older fathers may also be at an increased risk of having a child

with this condition. It is well known that the extra chromosome in trisomy 21

could either originate in the mother or the father. Most often, however, the

extra chromosome comes from the mother.

In Down syndrome, 95 percent of all cases are caused by this event:

one cell has two 21st chromosomes instead of one, so the resulting fertilised

egg has three 21st chromosomes. Recent research has shown that in these

cases, approximately 90 percent of the abnormal cells are the eggs. Three to

four percent of all cases of trisomy 21 are due to Robertsonian Translocation.

In this situation, two breaks occur in separate chromosomes, usually the 14th

and 21st chromosomes. There is rearrangement of the genetic material so that

some of the 14th chromosome is replaced by extra 21st chromosome. So

while the number of chromosomes remain normal, there is a triplication of the

21st chromosome material. Some of these children may only have triplication

of part of the 21st chromosome instead of the whole chromosome, which is

called a partial trisomy 21. Translocations resulting in trisomy 21 may be

inherited, so it’s important to check the chromosomes of the parents in these

incidences to see if either may be a “carrier.”

The remaining cases of trisomy 21 are due to mosaicism. These people

have a mixture of cell lines, some of which have a normal set of

chromosomes and others which have trisomy 21. In cellular mosaicism, the

mixture is seen in different cells of the same type. In tissue mosaicism, one

set of cells (such as all blood cells) may have normal chromosomes, where as

another type, such as all skin cells, may have trisomy 21.

Clinical Description of the Disorder

Although individuals with Down syndrome have distinct physical

characteristics, generally they are more similar to the average person in the

community than they are different. The physical features are important to the

physician in making the clinical diagnosis, but no emphasis should be put on

those characteristics otherwise. Not every child with Down syndrome has all

the characteristics; some may only have a few, and others may show most of

the signs. Some of the physical features in the children include flattening of

the back of the head, slanting of the eyelids, small skin folds at the inner

corner of the eyes, depressed nasal bridge, slightly smaller ears, small mouth,

decreased muscle tone, loose ligaments, small hands and feet and infantile

genitalia. About fifty percent of all children have only one line across the

palm, and there is often a large gap between the first and second toes. The

physical features observed in children with Down syndrome (and there are

many more than described above) usually do not cause any disability in the

child. These children are usually smaller, and their physical and mental

developments are slower, than other children who do not have this syndrome.

The majority of children with Down syndrome function in the mild to

moderate range of mental retardation. However, some children are not

mentally retarded at all; they may function in the borderline to low average

range; others may be severely mentally retarded. There is a wide variation in

mental abilities and developmental progress in children with Down syndrome.

Also, their motor development skills slow; and instead of walking by 12 to 14

months as other children do, children with Down syndrome usually learn to

walk between 15 to 36 months of age. Language development is also

remarkably delayed.

Treatment

Although many medications and various therapies have been given as

treatment for people with Down syndrome, there is no effective medical

treatment available at the present time. However, recent advances in

molecular biology make it feasible now to examine the genetic basis for

Down syndrome. Once we identify the genes on chromosome 21 ( many

already have been discovered) and once we find the mechanism of how these

genes interfere with normal developmental sequences, and if one could

counteract these specific actions, a rational approach to medical therapy

could emerge.

Detection

Down syndrome can easily be detected before its symptoms appear, by

screening the mother for chromosomal defects. This is done by

amniocentesis. By looking at the karyotype researchers can identify Down

syndrome prenatally. There has been no discovery on how to detect a carrier

for Down syndrome.