For New Parents
About Down Syndrome Nondisjunction Mosaicism Translocation Diagnosis Today
About Down Syndrome top
The human body is made of cells; all cells contain a center, called a nucleus, in which genetic material is stored. This genetic material, known as genes, carries the codes responsible for all our inherited characteristics. Genes are grouped along rod-like structures called chromosomes. Normally, the nucleus of each cell contains 23 pairs of chromosomes, half of which are inherited from each parent. In Down syndrome, however, the cells usually contain not 46, but 47 chromosomes; with the extra chromosome being a number 21. This excess genetic material, in the form of additional genes along the 21st chromosome, results in Down syndrome.
Because 95 percent of all cases of Down syndrome occur because there are three copies of the 21st chromosome, it is referred to as "trisomy 21." Chromosomes may be studied by examining blood or tissue cells. Individual chromosomes are identified, stained and numbered from largest to smallest. The visual display of the chromosomes is known as a karyotype.
Down syndrome affects people of all ages, races and economic levels. It is the most frequently occurring chromosomal abnormality, occurring once in approximately every 800 to 1,000 live births.
What is the chance of having a second child with Down syndrome? In general, it is estimated that the chance of having a second child with trisomy 21 or mosaic Down syndrome is about 1 in 100. The incidence is higher if one parent is a carrier of a translocated cell.
Down syndrome is usually caused by an error in cell division called non-disjunction. However, two other types of chromosomal abnormalities, mosaicism and translocation, are also implicated in Down syndrome - although to a much lesser extent. Regardless of the type of Down syndrome which a person may have, all people with Down syndrome have an extra, critical portion of the number 21 chromosome present in all, or some, of their cells. This additional genetic material alters the course of development and causes the characteristics associated with the syndrome.
Nondisjunction top
Nondisjunction is a faulty cell division which results in an embryo with three number 21 chromosomes instead of two. Prior to, or at, conception, a pair of number 21 chromosomes, in either the sperm or the egg, fail to separate. As the embryo develops, the extra chromosome is replicated in every cell of the body. This faulty cell division is responsible for 95 percent of all cases of Down syndrome.
Why nondisjunction occurs is currently unknown, although it does seem to be related to advancing maternal age. Many people are surprised to find out that 80 percent of children born with Down syndrome are born to women under 35 years of age. This is because younger women have higher fertility rates. It does not contradict the fact that the incidence of births of children with Down syndrome increases with the age of the mother.
Although nondisjunction can be of paternal origin, this occurs less frequently. Because this error in cell division is often present in the egg prior to conception, and women are born with their complete store of eggs, it has been postulated that some environmental factors may be implicated in nondisjunction. However, despite years of research, the cause (or causes) of nondisjunction, is still unknown. There seems to be no connection between any type of Down syndrome and parents' activities before or during pregnancy.
Mosaicism top
Mosaicism occurs when nondisjunction of the 21st chromosome takes place in one of the initial cell divisions after fertilization. When this occurs, there is a mixture of two types of cells, some containing 46 chromosomes and some containing 47. Those cells with 47 chromosomes contain an extra 21st chromosome. Because of the "mosaic" pattern of the cells, the term mosaicism is used. Mosaicism is rare, being responsible for only one to two percent of all cases of Down syndrome.
Some research has shown that individuals with mosaic Down syndrome are less affected than those with trisomy 21; however, broad generalizations are not possible due to the wide range of abilities that people with Down syndrome possess.
Translocation top
Translocation is a different type of chromosomal problem and occurs in only three to four percent of people with Down syndrome. Translocation occurs when part of the number 21 chromosome breaks off during cell division and attaches to another chromosome. While the total number of chromosomes in the cells remains 46, the presence of an extra part of the number 21 chromosome causes the features of Down syndrome. As with nondisjunction trisomy 21, translocation occurs either prior to or at conception.
Unlike nondisjunction, maternal age is not linked to the risk of translocation. Most cases are sporadic, chance events. However, in about one-third of cases, one parent is a carrier of a translocated chromosome. For this reason, the risk of recurrence for translocation is higher than that of nondisjunction. Genetic counseling can be sought to determine the origin of the translocation.
How is Down syndrome diagnosed in a newborn? top
The diagnosis of Down syndrome is usually suspected after birth as a result of the baby's appearance.
There are many physical characteristics which form the basis for suspecting an infant has Down syndrome. Many of these characteristics are found, to some extent, in the general population of individuals who do not have Down syndrome. Hence, if Down syndrome is suspected, a karyotype will be performed to ascertain the diagnosis. Some infants with Down syndrome have only a few of these traits, while others have many. Among the most common traits are:
* Muscle hypotonia, low muscle tone
* Flat facial profile, a somewhat depressed nasal bridge and a small nose
* Oblique palpebral fissures, an upward slant to the eyes
* Dysplastic ear, an abnormal shape of the ear
* A single deep crease across the center of the palm
* Hyperflexibility, an excessive ability to extend the joints
* Dysplastic middle phalanx of the fifth finger, fifth finger has one flexion furrow instead of two
* Epicanthal folds, small skin folds on the inner corner of the eyes
* Excessive space between large and second toe
* Enlargement of tongue in relationship to size of mouth
Most children with Down syndrome have mild to moderate impairments but it is important to note that they are more like other children than they are different. It is important to remember that while children and adults with Down syndrome may experience developmental delays, they also have many talents and gifts and should be given the opportunity and encouragement to develop them. Early Intervention services should be provided shortly after birth. These services should include physical, speech and developmental therapies. Most children attend their neighborhood schools, some in typical classes and others in modified education classes. Some children have more significant needs and require a more specialized program. Some high school graduates with Down syndrome participate in post-secondary education. Many adults with Down syndrome are capable of working in the community, but some require a more structured environment. top
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