Cause of Down syndrome still unknown

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NEW YORK – Over the last 100 years, individuals with Down syndrome have become increasingly active members of society.
With more than 350,000 cases of Down syndrome in the United States alone, these individuals prove they’re integral and valuable members of society, with more and more graduating from high school, attending college, and living independently.
But what is Down syndrome? What are its causes, and what can people expect?
What is it? Occurring in one out of every 733 live births, Down syndrome is one of the more frequently occurring chromosomal abnormalities. Down syndrome is a lifelong condition wherein a person is born with distinct physical features, such as a flat face and short neck.
In addition to those physical aspects, an individual with Down syndrome will also have a certain degree of cognitive disability.
As mentioned, Down syndrome is a chromosomal abnormality. Human beings are made up cells, each of which contains a center known as the nucleus.
Inside that nucleus is where a person’s genes are stored. These genes are essentially what makes a person, holding all of a person’s inherited characteristics.
Chromosomes. The genes are grouped in structures called chromosomes. In the nucleus of a typical cell, genes are grouped in pairs of 23 chromosomes (for a total of 46), half of which are inherited from each parent.
However, in the late 1950s, French physician Jerome Lejeune identified the chromosomal abnormality shared by all individuals with Down syndrome. Rather than 46 chromosomes per cell, individuals with Down syndrome have 47 chromosomes.
That extra chromosome (which was later determined to be an extra partial or complete 21st chromosome) is responsible for the characteristics associated with Down syndrome.
This extra chromosome is the result of the cell not properly dividing prior to or at conception. The error is then replicated throughout all of the body’s cells.
Not sure why. Exactly why the error in cell division occurs is still a mystery. A common misconception is that there is something the mother does during the pregnancy that results in her child being born with Down syndrome.
That is not the case, as researchers have determined environmental factors play no role in whether or not a child will be born with Down syndrome.
One factor that has been linked to Down syndrome is maternal age. In general, the older a woman is when she becomes pregnant the greater the chance for her child being born with Down syndrome.
In women under 35, the chance of conceiving a child with Down syndrome is one in 350, according to the National Down syndrome Society. By 35, the chances are one in 30.
Still, because younger women have higher fertility rates, 80 percent of children with Down syndrome are born to women under 35.
Different types. While 95 percent of all Down syndrome cases fall into the same category, there are different types. The most common is known as trisomy 21 and results from the aforementioned error in cell division known as nondisjunction.
Either before or at conception, the pair of 21st chromosomes fails to separate, resulting in the extra 47th chromosome. That error is then replicated in cells throughout the rest of the body.
A far less common form of Down syndrome is mosaicism. Mosaicism also occurs as a result of nondisjunction. But unlike in trisomy 21, mosaicism does not spread throughout all of a body’s cells, leaving some cells with the normal 46 chromosomes and others with the extra chromosome.
The least common form of Down syndrome, mosaicism only occurs in about 1 to 2 percent of all cases.
Translocation is another type of Down syndrome that actually does not lead to an extra chromosome. Those with translocation have 46 chromosomes per cell, but an error occurs during cell division that forces part of chromosome 21 to break off.
That part typically attaches to chromosome 14, but the presence of that extra part results in symptoms of Down syndrome.
Educate yourself. To learn more about Down syndrome, visit the National Down Syndrome Society.

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