THE SCIENCE OF DOWN SYNDROME


According to the Centers for Disease Control and Prevention, approximately one in every 700 babies in the United States is born with Down syndrome, making Down syndrome the most common chromosomal condition. About 6,000 babies with Down syndrome are born in the United States each year. Every cell in the human body contains a nucleus, which holds all the genetic information that codes our inherited traits. These codes are grouped structures called chromosomes, with each nucleus containing 23 chromosomes. Down syndrome occurs when there is a partial or full extra copy of chromosome 21.


The added genetic material changes an individual’s course of development and causes the physical characteristics of Down syndrome. Some of the physical characteristics include low muscle tone, small stature, an upward slant of the eyes, and a single deep crease across the center of the palm. However, it is crucial to remember that every individual with Down syndrome is a unique person, so they may have these characteristics in varying degrees, or not at all.


There are three types of Down syndrome: trisomy 21, translocation and mosaicism. Trisomy 21 accounts for 95% of all cases and is caused by an error in cell division referred to as “nondisjunction.” Nondisjunction results in an embryo with three copies of chromosome 21 instead of the average two. Before or during conception, a pair of 21st chromosomes fail to separate, which can happen in either the sperm or the egg. While the embryo continues to develop, the extra chromosome is replicated in each and every cell in the body.


In translocation, which accounts for about 4% of cases of Down syndrome, the total number of chromosomes in the cells remains 46. However, an additional full or partial copy of chromosome 21 attaches to another chromosome, typically chromosome 14. The presence of the full or partial extra chromosome 21 results in the characteristics of Down syndrome.


Mosaicism (also known as mosaic Down syndrome) is diagnosed when there is a combination of two kinds of cells, some containing the typical 46 chromosomes and others containing 47. The cells with 47 chromosomes have an extra chromosome 21. Mosaicism is the least common form of Down syndrome, only accounting for 1% of all cases. The reason behind the additional full or partial chromosome is still unknown. Maternal age is the only factor that has been linked to an increased chance of having a baby with Down syndrome. However, due to higher birth rates in younger women, 80% of children with Down syndrome are born to women under 35 years of age. There is no definitive scientific research that indicates that Down syndrome is caused by environmental factors or the parents’ activities before or during pregnancy.•


AMY AND THE ORPHANS UPSTAGE GUIDE


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