Trisomies occur when cells have three chromosomes instead of two, with Down syndrome being the most common mutation affecting the 21st chromosome. Edwards syndrome affects the 18th chromosome and Patau syndrome affects the 13th. Advancing maternal age increases the risk, but prenatal vitamins before conception can reduce it. Most trisomies are detected before delivery through ultrasound and prenatal screening, and the risk increases with the mother’s age.
The family of genetic conditions known as “trisomies” occur when some cells have three, rather than two, chromosomes. Trisomies can occur on any of the 23 chromosomes in the human body and are usually named by number based on the chromosome to which they are attached. The most common mutation affects the 21st chromosome and is usually diagnosed as Down syndrome. Edwards syndrome affects the 18th chromosome and Patau syndrome affects the 13th. Some conditions, known as complete trisomies, affect nearly all chromosomes, while so-called ‘mosaic’ conditions affect only a smattering. All of these typically present with physical and mental symptoms that can range from mild to severe and even life-threatening. Most of the time there is nothing expectant parents can do to prevent their children from developing any of these conditions. Advancing maternal age makes it more likely to occur, and taking prenatal vitamins before conception can reduce the risk, but nothing is foolproof.
Understanding trisomies in general
Trisomy is a genetic condition in which cells have an extra chromosome. Normally, each cell in the human body carries 23 pairs of chromosomes. At conception, as cells begin to divide, an extra chromosome may attach to a pair of chromosomes. This creates cells with 47 chromosomes instead of 46. The extra chromosome is usually expressed with distinctive physical and mental characteristics.
Down syndrome
Trisomies are typically numbered based on the position of the extra chromosome out of the 23 in the chain. Trisomy 21, also called Down syndrome, is the most common mutation and occurs when there are three chromosomes in the 21st position. Down syndrome typically manifests both physically and mentally, with those affected having broader faces, distinguishable features and low muscle tone as well as a severely limited intellect. Heart and organ problems are also common, but many if not most people with this extra chromosome survive into adulthood. However, they typically need a lot of care for life.
Edwards syndrome
Edwards syndrome, or Trisomy 18, is the second most common chromosomal mutation. It is estimated to occur in 1 in 3,000 live births. Edwards syndrome is usually fatal, with less than 10% of children born with this syndrome living more than a year. The syndrome causes heart defects, kidney abnormalities, and severe developmental delays.
Syndrome in Patau
Patau syndrome is the result of an extra chromosome in the 13th position and is also known as trisomy 13. Like Edwards syndrome, fewer than 10 percent of babies born with Patau syndrome live more than a year, and many die shortly after birth if they were born alive. This syndrome occurs in about 1 in 10,000 live births. It can cause structural brain problems, severe mental retardation, and heart defects.
Less common conditions
Trisomies can and do occur on any chromosome other than 21, 18, or 13, but rarely result in a live birth. There are isolated cases of children with rarer trisomies surviving for several years. A mutation on the 16th chromosome is thought to occur in about 1% of all pregnancies, but these pregnancies usually result in miscarriages during the first trimester due to severe defects in the fetus.
Complete trisomies occur when every cell in the body has the extra chromosome. In mosaic trisomies, only some cells contain the extra chromosome. People with partial trisomies have only part of the extra chromosome in their cells. Both mosaic and partial trisomies can be hereditary, while complete trisomies are not. Symptoms and survival rates vary depending on the extent of the mutation.
Causes and detection
Most of the time, modern ultrasound technology and prenatal screening can detect probable cases of the chromosomal mutation long before delivery. However, some mutations aren’t discovered until after birth. In general, the risk of delivering a baby with a trisomal mutation increases exponentially with the mother’s age at birth. Most medical experts recommend that your risk increases every year after age 35. Some vitamin and mineral deficiencies may also be linked to developing problems, but the research to support this is not conclusive. More often than not, these kinds of mutations are simply a matter of luck.
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