9 Rare Genetic Trisomies

Beyond Down Syndrome (Trisomy 21)

chromosomes, human karyotype, down's syndrome, trisomy 21
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Trisomy is when three copies of a chromosome are present instead of two (all chromosomes normally come in pairs). While most parents-to-be are familiar with Down syndrome and will undergo prenatal screening to detect it, there are other, potentially more serious trisomies that may occur, including Edwards syndrome, Patau syndrome, and others. Some may cause few, if any, symptoms, while others can lead to severe defects that make life—or even the pregnancy—unsustainable.

Understanding Trisomies

A gene is essentially a packaged bundle of chromosomes that contain all of the coded information related to our physiological makeup and metabolic function. Each gene typically contains 46 chromosomes, 23 of which we inherit from our mothers and fathers, respectively.

Of these, 22 pairs are autosomes, which determine our unique biological and physiological features. The 23rd pair is sex chromosomes (known as X or Y), which designate whether we are biologically female or male.

In rare instances, a coding error may occur when a cell divides during fetal development. Instead of splitting cleanly into the two identical chromosomes, the newly divided chromosome will have extra genetic material. This can lead to either a full trisomy (in which a complete third chromosome is created) or a partial trisomy (in which only part of the chromosome is copied). From this point forward, the error will be repeated and repeated as the cell continues to divide.

Down syndrome, the most common genetic disorder in humans is referred to trisomy 21 because there is an extra copy of chromosome 21 in a gene. Other genetic disorders are similarly named.

Causes and Consequences

Trisomies affecting the sex chromosomes—in which females typically have two X chromosomes (XX) and males have an X and Y chromosome (XY)—tend to be less severe. Autosomal trisomies often cause serious physical and intellectual disabilities, particularly full autosomal trisomies for which early death is common.

In addition to birth defects, trisomies can undermine the viability of a pregnancy. In fact, it is believed that more than half of all miscarriages are directly associated with a chromosomal defect. Of these, no less than 60 percent are due to trisomies.

No one knows for sure why chromosome 21 is so vulnerable to trisomy. Of all the trisomies identified by researchers, Down syndrome is known to affect nearly one of every 1,000 births worldwide.

These other trisomies are far less common but are worth knowing about.

Edwards Syndrome (Trisomy 18)

Edwards syndrome (trisomy 18) is rare, affecting only one of every 20,000 births. Around 95 percent of cases are caused by an extra chromosome 18. The remaining 5 percent of cases are due to an error known as translocation in which the building blocks of one chromosome is inserted into another.

Edwards syndrome is characterized by low birth weight, an abnormally small head, and defects in the heart, kidneys, lungs, and other organs. While a few children with Edwards syndrome survive to adolescence, the majority die within the first year (and often the first days) of life.

Patau Syndrome (Trisomy 13)

Patau syndrome (trisomy 13) is the third most common autosomal disorder among newborns after Down syndrome and Edwards syndrome. Most cases are related to a full trisomy; a very small proportion is caused by translocation or a similar condition known as mosaicism in which the chromosomal building blocks are rearranged.

Children with Patau syndrome will often have cleft lips and palates, extra fingers or toes, heart defects, severe brain abnormalities, and malformed or rotated internal organs. The severity of symptoms is such that a baby with Patau syndrome rarely lives past the first month.

Warkany Syndrome (Trisomy 8)

Warkany syndrome (trisomy 8) is a common cause of miscarriage and usually results in newborn death within the first months. Babies born with Warkany syndrome typically have a cleft palate, distinctive facial features, heart defects, joint malformation, abnormal or missing kneecaps, and an abnormally curved spine (scoliosis).

Babies with mosaic trisomy 9 have a far greater chance of surviving to adulthood. While some will have moderate intellectual deficits, others may have normal to above-normal intelligence. Mosaic trisomy 9 is also associated with an increased risk of myeloid leukemia.

Trisomy 16

Trisomy 16 is the most common autosomal trisomy seen in miscarriages, accounting for at least 15 percent of first-trimester pregnancy losses. Full trisomy 16 is incompatible with life. While most fetuses with this abnormality are spontaneously aborted by the 12th week of gestation, a few have survived into the second trimester.

By contrast, the chances of survival of children with mosaic trisomy 16 used to be considered bleak with most deaths occurring in early infancy. Advances in genetic research have since shown that some children previously unidentified with mosaic trisomy 16 have no abnormalities of any sort and that the risk of miscarriage and birth defects is directly related to the number of cells carrying the chromosomal mutation.

With that being said, more than half of babies with mosaic trisomy 16 will have fetal abnormalities, including musculoskeletal defects, distinctive facial features, undersized lungs, and an atrial septal defect (a hole between the upper chambers of the heart). Males will often have hypospadias in which the opening of the urethra develops on the shaft of the penis rather than at the end. Development delays may occur but are less common than other trisomies.

Trisomy 22

Trisomy 22 is the second most common chromosomal cause of miscarriages. Survival beyond the first trimester is rare in babies with full trisomy 22. The severity of physical and organ defects is such that babies carried to term are unable to survive for more a few hours or days.

Some babies with mosaic trisomy 22 do survive. The severity of birth defects is determined by the number of cells with the mutated chromosomal copy. Characteristic detects include heart abnormalities, kidney problems, intellectual disability, muscle weakness, and cognitive and developmental delays.

Trisomy 9

Trisomy 9 is a rare disorder in which a full trisomy is usually fatal within the first 21 days of life. Newborns with trisomy 9 will have a smaller head, distinctive facial features (including a bulbous nose and sloping forehead), a deformed heart, kidney problems, and often severe muscle and skeletal malformations.

Babies born with partial or mosaic trisomy 9 have a far greater chance of survival. This is especially true with mosaic trisomy 9 in which organ defects tend to be less severe and intellectual disabilities don't necessarily impede basic language, communication, or social-emotional development. Since the disorder was first identified in 1973, fewer than 60 cases of mosaic trisomy 9 have been positively identified in medical literature.

Klinefelter Syndrome (XXY Syndrome)

Klinefelter syndrome, also known as XXY syndrome, is a condition affecting males caused by an additional X chromosome. Men with Klinefelter syndrome typically produce little testosterone, resulting in reduced muscle mass, facial hair, and body hair. Characteristic symptoms include small testicles, delayed development, breast enlargement (gynecomastia), and reduced fertility.

The severity of symptoms can vary dramatically. Some men may also have learning disabilities, which are typically language-oriented, although intelligence will usually be normal. Testosterone replacement therapy is often used to treat the disorder alongside assisted fertility treatments for men wanting to father children.

Triple X Syndrome (XXX Syndrome, Trisomy X)

Some girls are born with triple X syndrome, involving an extra X chromosome. Triple X syndrome, also known as XXX syndrome, is not associated with physical features and often causes no medical symptoms at all. A small proportion of affected women may have menstrual irregularities as well as learning disabilities, delayed speech, and compromised language skills. However, most will develop normally and without impediment.

XYY Syndrome

Most boys born with an extra Y chromosome have no distinctive physical features or medical issues. If anything, men with XYY syndrome can sometimes be taller than average and may have an increased risk of learning disabilities, as well as delayed speech and language skills. The impairment, if any, tends to be mild. Most men with XYY syndrome have normal sexual development and are able to conceive children.

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