What Are the Different Human Trisomies?

It is possible to have three of every chromosome, but many are fatal

Mother and baby girl with Down Syndrome celebrating birthday. Credit: JGI/Tom Grill / Getty Images

Down syndrome is the most common liveborn chromosome abnormality in humans, but did you know that almost any of our 23 pairs of chromosome can be seen in trisomic form? However, while any chromosome can be seen in trisomic form, very few trisomies are compatible with life. Some chromosomes –– 13, 18, 21, X, and Y –– can be lived with. While trisomies of chromosomes 15, 16, and 22 are often seen in miscarriages.

The other chromosomes –– 1 through 12,14,17,19 and 20 –– are almost never seen as full trisomies. Trisomies in these chromosomes can cause early miscarriage –– a miscarriage that occurs before it can be evaluated or treated.

Chromosomes and Miscarriage

Chromosomal anomalies are known to be the single most common cause of miscarriages. It is believed that the rate of miscarriages caused by chromosome abnormalities is higher than 50 percent.

In miscarriages, the majority of chromosomal anomalies (95 percent) are due to an abnormal number of chromosomes. About 60 percent are due trisomies and a further 20 percent are found to be missing a sex chromosome –– a condition called Turner syndrome. Another common chromosomal problem is triploidy. In triploidy, the fetus receives a full extra set of chromosomes, totaling of 69 chromosomes. 

While Down syndrome is the most common trisomy seen in newborns, it too carries a significant risk of miscarriage.

Approximately 25 percent of fetuses with trisomy 21 will either miscarry or be stillborn. 

Trisomy 18 (Edwards syndrome)

Trisomy 18 occurs in approximately 1 in 6,000 live births. making it about ten times rarer than Down syndrome. Most cases of trisomy 18 (95 percent) are caused by an extra number 18 chromosome. The remaining 5 percent are due to a translocation involving chromosome 18.

Unfortunately, children with trisomy 18 have severe physical birth defects. About 90 percent of infants with trisomy 18 will have some sort of heart defect.

Newborns with Edwards syndrome may also have kidney defects, and lung and diaphragm abnormalities. Because of severe physical birth defects, most infants with trisomy 18 (90 percent) will die in the first days of life. Those infants that do survive, have profound mental retardation. Most do not survive beyond the first few months of life, but some children do survive until adolescence.

Trisomy 13 (Patau syndrome)

Trisomy 13 (Patau syndrome) is the third most common autosomal abnormality among newborns, following Down syndrome (trisomy 21) and Edwards syndrome (trisomy 18). Trisomy 13 occurs in about 1 out of every 16,000 newborns. Most cases result from total trisomy 13 with a very small proportion of trisomy 13 being caused by mosaicism and translocation. Some trisomy 13 fetuses detected in mid-trimester do not survive to term or are stillborn.

Children with trisomy 13 can have cleft lip and palates, extra fingers and toes, malformed and rotated internal organs, severe congenital heart defects, and severe brain abnormalities.

Because of the severity of their organ defects, more than 80 percent of children born with trisomy 13, die in the first month. Less than 5 percent of infants with trisomy 13 survive into childhood.

47, XXY syndrome (Klinefelter syndrome)

Klinefelter syndrome, 47,XXY or XXY syndrome, is a condition caused by an extra X chromsome.

The condition occurs in about 1 in 500 males (1 in 1000 births). Affected individuals have two X chromosomes and one Y chromosome. Many individuals are unaware that they have Klinefelter syndrome as the differences due to having an extra X chromosome are not very obvious and often go undetected.

The main problems seen in Klinefelter syndrome are small testicles and reduced fertility. A variety of other physical and behavioral differences are common; however, the severity of these symptoms can vary from person to person. 

47,XYY Males

About 1 in 1,000 boys are born with an extra Y chromosome and have a 47,XYY karyotype. Most often, this extra Y chromosome causes no unusual physical features or medical problems. Males with 47,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. Developmental delays and behavioral problems are also possible, but these characteristics vary widely among affected boys and men.

Most males with 47,XYY syndrome have normal sexual development and are able to conceive children.

47,XXX (triplo-X, trisomy X, and XXX syndrome)

About 1 in 1000 girls are born with Triple X syndrome. Triple X syndrome often has no associated physical features or medical problems. A small proportion of women with the condition may have menstrual irregularities as well as learning disabilities, delayed speech, and language skills. Many individuals with 47,XXX syndrome have completely normal physical and mental development.

Trisomies More Likely to Cause Miscarriage 

Trisomy 15. Trisomy 15 is rare. Most pregnancies with trisomy 15 end in early miscarriage. In pregnancies that have progressed, the fetus often has abnormalities of their facial and cranial features, hands and feet, and growth delays. Trisomy 15 has been linked to Prader Willi syndrome.

Trisomy 16. Trisomy 16 is the most common autosomal trisomy seen in miscarriages and accounts for at least 15 percent of first trimester miscarriages. Most pregnancies with trisomy 16 are lost around 12 weeks although a small percent may be lost in the second trimester. A few fetuses with mosaic trisomy 16 have survived until birth. Most of these infants have growth failure, psychomotor issues, and die early in infancy.

Trisomy 22. Complete trisomy 22 is the second most common chromosomal cause of miscarriages. Survival beyond the first trimester of pregnancy is rare. Complete trisomy 22 is very rare. Most fetuses with complete trisomy 22 die before or shortly after birth due to severe birth defects.

Sources:

Hay, William W., Deterding, Robin R. , Levin, , Myron J., Sondheimer , Judith M., Current Pediatric Diagnosis & Treatment, The McGraw-Hill Companies, Inc. Eighteenth Edition. 2007

Nelson-Anderson, D., Waters, C., Genetic Connection A Guide to Documenting your Individual and Family Health History Sonters Publishing, 1995

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