What Is Mosaic Trisomy 8?

Medically Reviewed by Amita Shroff, MD on July 29, 2023
Written by WebMD Editorial Contributors
3 min read

Mosaic trisomy 8 is a rare genetic condition caused by an extra chromosome. Sometimes called trisomy 8 mosaicism, this condition develops well before birth. It's a result of an abnormality in how cells divide and replicate from the earliest stages of pregnancy. It's a spontaneously occurring condition. It is not due to anything an expectant parent does during pregnancy.‌

The condition is not usually life-threatening for the baby and doesn't present a danger during pregnancy. Once the baby is born, they will likely have physical disabilities. Some people with mosaic trisomy 8 also have intellectual disabilities.

Chromosomes are thread-like structures that contain our DNA. DNA is the inherited genetic material that you get from your parents. A copy of your entire chromosomal structure is present in every cell in your body. Chromosomes come in pairs, with one from each of your biological parents. Humans have 23 pairs of chromosomes.

Trisomy and mosaicism are situations where your chromosomes don't develop in a typical way. Chromosomes are part of the genetic instruction manual that tells each of your cells how to develop and what to do. Unusual chromosomal structure or variable distribution of chromosomes can result in physical and cognitive differences.‌

Trisomy. Sometimes, you can have an extra chromosome. Instead of the typical pair, there will be three chromosomes in a row. This is called trisomy. The presence of an extra chromosome can result in lifelong physical or cognitive differences. Down syndrome is an example of a trisomy condition (Trisomy 21).‌

Mosaicism. Trisomy can affect the DNA in every cell in your body. However, in some cases, not all cells have the extra chromosome. This mix of typical cells and cells with a chromosomal trisomy is called mosaicism.

The formation of the extra chromosome and the distribution of chromosomes in cells happens while a baby is developing in the womb. Most of the time, they occur spontaneously with no known trigger. They are not the result of any genetic condition either biological parent might have.

The age of the gestational parent is the main risk factor for this and all trisomy conditions. People who are pregnant in their 30s and 40s have a higher rate of babies with trisomy conditions. You may want to talk with your doctor about prenatal testing to screen for genetic disorders.

In mosaic trisomy 8, there is an extra copy of the 8th chromosome in some of your cells. It's not a common condition, occurring in only about 1 baby in 25,000 to 50,000 live births. It shows up more frequently in boys than in girls. There have been about 120 cases of mosaic trisomy 8 described in the medical literature.

People with mosaic trisomy 8 tend to have physical differences. Some of the more common physical characteristics of the condition include:

  • Distinctive facial features such as a prominent forehead, widely-spaced eyes, deeply set eyes, and broad upturned nose
  • Small jaw and teeth
  • Highly arched or cleft palate
  • Short neck with extra skin folds
  • Distinctive body shape with a narrow chest, shoulders, and pelvis
  • Kidney problems 
  • Heart abnormalities
  • Bent fingers and stiff joints
  • Missing or malformed kneecap
  • Spinal issues such as scoliosis
  • Eye conditions such as strabismus 

Cognitive abilities in people with mosaic trisomy 8 are variable. Some people have typical intelligence with no learning differences. Others have intellectual disabilities, with an IQ between 50 and 75.

Experts have found that people with mosaic trisomy 8 may have an increased risk of certain health conditions. They may be susceptible to infections. The condition may increase the risk for certain cancers such as Wilms tumor, myelodysplasia, and acute myeloid leukemia. Life expectancy for people with mosaic trisomy 8 is normal unless other health conditions such as cancer arise.

There is no treatment for genetic conditions such as trisomy or chromosomal mosaicism. You can't change or repair the structure of chromosomes. Mosaic trisomy 8 is a lifelong condition.

There are treatments to help manage symptoms caused by the condition. Some people benefit from occupational therapy and physical therapy to assist with their physical disabilities. People who need accommodations in school due to cognitive disabilities can work with teachers and administrators to get proper services.