Medical Genetics: Mosaicism
What is mosaicism?
Mosaicism is when a person has 2 or more genetically different sets of cells in his
or her body. Chromosomes are stick-shaped structures in the middle of each cell in
the body. Each cell has 46 chromosomes grouped in 23 pairs. A person with mosaicism
may have some cells in his or her body with 46 chromosomes, but other cells with 47
chromosomes. This can cause health problems in the body.
What causes mosaicism?
Mosaicism may be caused by an error in mitosis. Mitosis (my-TOH-sis) is the dividing
of body cells. It’s how a baby in the womb grows. Mitosis causes the number of chromosomes
to double to 92, and then split in half back to 46. This process repeats constantly
as the baby grows. Mitosis continues throughout your lifetime. It replaces skin cells,
blood cells, and other types of cells that are damaged or naturally die.
If there is an error in mitosis, a cell doesn’t split evenly into two cells. The result
is that some cells have the normal number of 46 chromosomes, and other cells have
more (47) or fewer (45) chromosomes.
Types of conditions caused by mosaicism
Mosaicism can cause many different kinds of disorders, including:
Ichthyosis with confetti. This is a disorder that causes red, scaly skin all over the body.
Klinefelter syndrome. This syndrome can cause low amounts of testosterone. This can lead to problems with
sexual development, and other issues.
Klippel-Trenaunay syndrome. This disorder causes a red birthmark called a port-wine stain. It also causes excess
growth of soft tissues and bones, and abnormal veins.
Mosaic Down syndrome. Down syndrome is a condition that causes intellectual disabilities and delays, weak
muscles, and flat facial features. It can also cause a heart defect, digestive problems,
thyroid problems, and other health issues.
Pallister-Killian mosaic syndrome. This is a developmental disorder that causes weak muscles, intellectual disability,
thin hair, patches of abnormal skin color, and other birth defects.
Ring chromosome 14 syndrome. This condition causes seizures, intellectual disability, and delayed speech and motor
SOX2 anophthalmia syndrome. This is a rare disorder that can cause a child to be born without eyeballs. It can
also cause seizures, brain problems, and delayed growth.
Triple X syndrome. This condition is caused by an extra X chromosome in each of a female’s cells. It
can cause learning disabilities, delayed language skills, and problems with motor
skills and muscles.
Trisomy 18. This condition can cause slow growth in the womb, heart defects, a small head, and
other defects. Only a small number of children with this condition live past their
Turner syndrome. This is a condition in females that causes short height, ovarian problems, lack of
fertility, and heart defects.