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 have 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 2 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, such as:

• 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 development.

• 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 first year.

• Turner syndrome. This is a condition in females that causes short height, ovarian problems, lack of fertility, and heart defects.