Genetic mosaicism is a condition in which an individual has two or more populations of cells with different genetic makeups, even though all those cells belong to the same person.

To understand this, imagine that the human body develops from a single fertilised egg, called a zygote. Normally, as this cell divides repeatedly, it produces daughter cells that carry essentially the same genetic information. These cells eventually develop into different tissues and organs, such as the brain, heart, skin and blood.

However, sometimes a genetic change occurs during one of these cell divisions after fertilisation. As a result, the cells that develop from the affected cell may carry the genetic alteration, while cells developing from unaffected cells retain the original genetic makeup.

This creates two or more genetically different cell populations within the same individual. This phenomenon is called genetic mosaicism.

For example, imagine that a genetic change occurs during the third cell division of an embryo. Some of the cells produced from that division may carry the alteration, while others remain genetically unaffected. As the embryo continues developing, both populations multiply and contribute to different tissues.

Mosaicism can involve different types of genetic changes, including:
– Chromosomal abnormalities, such as an additional chromosome in some cells.
– Structural chromosome changes, such as deletions or duplications.
– Single-gene variants affecting only a proportion of cells.
– Changes in chromosome number, including mosaic aneuploidy.
One well-known example is mosaic Down syndrome, where an individual has two cell populations: some cells have 46 chromosomes, while others have 47 chromosomes with an additional copy of chromosome 21.

The proportion and distribution of affected cells can vary considerably between individuals and tissues.

Importantly, mosaicism does not always cause health problems. Some individuals may have noticeable developmental or medical features, while others may have very mild manifestations or remain asymptomatic.

Therefore, genetic mosaicism is essentially about genetic differences between cells within the same individual, rather than differences between individuals.