Mosaic (genetics) facts for kids
In genetics, a mosaic means that an individual has two different sets of genotypes (genetic instructions) in their body. This happens even though they started from just one fertilized egg. So, a mosaic person or animal has cells with slightly different genetic information, all coming from that single original cell.
This interesting situation can happen because of a few things:
- Changes during mitosis (when cells divide)
- A gene mutation (a change in the genetic code) that happens while the body is developing
- A change in a chromosome during development
- X-inactivation: In female mammals, one of their two X chromosomes is randomly switched off in each cell.
A scientist named Curt Stern discovered this idea. In 1936, he showed that genetic mixing, which usually happens when new life begins (in meiosis), can also happen when body cells divide (in mitosis). When this occurs, it creates "somatic" (body) mosaics, meaning organisms with different types of genetic tissue.
Contents
Mosaics vs. Chimeras
Sometimes, people confuse mosaics with chimeras, but they are different. A mosaic comes from a single fertilized egg that develops different cell lines. A chimera, however, comes from two separate fertilized eggs that somehow combine into one individual.
Different Eye Colors
One easy way to see something similar to mosaicism is with eye colours. When someone has different colored eyes, or even different colors within one eye, it's called heterochromia iridis. This means 'different colored iris'. It can be caused by genetics or by an injury. For example, the singer David Bowie had eyes that looked different because of an injury that made one pupil stay wide open.
On this page, we are only talking about genetic mosaicism.
X-inactivation in Females
This is a natural process that happens in female mammals. Females have two X chromosomes, while males have only one. The two X chromosomes in a female are usually not exactly the same. They have the same genes, but at certain spots (called loci), they might have different versions of those genes (called alleles).
Early in the embryo's development, each cell randomly picks one of its two X chromosomes to switch off. This X chromosome stays switched off for the entire life of that cell and all the cells that come from it. This process is reversed in female germline cells, so all egg cells have an active X chromosome.
You can see this process clearly in calico cats and tortoiseshell cats. These female cats have different colors because the genes for their coat color are on the X chromosome. X-inactivation causes patches of cells to have one X chromosome active, and other patches to have the other X chromosome active, leading to their unique patterns.
X-inactivation is an epigenetic change. This means it's a way genes are switched on or off without changing the actual genotype (the genetic code itself). The cells that come from the embryo keep the same X-inactivation pattern. This can sometimes cause mild symptoms in female 'carriers' of certain genetic conditions linked to the X chromosome.
Mutations in Body Cells
Mutations (changes in DNA) that happen in body cells (called somatic mutations) can also lead to mosaicism. This means groups of cells in the body will have slightly different genetic information.
Somatic mutations are common at the beginning and end of human life. Research into Cancer has shown that these types of mutations are responsible for many kinds of cancer, like leukemia and other tumors.
Related pages
Images for kids
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Tulip flower showing mosaicism
See also
In Spanish: Mosaico genético para niños
