Depending on the variant and how many cells are affected, somatic mosaicism may or may not cause health problems. When a proportion of somatic cells have a gene variant and others do not, it is called somatic mosaicism. As cells grow and divide, cells that arise from the cell with the altered gene will have the variant, while other cells will not. In mosaicism, the genetic change is not present in a parent’s egg or sperm cells, or in the fertilized egg, but happens later, anytime from embryonic development through adulthood. Variants acquired during development can lead to a situation called mosaicism, in which a set of cells in the body has a different genetic makeup than others. De novo variants are one explanation for genetic disorders in which an affected child has a variant in every cell in the body, but the parents do not, and there is no family history of the disorder. (It is often impossible to tell exactly when a de novo variant happened.) As the fertilized egg divides, each resulting cell in the growing embryo will have the variant. In other cases, the variant occurs in the fertilized egg shortly after the egg and sperm cells unite. In some cases, the variant occurs in a parent’s egg or sperm cell but is not present in any of their other cells. Some genetic changes are described as new (de novo) variants these variants are recognized in a child but not in either parent. Non-inherited variants can be caused by environmental factors such as ultraviolet radiation from the sun or can occur if an error is made as DNA copies itself during cell division. These variants cannot be passed to the next generation. Because non-inherited variants typically occur in somatic cells (cells other than sperm and egg cells), they are often referred to as somatic variants. Non-inherited variants occur at some time during a person’s life and are present only in certain cells, not in every cell in the body.Any variants that are present in that DNA will be present in the cells of the child that grows from the fertilized egg. When an egg and a sperm cell unite, the resulting fertilized egg cell contains DNA from both parents. These variants are also called germline variants because they are present in the parent’s egg or sperm cells, which are also called germ cells. Inherited (or hereditary) variants are passed from parent to child and are present throughout a person’s life in virtually every cell in the body.Gene variants can be inherited from a parent or occur during a person’s lifetime: Variants can affect one or more DNA building blocks (nucleotides) in a gene. This type of genetic change used to be known as a gene mutation, but because changes in DNA do not always cause disease, it is thought that gene variant is a more accurate term. A gene variant is a permanent change in the DNA sequence that makes up a gene.
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