What is a gene variant and how do variants occur? Show A gene variant is a permanent change in the DNA sequence that makes up 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. Variants can affect one or more DNA building blocks (nucleotides) in a gene. Gene variants can be inherited from a parent or occur during a persons lifetime:
Some genetic changes are described as new (de novo) variants; these variants are recognized in a child but not in either parent. In some cases, the variant occurs in a parents egg or sperm cell but is not present in any of their other cells. In other cases, the variant occurs in the fertilized egg shortly after the egg and sperm cells unite. (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. 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. 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. In mosaicism, the genetic change is not present in a parents egg or sperm cells, or in the fertilized egg, but happens later, anytime from embryonic development through adulthood. As cells grow and divide, cells that arise from the cell with the altered gene will have the variant, while other cells will not. When a proportion of somatic cells have a gene variant and others do not, it is called somatic mosaicism. Depending on the variant and how many cells are affected, somatic mosaicism may or may not cause health problems. When a proportion of egg or sperm cells have a variant and others do not, it is called germline mosaicism. In this situation, an unaffected parent can pass a genetic condition to their child. Most variants do not lead to development of disease, and those that do are uncommon in the general population. Some variants occur often enough in the population to be considered common genetic variation. Several such variants are responsible for differences between people such as eye color, hair color, and blood type. Although many of these common variations in the DNA have no negative effects on a persons health, some may influence the risk of developing certain disorders.
The Centre for Genetics Education provides a fact sheet discussing variations in the genetic code. An introductory explanation of what variants are and how they occur is provided by Your Genome from the Wellcome Genome Campus. KidsHealth from Nemours offers an introduction to genes, genetics, and genetic changes. Additional information about genetic alterations is available from the University of Utah fact sheet "What is Mutation?" Using animations and videos, DNA From the Beginning (Cold Spring Harbor National Laboratory) describes the early experiments that helped researchers understand genetic variants. Other chapters in Help Me Understand Genetics Genetics Home Reference has merged with MedlinePlus. Genetics Home Reference content now can be found in the "Genetics" section of MedlinePlus. Learn more The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health. |
