What is a Gene Mutation? How Do Mutations Occur?
A gene mutation is permanent changes in the DNA sequence that makes up the gene. A mutation can affect anywhere from a tiny block of DNA to a large part of the chromosome that contains many genes. Gene mutations fall into two main classes:
1) Hereditary Gene Mutations: Inherited gene mutations inherited from a parent occur in almost every cell in an individual’s life. This mutation is also called germline mutations because it originates from egg and sperm cells called germ cells.
2) Acquired (Somatic) Gene Mutations: Somatic gene mutations are a type of mutation that occurs at a specific time in an individual’s life and only in certain cells of the body.
These mutations can be caused by environmental factors such as radiation from UV rays from the sun, or from an error that occurs when DNA replicates itself during cell division. Mutations that occur in somatic cells (body cells other than sperm and eggs) are not passed on to future generations. Mutations called de novo (new) mutations can be both hereditary and somatic. In some cases, only the sperm or egg cells are mutated, while in some cases, a gene mutation may occur in the fertilized egg that occurs shortly after the egg and sperm unite. (Usually it’s impossible to tell when de novo mutations occur.)
As the fertilized egg divides, every cell in the growing embryo has a mutation. De novo mutations may explain why a child who carries the mutation in all body cells has genetic diseases that have never been found in their family or family history. Somatic gene mutations that occur in a single cell early in embryonic development can lead to a condition called mosaicism. These genetic changes are not found in the parents’ sperm or egg cells or in the fertilized egg cell. However, it occurs when the embryo is still made up of several cells. Since all cells divide during growth and development, cells derived from the cell containing the mutated gene will have a gene mutation, while other cells will not have mutations.
Whether mosaicism causes health problems depends on the mutation and how many cells are affected by the mutation. While gene mutations that cause disease in the general population are very rare, other genetic changes are more common. Mutations seen in more than 1% of the population are called polymorphisms. It is common enough to be considered a normal variation in DNA. It is the cause of many normal differences such as eye color, hair color and blood type in humans. Although polymorphism does not have a negative effect on human life, some variations may affect the risk of developing certain disorders.