A mutation, in regard to genetics, is a permanent change in the DNA sequence that makes up a gene. Mutations can range in size, and can affect a single building block of DNA (base pair) to a large segment of a chromosome that impacts multiple genes. Gene mutations can be put into two different categories:
Hereditary mutations: these are inherited from a parent and are present throughout an organism's life. These mutations can also be called germline mutations because they are present in the parent's egg or sperm cells, which are called germ cells.
Acquired (somatic) mutations: occur at some point in an organisms life (therefore are not inherited) and are only present in certain cells, unlike hereditary mutations. These changes are usually caused by environmental factors or can occur when a mistake is made during DNA cell division. These types of mutations in somatic cells (or all other cells besides sperm and egg cells) can also not be passed to the next generation.
Somatic mutations that happen in a single cell early development stages can lead to mosaicism. These changes are not present in the parent's cells or the fertilized egg, but happen when the embryo starts to develop into multiple cells. As the cells continue to grow and divide, cells with the altered gene may have the mutation while others may not. Depending on the mutation and how many cells are affected, mosaicism may or may not case health problems.
Most disease-causing mutations are uncommon, but genetic alterations that occur in more than 1% of the population are called polymorphisms. Polymorphisms are common and considered a normal genetic mutation are responsible for many normal differences like eye color, hair color and blood type. Usually polymorphisms have no negative health effects, but there is always the chance. 
1. "What Is a Gene Mutation and How Do Mutations Occur?" Genetics Home Reference. Web. 10 Mar. 2015. http://ghr.nlm.nih.gov/handbook/mutationsanddisorders/genemutation.
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