Mutations in genes are usually a result of imperfect gene copying in meiosis (the production of egg cells and sperm cells), causing the offspring to have genes that were not present in the parents. An example of this is achondroplasia (the most common form of dwarfism). The cause of achondroplasia is a dominant gene. A dominant gene is one that you only have to have a single copy of in order to have the trait, and a recessive gene is one that you have to have two copies of in order to have the trait. For example (this is a slight over-simplification), if you have one copy of the blond gene and one copy of the brunette gene, you'll have brown hair because the brunette gene is dominant and the blond gene is recessive. The significance of this is that there have been observed cases of people with achondroplasia whose parents were not dwarfs. This means that neither parent had a copy of the gene for achondroplasia (if either of them had it, he/she would be a dwarf as well), but the child does have a copy, and therefor has the trait for dwarfism. This provides a real-life example of how new genes develop.
