Genetic disorders can be grouped into three different categories. Single gene disorders result from a change or mutation in a single cell. Cystic fibrosis is an example of this type of disorder. Chromosome disorders are caused by too few or too many genes located on chromosomes, or other changes in chromosomes. Down syndrome is a type of chromosome disorder. Small changes in genes along with environmental factors cause complex, or multifactorial, disorders. Both cancer and heart disease fall into this third category of genetic disorders.