We inherit our DNA from our biological parents—half from our father and half from our mother. Comprehension of how genetic information is passed on from generation to generation aids our understanding of genetic inheritance. Errors that can arise during cell division both to create gametes and to maintain the body can lead to changes of evolutionary advantage or to detriment. Chromosomal aberrations are extremely common and affect at least 7.5% of all conceptions. They include too many or too few chromosomes, missing pieces of chromosomes and mixed-up pieces of chromosomes. In most cases, the pregnancy ends with spontaneous miscarriage, with a live birth frequency of only 0.6%. Aberrations can be classified into numerical errors and structural errors. Only approximately 90% of chromosomally normal conceptions reach full term. The impact of errors that lead to medical disorders and the consequences on people’s lives is the reason for this book. This chapter describes the process of cell division and the creation of gametes and gives insight into how errors arise.