ABSTRACT
Introduction Genetic variability and epigenetic factors affect reproduction and fertility from one generation to the next. This starts with sex determination and the onset of gametogenesis and finishes at the end of the reproductive period. From each generation to the next, the haploid complement of the human genome is transmitted. The diploid complement contains 23 pairs of chromosomes, each containing hundreds or thousands of genes. The estimated 25000 genes each make on average several proteins. The nuclear genome involves three billion base pairs which are subject to countless variations that may affect health and disease. Furthermore, there is a small mitochondrial genome. Sexual reproduction and genetics are firmly con-
nected. Sex determination, which takes place during embryogenesis, is determined by the chromosomal sex. Sexual reproduction is characterised by the formation of haploid gametes which combine at fertilisation to give rise to a new diploid individual. The gametes are all genetically unique. During meiosis new combinations of the genetic characteristics are formed, which are present on the homologous chromosomes.
