Humans, compared to animals, display a low-efficiency reproduction, i.e. they have lower chances to reproduce within a single reproductive cycle and generate a limited off-spring per couple. Nevertheless, humans are the winners of the evolution race, likely because they exchange low reproductive efficiency with higher genetic and phenotypic variability. Accordingly, the system grows to the highest level of complexity with sharply regulated interactions and, in parallel, plenty of physiologic and pathologic variants.

The environment has always been known as a main modulator of reproductive functions. Mammalians are supposed to regulate their reproductive output on the ability of the environment to sustain their offspring. Accordingly, the evolution has provided a variety of regulating pathways connecting the metabolism with the endocrine system, the gametogenesis and the embryogenesis.

The information for the development of the functional and morphologic phenotype of any individual is genetically encoded, i.e. it is written into long molecules called genes. We use to call genetics all that relates to the assortment of gene variants that constitute our inherited baggage whereas we call genomic the profile of activation of these genes, i.e. whether they are expressed or silenced.