Epidemiology and Genetics of Dizygotic Twinning

In this chapter, we discuss features of DZ twinning (DZT) and offer an overview of the recent findings regarding the genetic influences on DZT. DZT is influenced by, among other factors, maternal age, parity, ancestry, having twins in the family/pedigree, and the use of medically assisted reproduction (MAR) techniques. DZT ratios differ substantially between ancestral groups, and this observation, together with the familial clustering of twins, likely points to the genetic influences on DZT. While candidate gene studies and linkage projects failed to identify genetic loci associated with DZT, genome-wide association studies (GWAS) now are successful. GWAS results in European populations identify biologically relevant loci near the follicle-stimulating hormone beta subunit (FSHB), gonadotropin-releasing hormone 1 (GNRH1), and the follicle-stimulating hormone receptor (FSHR) and several more genes with a hitherto unknown relation with female fertility. The DZT trait shows genetic correlations with indices of female fertility and infertility such as number of children ever born and polycystic ovary syndrome (PCOS). Further research into the genetics of DZT should optimize the use of additional omics data, focus on including other ancestries than European, and aim at understanding the relation between DZT and female fertility and infertility.