Genetics of Sexual Dimorphism in Humans

Abstract

Sexual dimorphism describes the suite of morphological, physiological and behavioural phenotypes that distinguishes males and females. In humans, the determination of sex is triggered early in embryogenesis by the presence or absence of a Y chromosome and its SRY locus, prompting a regulatory cascade of genetic, epigenetic and hormonal changes. A distinct male and female genetic state precipitates the development of sexually dimorphic characters including primary and secondary sexual traits. Recent genomic studies have also found sexually dimorphic differences in gene expression across surveyed adult tissues that may be modulated by sex‐specific cis‐regulatory elements, alternative splicing and differentially methylated sites. Such sexual dimorphisms at both phenotypic and molecular levels have important implications in disease onset, progression and treatment. New molecular, genomic and clinical data mining tools are providing novel insight into the emerging field of gender medicine with a focus to understand sex differences in disease. Sexually dimorphic characters, starting with our earliest eukaryotic ancestors, can evolve via sexual conflict with direct and indirect consequences on the patterning of the genomic landscape. Key Concepts Anisogamy evolved early in eukaryotic evolution from an isogamous ancestor. Human sex determination is based on the sex chromosome inherited from the father. The presence or absence of the Y‐linked SRY locus triggers a cascade of genetic and hormonal signals in the early fetus. Androgen and testosterone, among the initial hormones produced in the fetus, decide the sexual fate of the early gonads that eventually develop into primary reproductive organs found in the adult. Secondary sexual characteristics that develop during puberty distinguish males from females at the morphological level. Sex‐biased gene expression may be modulated by cis‐regulatory elements (CREs) and sex‐specific alternative splicing. Differences in the male and female epigenomes include distinct methylated patterns such as imprinting and result in differential gene expression. Sexual selection and sexual conflict may play a role in the origins and maintenance of sexually dimorphic traits. Sexual dimorphism at the genetic and cellular levels is becoming an important focus of disease research owing to the recent policy shifts in government funding to address gender health disparities.

Gender and age effects on dental and palatal arch dimensions among full siblings

“…1C) Maximum distance between cuspal tips of upper and lower canines, first and second premolars, and first molars [12,13] Tooth-size arch length discrepancy Difference between arch length and the total mesiodistal width of the tooth anterior to first molars Palatal arch dimension [14] (Fig. 1D) Sagittal plane: palatal length (mm) -horizontal projection of IP-M line; palatal slope (°) -the slope of the maximum palatal height versus the horizontal axis; maximum palatal height (mm) Horizontal plane: angles between the IP-RP and the IP-M lines (°) Frontal plane: palatal width and mid-palatal height (distance; mm) at the first permanent molar (16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26), second premolars (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25), first premolars (14-24), and canines (13-23)…”

“…At the phenotypic and molecular levels, such sexual dimorphism has important implications for disease onset, progression, and treatment. Recent research has provided novel insights into sex/gender medicine, focusing on sex/gender-related differences in disease [15].…”

“…As for the PAD (Fig. 1D), the incisive papilla (IP), the posterior-most limit of the palatal raphe (PR), the palatal sulci of the first molars (16,26), the gingival margins of the premolars (15,25,14,24), and the gingival margins of the canines (13,23) were identified and marked accordingly. The intermolar (16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26) line and its perpendicular line starting from IP were traced.…”