Biological sex refers to the classification of an individual as male or female based on biological characteristics. These encompass reproductive organs, chromosomes, and hormones. Sex is assigned at birth by observing external genitalia, but is fundamentally determined by genetic and developmental factors. This biological classification is distinct from how a person identifies or expresses themselves.
Genetic Blueprint of Biological Sex
The foundation of biological sex lies in an individual’s genetic blueprint, specifically the sex chromosomes. Humans possess 46 chromosomes in each cell, with two designated as sex chromosomes: X and Y. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
The presence or absence of the Y chromosome directs sex determination. The SRY gene on the Y chromosome instructs the creation of the SRY protein. This protein binds to DNA, influencing other genes. It triggers male gonad (testes) development and prevents female reproductive structure formation. Without the SRY gene, as in XX individuals, female gonads (ovaries) develop.
How Biological Sex Develops
After genetic determination, developmental events lead to physical sex manifestation. In embryos with a Y chromosome, the SRY gene differentiates bipotential gonads into testes around the seventh week. These testes then produce hormones, primarily testosterone and Anti-Müllerian Hormone (AMH).
Testosterone promotes internal male reproductive organ development from Wolffian ducts (e.g., epididymis, vas deferens, seminal vesicles). Its metabolite, dihydrotestosterone (DHT), forms external male genitalia (penis and scrotum). AMH causes Müllerian duct regression, preventing female reproductive structure development. Without these male hormones, bipotential gonads become ovaries, and Müllerian ducts form the female reproductive tract.
During puberty, hormonal changes drive secondary sex characteristic development. The hypothalamus releases gonadotropin-releasing hormone (GnRH), stimulating the pituitary gland to produce luteinizing hormone (LH) and follicle-stimulating hormone (FSH). In males, LH stimulates Leydig cells in the testes to produce testosterone, leading to increased muscle mass, bone density, body hair, and voice deepening. In females, FSH stimulates ovarian follicles, and LH acts on theca cells to produce androstenedione, converted to estrogen. Estrogen promotes breast development, pelvic widening, uterine and vaginal growth, and menstruation.
Variations in Biological Sex Development
Biological sex development does not always follow XX or XY patterns, leading to variations in sex characteristics, or intersex conditions. These involve differences in chromosomes, gonads, or reproductive anatomy that do not fit male or female definitions. They are estimated to occur in up to 1-2% of the population.
Variations can stem from differences in genes, hormone response, or both. For example, XX individuals may have more male-appearing external genitalia due to conditions like congenital adrenal hyperplasia (CAH), where increased androgen exposure before birth affects development. Conversely, XY individuals might have incompletely formed or female-appearing external genitalia, as seen in complete androgen insensitivity syndrome (AIS), where cells cannot respond to male hormones despite their production. Other variations include differing chromosome combinations, such as XXY (Klinefelter syndrome) or a missing X chromosome (Turner syndrome), affecting hormone levels and sexual development.
Biological Sex Versus Gender
Biological sex refers to physical characteristics at birth, including chromosomes, hormones, and reproductive anatomy. It is assigned at birth based on observed physiological markers. Males have XY chromosomes, higher testosterone, and testes; females have XX chromosomes, higher estrogen, and ovaries.
Gender, in contrast, is a social construct encompassing identity, roles, and behaviors within society. It involves how a person perceives and expresses themselves, which may not align with their assigned biological sex. Unlike biological sex, gender is not a binary concept and exists across a broad spectrum. While biological sex is rooted in observable attributes, gender is shaped by cultural norms, societal expectations, and personal identification.