A baby’s biological sex is determined by genetics, specifically the chromosomes inherited from each parent. This process begins at conception, when genetic material from the egg and sperm establishes an individual’s foundational blueprint.
The Chromosomal Blueprint
Humans typically possess 23 pairs of chromosomes within each cell, totaling 46. Of these, 22 pairs are autosomes, carrying genetic information for general body functions. The remaining pair, the sex chromosomes, are designated X or Y.
Individuals with two X chromosomes (XX) develop as female, while those with one X and one Y chromosome (XY) develop as male. During reproductive cell formation (gametes), each parent contributes one chromosome from their sex chromosome pair. This ensures the offspring receives one sex chromosome from each biological parent.
The Father’s Genetic Contribution
The mother’s egg cell consistently carries a single X chromosome. The offspring will always receive an X chromosome from the biological mother. The biological father’s sperm cell, however, carries either an X or a Y chromosome.
The specific sex chromosome carried by the fertilizing sperm determines the offspring’s biological sex. If an X chromosome-carrying sperm fertilizes the egg, the resulting combination will be XX, leading to female development. Conversely, if a Y chromosome-carrying sperm fertilizes the egg, the combination will be XY, leading to male development. This makes the biological father the determinant of the baby’s sex. The Y chromosome contains a gene called SRY (Sex-determining Region Y), which initiates testes development and male characteristics.
Understanding Sex Development Variations
While the X and Y chromosome system describes the most common pathway for sex determination, biological sex development can be more complex. Variations in sex development (VSD), sometimes referred to as intersex conditions, encompass situations where an individual’s chromosomes, gonads, or anatomy do not fit typical male or female definitions. These variations are natural biological occurrences, not disorders, and arise from different genetic or hormonal factors.
Such variations can include atypical chromosome combinations, like XXY or XO, where individuals may have an extra or missing sex chromosome. Other causes can involve genetic mutations affecting hormone production or the body’s response to hormones, leading to differences in genital formation or internal reproductive organs. These conditions highlight the spectrum of human biological diversity, extending beyond the typical binary understanding of sex.