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Almost every living organism transmits DNA to its offspring during reproduction. The particular combination of DNA an individual inherits is called a genotype. While the genotype is the biological blueprint, the phenotype is the observable manifestation of that blueprint—traits ranging from blood type to a flower’s petal color or a person’s sensitivity to cilantro.

TL;DR

Genotype refers to DNA, phenotype to its expression. In humans, females carry two X chromosomes (XX) and males carry one X and one Y (XY). The presence or absence of the Y chromosome and its SRY gene determines the development of sex organs, but gender identity can differ from biological sex.

Sex Determination Begins In Utero

Human reproduction is sexual, involving the fusion of a spermatozoon and an ovum to form a zygote. The sex of the embryo is dictated solely by the pair of sex chromosomes: an X from the mother and either an X or a Y from the father. The SRY gene on the Y chromosome triggers the release of hormones that promote male genital development. Embryos lacking a Y chromosome develop female reproductive structures.

Females and Sex‑Linked Traits

Both X chromosomes contain thousands of genes, whereas the Y chromosome carries only a handful. Many sex‑linked traits—such as red‑green color vision—are located on the X chromosome. Because the trait is recessive, a female requires two copies of the defective gene to express color blindness, whereas a male needs only one. Consequently, red‑green color blindness is far more common in males. Over 1,000 sex‑linked traits, including hemophilia and male‑pattern baldness, disproportionately affect males.

The Spectrum Between Male and Female Phenotypes

While the XX genotype is typically associated with female phenotypes and the XY genotype with male phenotypes, numerous exceptions exist. Gender identity—the personal sense of being male, female, or non‑binary—does not always align with chromosomal sex. Non‑binary identities encompass a wide range of experiences beyond the traditional male/female categories. Intersex individuals may have chromosomal variations (e.g., three sex chromosomes or missing one) or ambiguous physical traits, illustrating that the biological and social dimensions of sex are diverse and fluid.

For more information about gender identity, sexual orientation, and related topics, please refer to the links in the References section.

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