Existence of biological sex as binary

Medical definition of sex
Farlex Medical Dictionary


 * The fundamental distinction, found in most species of animals and plants, based on the type of gametes produced by the individual; also the category to which the individual fits on the basis of that criterion.
 * chromosomal sex the sex as determined by the presence of the XX (female) or the XY (male) genotype in somatic cells, without regard to phenotypic manifestations. Called also genetic sex.
 * endocrinologic sex the phenotypic manifestations of sex determined by endocrine influences, such as development of breasts and genital organs.
 * genetic sex chromosomal sex.
 * gonadal sex the sex as determined on the basis of the gonadal tissue present (ovarian or testicular)

Merriam-Webster Medical Definition


 * Either of the two major forms of individuals that occur in many species and that are distinguished respectively as male or female especially on the basis of their reproductive organs and structures
 * the sum of the structural, functional, and sometimes behavioral characteristics of organisms that distinguish males and females

Sex is defined as the type of gamete reproductive anatomy is organized to produce. Male bodies develop towards the production of small gametes or sperm; female bodies develop towards the production of large gametes. There is no intermediate gamete.

99.98 percent of births are unambiguously male or female

sex is genetically determined by a functioning SRY gene located on the Y chromosone. With SRY activation, the gonads differentiate into testis. The fetus develops anatomy to support the production of small gametes. In the absence of SRY activation, the gonads differentiate into ovaries, and the fetus develops anatomy to support the production of large gametes. This is why developmental biologists refer to SRY as the "master switch" gene for mammalian sex determination, because without its activation, the fetus develops as a female.


 * 1) On very rare occasions, a fetus may develop with two X chromosomes and one Y, or three Xs and one Y, or even four Xs and one Y. Despite the extra X chromosomes, all these cases develop as males thanks to the presence of SRY on the Y chromosome.
 * 2) Sometimes, a translocation of the SRY gene results in it being placed on an X chromosome in a fetus with two X chromosomes. Thanks to the presence of SRY, the fetus develops testicular tissue despite having no Y chromosome.
 * 3) In exceptional cases, a fetus may develop a female phenotype with an XY karyotype. Because the SRY gene remained inactive, they developed as females.

two gammete types