Popular understanding sees biological sex as a simple binary of female or male, and popular synonyms for biological sex are “anatomical sex,” or “physical sex” or just “sex.” And most legal definitions of ‘man’ and ‘woman’ still rely on this stark division.
However, the complex arrangement of sex traits in humans does not always support this simple binary. In some people, sex traits like chromosomes, external genitals, gonads (ovaries or testes), sex hormones, and secondary sex characteristics do not align the same way (not ‘all female’ or ‘all male’) . For example, some people are born with female chromosomes (XX) but have testes; some born with male chromosomes (XY) have ovaries.
In other individuals, a sex trait like external genitalia may fall outside the accepted standard for male or female, or fall in between, or may be ambiguous. And some individuals may possess both female and male sets of one characteristic – for example, they have both XX and XY chromosomes, or they are born with both ovarian and testicular tissue.
Together, these variations are known as intersex conditions. Cases where an individual’s anatomical sex is inconsistent with their chromosomal or gonadal sex are known as differences of sex development, or disorders or sex development (DSD). A 2006 scientific article estimated the frequency of DSDs at 1 in 4500 (Hughes 2006).
As advancements are made in the discovery and understanding of DNA, researchers are discovering smaller variations in gene sequencing that have lesser, but discernible effects.
Resources:
Nature, February 18, 2015. “Sex redefined.”
https://www.nature.com/news/sex-redefined-1.16943