What is the basis for determining sex?

The presence of sex chromosomes is used to determine sex. 

The typical human karyotype consists of 46 chromosomes arranged in 23 pairs. 

The sex chromosomes, which determine an individual's sex, are two of these pairs. The two sex chromosomes in females are both X chromosomes. Males have one X chromosome and one Y chromosome. 

The presence of two X chromosomes or an X and a Y chromosome determines whether female or male sexual characteristics develop. 

The genes on the sex chromosomes direct the development of specific male or female reproductive organs, secondary sexual characteristics, and other sexually dimorphic traits.