What characteristic primarily determines an individual's sex?

The characteristic that primarily determines an individual's sex is the sex chromosomes. In many organisms, including humans, sex is typically determined by the presence of specific sex chromosomes. Humans have two types of sex chromosomes: X and Y. Females usually have two X chromosomes (XX), while males have one X and one Y chromosome (XY).

The presence of the Y chromosome, which carries the SRY gene, initiates male sex determination during embryonic development. This leads to the development of male characteristics. In contrast, the absence of a Y chromosome (having two X chromosomes) leads to the development of female characteristics.

Other options relate to genetic and phenotypic traits but are not the primary determining factor for sex. Autosomes carry genes unrelated to sex determination, phenotype refers to the observable characteristics resulting from the genotype, and alleles are different versions of a gene but don't directly dictate sex. Thus, sex chromosomes are the key element in defining an individual's biological sex.