In some organisms, sex is genetically determined, but there are no obvious differences in the chromosomes of males and females: there are no sex chromosomes. These organisms have genic sex determination: genotypes at one or more loci determine the sex of an individual. Scientists have observed genic sex determination in some plants, fungi, protozoans, and fishes.

It is important to understand that even in chromosomal sex-determining systems, sex is actually determined by individual genes. In mammals, for example, a gene (SRY, discussed later in this chapter) located on the Y chromosome determines the male phenotype. In both genic sex determination and chromosomal sex determination, sex is controlled by individual genes; the difference is that with chromosomal sex determination, the sex chromosomes that carry those genes look different in males and females.