A long-held paradigm of reproductive biology is that sexual development is a two-step process; chromosomal sex directs the embryonic gonads to form ovaries or testes, which then secrete sex hormones to feminise or masculinise the rest of the body. Yet, the concept has increasingly been challenged by curious data that do not fit the model. Evidence from gynandromorphic birds supports direct genetic effects upon sexual differentiation in addition to sex steroid hormones. Gynandromorphs are bilateral sex chimeras, male on one side of the body and female on the other. We examined a naturally occurring gynandromorphic chicken that was chromosomally ZZ on one side of the body and largely ZZ on the other, but with 10% ZW cells, based on karyotyping. The gonads of this bird at sexual maturity were ambiguous. The right gonad was a testis, with SOX9+ Sertoli cells, DMRT1+ germ cells and active spermatogenesis. The left gonad was atypical . Histologically, it was primarily testicular, but with a small number of peripheral follicles. The bird had very low levels of serum 17b-estradiol and high levels of testosterone. Yet the bird was female on one side of the body. Despite the low percentage of ZW cells on that side, the bird was asymmetrically female. This indicates that sexually dimorphic structures such as the wattle, spur and feathering must be at least partly independent of sex steroid effects. Given the lack of chromosome-wide dosage compensation in birds, various sexually dimorphic features may arise due to Z gene dosage differences between the sexes. In the gonads, the Z-linked gene, DMRT1, directs testis formation. Monoallelic DMRT1deletion causees ovary formation yet the somatic tissues remain “male”. This again supports the notion of cell autonomous sex in birds. Our lab is currently conducting studies to determine how DMRT1 induces testis formation in the chicken.