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7430580 
Journal Article 
Sex Determination and Differentiation in Birds 
Kuroiwa, A; , 
2018 
391-405 
The sex of birds is genetically determined. Females have a heterogametic sex chromosome constitution (ZW), whereas males are homogametic (ZZ). Genes carried on these sex chromosomes control gonadal differentiation and development during embryogenesis. There are two hypotheses for the mechanisms of sex determination. One proposes that the dosage of genes on the Z chromosome determines the sexual differentiation of undifferentiated gonads, and the other proposes that W-linked genes dominantly determine ovary differentiation or inhibit testis differentiation. Z-linked dmrt1, which is a strong candidate for an avian sex-determining gene, supports the former hypothesis. Although no candidate for the W-linked gene has been identified, extensive evidence for spontaneous sex reversal in birds and aneuploid chimeric chickens with an abnormal sex chromosome constitution strongly supports the latter hypothesis. Undifferentiated and bipotential gonads differentiate into either testes or ovaries via several genes. The developed gonads release sex hormones to masculinize or feminize the brain and body. However, this process cannot explain spontaneous sex-chimeric birds, i.e., gynandromorphs, in which one side of the bird appears male and the other female. This chapter introduces the sex-determining mechanism as well as the genes and sex hormones mainly involved in gonadal differentiation and development of the chicken.