Abstract: The chicken (Gallus gallus) is an important model organism in genetics, developmental biology, immunology and evolutionary research. Moreover, besides being an important model organism the chicken is also a very important agricultural species and an important source of food (eggs and meat). The availability of the draft chicken genome sequence provided many possibilities to in detail study a variety of genomic changes during evolution using a comparison between chicken and mammals. For example, compared to mammals, the use of a Z/W sex determination system is a special aspect of the avian genome where the female is the heterogametic sex (ZW) and the male is the homogametic (ZZ) sex. A comparison of the genomic sequences of platypus, chicken and human showed that sex chromosomes evolved separately in birds and mammals.