The secrets of bird sex revealed

University of Edinburgh scientists have discovered why some chickens are half male and half female, a question that has remained unanswered until now.

Their "revolutionary" findings have been published in the prestigious journal Nature.

The phenomenon, which the scientists have named cell autonomous sex identity (CASI), is a naturally occurring anomaly which causes chickens to be white (male) on one side and brown (female) on the other.

Dr Michael Clinton of the Roslin Institute at the University of Edinburgh, who led the study, said: "This research has completely overturned what we previously thought about how sexual characteristics were determined in birds. We now believe that the major factors determining sexual development are built into male and female cells and derive from basic differences in how sex chromosome genes are expressed. Our study opens a new avenue for our understanding of sexual development in birds.”

The previous consensus was that birds' sex chromosomes control whether a testis or ovary forms, and that sexual traits were determined by hormones. The research into CASI shows that differences between male and female cells control the development of sexual traits.

Dr Clinton added: "It also means we must now reassess how this developmental process occurs in other organisms. There is already some evidence that organs such as the heart and brain are intrinsically different in males and females and birds may provide a model for understanding the molecular basis for these gender differences."

The findings may also be relevant to behavioural differences between males and females, as well as their differing susceptibility to disease. They believe the research could also lead to improvements in poultry production. The group has been granted £800,000 from the Biotechnology and Biological Sciences Research Council (BBSRC), the UK's leading biosciences agency, to pursue research into the molecular mechanisms underlying the differences between male and female cells.