The reason behind it is Males get BB while females only get B_. B = Barring Gene.
http://home.earthlink.net/~100chickens/id56.html
http://www.gfhev.de/abstracts_kongresse/2005Abstracts.pdf
http://www.gnxp.com/
http://www.genetics.org/cgi/content/full/168/3/1507...