Is this a roo?
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It is too early yet to determine gender. The combs on white leghorns tend to develop quite rapidly, in pullets as well as cockerels -- more than size of the comb the color of the comb is what I consider and in your bird it is still a nice pale comb, if it starts to pink up(and/or you see wattle development that is also pinking up) before about 12/14 weeks that is when to be concerned.