It is hard to say for certain on the breed given the angle of the photo. It could be a BCM that is partridge (which I believe is a DQ) but to me the head shape looks "off" from what I'd expect from that breed. Several other breeds or mixes are more likely, thought. A shot of the body would help.
How old is this bird? My initial impression is it's a pullet.
What color are this birds legs? Are they feathered?
ETA: Just read your original post about sexing your BCM's. If you bought these chicks from a reputable breeder then this bird looks to be partridge--if she has the genetics she will still lay gorgeous dark eggs and is perfect for a breakfast pen, but not a breeding pen. If you got them from someone who wasnt a reputable breeder I'd say there is a very high likelihood that a few roosters jumped in the pen