Now in human chimerism, there can be very visible markers. For example, in some reports of human chimerism (which is very rare as I understand things), people have documented checkerboard skin patterns on the chest where there is essentially a line going down the middle of the body, and where alternating squares (perhaps one inch by one inch blocks) of skin have light and dark pigmentation. To a casual observer, the checkerboard pattern may look perfectly square. In other reported cases, the person has adopted both male and female DNA. Again with a line going down the middle of the body from the head to the belly button, one half of the body may have male organs versus female. Surgery may be required to adjust the person to reconstruct the person as one sex or the other.

Source: http://steveshu.typepad.com/steve_shus_weblog/2006/01/musings_on_huma.html