Earlier PZ Myers disputed the claim that the recently-notorious 4-legged duck was the product of “genetic changes” as reported by the media. Taking a look at the morphology of the poor bird’s extra legs, it much more likely that the duck suffers from a phenomenon known as “parasitic limb” or “parasitic twin” in humans. For example, this child was born in Detroit with an extra leg, which was actually the remnant of a twin who never properly separated during development (below).
A parasitic twin results from the same general process which results in normal identical (monozygotic) twins: the division of the egg following fertilization. However, the process goes awry for some reason and the division was quite incomplete (nearly complete division would result in conjoined twins). This process usually occurs (in people) at about 13-14 days after fertilization. One of the “twins” becomes vestigial to the other, healthy and developed twin—called “parasitic” due to its complete reliance on the physical support of the healthy twin. When this occurs in people, the extra limbs, etc, are normally removed if it does not impinge on the health of the normal twin.