[constant thinker]

I saw an article about a duckling that has 4 legs instead of 2.
http://www.comcast.net/news/strange/index.jsp?cat=STRANGE&fn=/2007/02/18/590075.html&cvqh=itn_fourlegs

Definitely an oddity and I thought I'd share it.

I think it's kind of cool, anyone else's thoughts on it?

Also that's an interesting mutation in my opinion.

[Yggdrasil]

It's always amazed me in biology that such simple mutations to homeobox genes can produce such weird but semi-functional phenotypes.  I mean, getting feet to grow where the wings should be or to get legs to grow where antennae should be seems like it would be an extremely difficult task.  However, the regulation of animal development is such that relatively simple mutations can create these strange phenotypes. 

[Borek]

Legs instead of antenna in arthropods doesn't look that strange - basically that's the same object (note how many different legs arthropods have; in a way they have only specialized legs, even for jaws).

In the case of duckling that's different thing, but, first of all - it doesn't look to me like these are instead of wings, rather the bird is a 'segment' longer. And that's where the answer most likely lies - we are segmented and some error during segmentation can lead to additional pair of legs. Just like these long 6 doors limos

Finally, when it comes to mammals (no idea if it is possible with birds, but perhaps - in the case of two yolks egg - it may happen) I can think of other possible explanation. When two (or more) animals start to develop in uterus they usually will be born separately as brothers/sisters. However, in some cases zigotes can merge resulting in grown chimera (see http://en.wikipedia.org/wiki/Lydia_Fairchild for the most intriguing case). While it generally doesn't mean you have an additional set of legs that belong to your brother, I recall seeing a report on TV of someone posessing some additional tissues from such a source. It was a chinese boy with deformed face  and these deforming tissues where what was of left of his twin. But the reference is very obscure - some TV documentary I have seen long ago.