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tgb comments on Open Thread, June 16-30, 2012 - Less Wrong
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What makes you think that the extra limbs were caused by mutations? I know very little about bovine biology, but if we were dealing with a human, I would assume that an extra leg was likely caused by absorption of a sibling in utero. I have never heard of a mutation in mammals causing extra limb development. (Even weirder is the idea of a mutation causing an extra single leg, as opposed to an extra leg pair.) The vertebrate body plan simply does not seem to work that way.
Pure speculation! However, this was a wide-spread occurrence not just one or two cows hinting at some systematic setup. I also don't remember the details as it was many years ago and I was quite young - it's possible that there was a pair of legs.
Forgive me, for my biology is a bit rusty.
A gene can become more common in a population without being selected for. However, invoking random genetic drift as an explanation is generally dirty pool, epistemically speaking. We should expect a gene that creates extra useless legs to be selected against. (Nutrients and energy spent maintaining the leg could be better used, the leg becomes more space for parasite invasion, etc.) Assuming that you were dealing with such cattle, you should assume that some humans were selecting for them. (No reason necessary. Humans totally do that sort of thing.)
I cannot think of any examples of a mutation causing extra limb development in vertebrates. However, certain parasites can totally cause extra limb development in amphibians. I doubt this is the case, but it is more likely than mutation.
Alternatively, consider there existing a selection effect on your observations. I wager that Indian cattle are less likely to be culled for having an extra leg that American cattle are. I'm just going off of stereotypes here, however.