It is not a "single" trait but a combination of a few. To breed for it is akin to breeding for high saddle peaks or redder color. You can selectively breed for those things by choosing animals with the "look" you like and breeding them together. However, in the end there will be babies with the desired characteristics you were looking for and some that don't (and there would be varying degrees in between). It isn't like, say, albinism where you know that specific animals carry that gene and will produce that trait repetedly.

I hope that helps.
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