FarmVille Blog explains more about Sheep Breeding

While the basics of Sheep Breeding in FarmVille'sEnglish Countryside are fairly easy to understand (check out our full guide if you've yet to begin), the aspects concerning which traits are carried down from which parent are a bit more complicated. While the basic phrase of "pattern comes from the father and color comes from the mother" holds true to a point, there is a whole possibility for variation with each and ever Lamb that you breed. To help clarify matters, Zynga has posted "a bit more on sheep breeding" over on the official FarmVille Blog.

First, for those users that have been "unlucky" enough to receive the same gender of Lambs three times in a row, you are supposedly guaranteed to receive a Lamb of the opposite gender the next time around. Unfortunately, this seems to be a bit more like "wishful thinking" than an actual guarantee, as users like myself are now on our fourth or fifth Lambs, all of which have been one gender or another (in my case, all females). This may just be because the "guarantee" has yet to roll out into the game's coding, so we'll give Zynga the benefit of the doubt here.