"so" as a conjunction

[stoney]

Is this sentence OK?

My dog is black, so is my cat.

Or should it be;

My dog is black, and so is my cat.

Thanks for any help on this.

[JSmiley]

The second sentence is the better one. In the first one I would imagine a conversation with a non-verbal participant:

Neighbor: "I saw a white animal in my garden taking a shit!"
You: "My dog is black."
Neighbor: <gives a look of disbelief>
You: "So is my cat!"

So, the first sentence would look better to me if it was: "My dog is black; so is my cat."

Also, if the first sentence continued: "My dog is black, so is my cat, and my bird is white." would look just fine. If you think like that, then the 'and' is just there as a consequence of being the last in a list.

Last word: I dont think that either are wrong, depends on your style.