Last time we were talking about how cloned animals, to some people's surprise, aren't just copies of their parent animals. In cloned pigs, for example, some come out bristly-haired, some smooth; some are shy, others aggressive.

How can animals share all the same genes and yet be so different?

Geneticists aren't sure yet. One possibility is that chemical conditions inside the uterus affect how the fetus develops. But there's also gene expression to consider. Gene expression refers to which genes are turned on and which are turned off.

Every gene in your body comes in duplicate. You get one from your mother and one from your father. Let's simplify the situation and pretend there's just one gene that gives you your sparkling personality.

For example, let's say your mother was a jewel and that your dad was a crank.

In this instance, presuming you're an angel, we can imagine that the "personality gene" your father gave you is in the "off" position, while the personality gene your mother gave you is in the "on" position. Would your clone have your same personality?

We don't know, because we don't know what position that switch will be in. In reality, there are many, many switches, leading to many possible combinations. A clone is a duplication of the genes, but not of the gene expression.

In other words, a clone could share genes and still look and act differently. They aren't carbon copies--they are separate organisms all to themselves.