What adults label aggression is really unintentional behavior in a small child. In other words, a one-year-old will not hit or bite in a premeditated way; she will only do so because she hasn't yet learned to use words to communicate. You'll need to remember that aggressive behavior in a one-year-old — hitting, biting, kicking, or yelling — is not necessarily a sign that a child has an aggressive personality. It is simply an early behavior form that all children try to see if it works.
One good way to look at phases is to consider all negative behavior as a phase (or a chance for instruction) and all positive behavior as a reality, that is, the person your child really is. Then, react expressively (with emotion) to positive behavior, but react with detachment, without emotion, to phase behavior.
One of the most interesting developmental occurrences you'll notice is “the phase.” A phase is a period during which your child will repeat a certain behavior or behavioral style. For example, she may yell more than usual for a few weeks or may be extra sweet and calm. It's amazing how suddenly a phase will come along and then how suddenly it will leave.
When your child begins to behave in a way that is surprising to you, and if she continues the behavior, it's best to relax your expectations for a short time in order to determine if the behavior is just a phase.
You should always, of course, react to your child and take an interest, but don't judge what she's doing as if her future depends on her behavior today. Chances are she's trying on a new behavior to determine what your reaction will be. If you overreact, she might find the behavior more intriguing. If you react with instruction, but little emotion, she'll see that it's not worth her time.