The following is a piece I wrote for Helium.com (I write under the pen name Jade LaFemme there)
A child's ability to understand the world around them is much more acute than many adults give them credit for. They may not be fully developed enough to be able to come right out and say "Well gee Mom, I heard you and Dad fighting last night and it really scared me", but they understand the emotions of what went on. They can feel the hostility, the tension, the anger. The younger the child, the sharper their ability to empathize.
A child's mind is like a sponge from the minute they draw their first breath; everything is new and fascinating. They learn rather quickly what noises and facial expressions adults make that will garner a certain type of response from them. Rage, sadness, delight; they store away it all from the moment their blurry little eyes finally clear. They register the good right along with the bad, and they hold onto it as they grow.
After a child turns two, they start to remember things. They think about how Mommy was putting on her make-up, and they imitate her while playing dress-up one day. The child grows older and hears Mommy talking to herself while looking in the mirror a different time, commenting on how she needs to go on a diet. Her daughter, not having any sense of perception, therefore no concept of what beauty is, thinks that maybe she should go on one too so she can be pretty like Mommy. A boy hears his father talk about his days in the Marines and how it was the best decision he ever made to join, and the boy eventually can be found playing "Army" with his school friends because he wants to grow up and be a Marine just like Daddy.
It isn't just what the parents aspire to be themselves that rubs off on their children, it's their mannerisms and neurosis as well. One day you will be going about your own life, minding your own business, and you'll catch yourself saying a phrase or making a noise that your mother did all the time. You could even wind up not realizing you have adapted that phrase or noise until someone points it out to you. You could say you will never wind up being an alcoholic or a drug addict constantly in and out of prison like your father was, then one day you find yourself offered a hit off of a pipe of some kind, and the life as you knew it is over.
Genetics are funny like that. They are nothing more than a grab bag and you won't know exactly what you've gotten until the day of realization comes to you, and you find yourself laughing at the irony of it all.