Pretend you are painting a picture.  You look at your paints, but see you only have red, blue, yellow, black, and white.  You want to paint the sky, but all you have is bright blue.  You want a light blue.  What can you do?  You can mix bright blue and white and get a lighter blue.  The new paint looks a bit like both of the colors you used.  It's certainly blue, but the white makes it a lot lighter than the color you had in your paint box.  So go ahead and paint that morning sky!

Those blue shades are just perfect, but what does this have to do with genes?

You can mix paints and get a combination of both colors.  You can mix milk and chocolate to get chocolate milk.  You can mix lots of things and end up with something that looks a bit like both things it came from.  This isn't true for people, though.  Your get half the directions for how you look from your mother and half from your father.  However, you will not look like a mix of them.  If your mother has black hair and your father has blonde hair, your hair color will not just be a dark blonde or light black.  It is not that simple.

More than a hundred years ago, people didn't really understand how children got traits such as hair color, size, and eye color.  Some thought that the blood of parents would mix, and this would make children look like a mix of their parents.  They thought that If a mother dog was brown and the father dog was white, then their puppies would be tan.  The plain brown or white could never show up again in that family because it had been changed to tan.  The puppies could only pass along tan to their puppies.  Blending theory was the idea that children's traits came from mixing their parents' traits.

Looks like somebody forgot to blend my ears.

However, blending theory clearly did not work.  Black and white cats do not have only gray kittens.  Tall and short parents do not end up with all middle-size kids. Many things about a child, such as nose length, eye shape, and blood type, come from mixing many different directions together.  There is not one simple gene that decides how your nose will look or how tall you will grow.  

A number of genes influence how you will look.  To influence means to have an effect on something, but does not mean that something will change.  If your favorite color is blue, you might be more likely to buy a blue shirt.  That does not mean that you would never buy a red shirt.  You are influenced to buy more blue things because you like blue.  It does not mean you will be wearing all blue for the rest of your life.

You carry many genes that can cause your children to look different from you.  It all depends upon what traits both the mother and father have.  Some traits, like brown eyes, are very common.  Even if a father has brown eyes, though, his children might have blue eyes.  His cells can carry the information for both color eyes.  Even if the mother also has brown eyes, there's still a chance the kids could have blue eyes.  Like many traits, eye color is set by more than one gene.  It's not as simple as people once believed!  

Here's something else that's not simple.  Each one of your body parts has a certain job to do.  Your eyes see, your heart pumps blood, your skin protects your body, and so on.  You might think that the directions inside the cells for each of these body parts are different.  Do your brain cells only carry information that makes brain cells work?  Do all the directions inside your nose cells tell you how to breathe and smell?  That might make sense, but it's not what happens.  Actually, all the cells in your body have exactly the same genes.  They all carry directions for every part of your body.

If all cells have the same directions, how can your ears and liver look different and do different jobs?   This happens because some of the information inside each cell is turned off.  A cell only uses the directions it needs to do its work.  It's just like the lights in a house.  Some lights might be turned on and some might be off.  That allows different things to happen in different rooms.

If the lights are switched off in your room, you can sleep.  If the lights are on in another room, your brother can read or play a game.  Genetic switches in your cells make it so that a cell inside your body can only have some information turned on.  This lets us have many different body parts that do many different jobs, even though they all have the exact same genes.  Scientists have been studying ways to manipulate these switches using drugs.  They hope that some diseases can be improved or perhaps even cured by turning certain switches on or off.

All right, who left the genetic switches on again?