What’s In Your Genes?
You might be aware that you have inherited certain things from your parents, and they from theirs, and so on. As a result, you might have your grandfather’s blue eyes, or your childhood pictures might look just like your Aunt Martha’s at that age. You and your siblings might all look alike, or you might have some that definitely take after one parent more than another.
You also might have heard words like “genes” and “chromosomes” in high school biology, and have a vague idea of how they work, and how they determine that your sister has red hair while yours is brown.
What you might not realize is that genes could determine more than just your physical characteristics; they can also determine whether or not you develop certain diseases and disorders, and they could even determine things like sexual orientation. 
Genes and Chromosomes
To understand how genes work, it’s important to understand what they are. Every human being has 23 pairs of chromosomes – for a total of 46 individual chromosomes. You inherit one pair from your mother and one from your father. 
Each individual chromosome is like a backpack filled with genes. The number of genes in each backpack could be in the thousands, and the contents of each backpack can vary. For this reason, it’s exceedingly rare for any two people – even those from the same parents – to inherit the exact same genes, in the exact same ratios. The only exception are identical twins, who develop with one egg and one sperm unite and split into two distinct embryos.
These minute differences in the combination of genes that you inherit is why you and your family members can strongly resemble each other, but still be physically different. These minute differences could also determine why some family members have certain behaviors and personality traits, a tendency to get certain illnesses, or differences in abilities, despite all growing up in the same environment.
How Genes Influence Who You Are.
In truth, scientists are still not sure of the full impact that genes have on making you who you are. As previously stated, each chromosome could have thousands of genes. While scientists know that those genes exist, they are still in the process of figuring out what they all are and what they do. The Human Genome Project  has been mapping and cataloguing human genes for several years now and they are just scratching the surface.
Some genes have been identified and linked to certain behaviors and conditions. For example, a mutation of the BRCA1 and BRCA2 genes has been positively linked to certain types of breast and ovarian cancer.  
However other diseases, like alcoholism, are strongly believed to have a genetic component, but scientists have not been able to isolate or identify a specific gene, or sequence of genes, responsible. 
Defying Your Genes
Genes simply provide a template for the body to follow. Some parts of the template could be very difficult to deviate from, while others are a bit more fluid.
For example, someone who inherited the BRCA1 or BRCA2 mutation, has a much greater likelihood of developing breast or ovarian cancer than someone who did not inherit those genes. They might be able to delay the onset of disease with diet and exercise, but because it’s actually written into their genes, and into the very cells that are affected by those genes, there is very little they can actually do to stop it short of removing the organs in question – as Angelina Jolie did in 2013.
On the other hand, those born with a genetic predisposition toward alcoholism could avoid developing the addiction by avoiding alcohol. This is because the tendency to become addicted might be written into their cells, but they actually have to introduce alcohol into the body to trigger it. If someone with a genetic predisposition to alcoholism does drink, they still have the possibility of avoiding addiction by learning to be aware of behaviors that signal the beginning of an addiction and how to counteract them.
You should also remember that your genes are only one factor in determining who you are; your environment also plays a large part. That’s why identical twins, separated at birth and raised in different environments, can often end up looking very different – even having different body types.
That’s also why two people raised in the same household can have very different interests and abilities. Because while their home and neighborhood is the same, they could have been exposed to different environments outside of their home and neighborhood – such as at school, or in social groups.
Of course all of this is an incredibly oversimplified explanation of a very complex process. However, it should give you a basic idea of what genes are, and how they can influence who you are.