The basics
Nearly every cell of your body contains a tightly wrapped copy of the entire instruction manual for assembling and running your body. These instructions are found in the molecules called DNA.
The instruction set is divided into 23 volumes of information, called chromosomes, which you inherited from your parents. You have two copies of each chromosome, one from your father and the other from your mother. Each chromosome consists of thousands of genes, which are the actual instructions that tell your body what to do. Imperfections — or mutations — in genes can predispose you to disease.
Each gene is the instruction set for assembling amino acids — the building blocks for proteins — into the tens of thousands of specific protein molecules that make up your body.
The molecular letters that make up your DNA are referred to as A, C, G and T. The reason we differ from each other is that we have inherited from our ancestors single-letter differences in the genetic code — for instance, a G instead of a C in the instructions for making a protein. Sometimes these minor changes have significant effects.
Each of your parents gave you half your DNA, and you'll give your children half of yours. Which half? That's completely unpredictable. The chromosomes are assembled slightly differently for each child. Except for identical twins, no two people get exactly the same mix of genetic code. So, right from conception, we differ from each other in the structure and function of our bodies — and the prospects for our lives.
