All of the entries in your genome encyclopedia are written in a special language-the DNA code. The DNA alphabet has only four letters, A, C, T, and G, representing the four different chemical bases. In total, each person has more than three billion DNA letters in each set of their 23 chromosomes.
The DNA Code
Although it may appear to be one long string of DNA letters, by studying the DNA code, researchers have learned that it also contains a complicated system of punctuation. Special codes signal when the DNA letters are part of a gene. Three letter combinations of DNA refer to specific amino acids. Within a gene, the amino acids provide instructions that can be read by special substances in the body to determine what type of protein needs to be made. The order of all these DNA letters is called your genome sequence. When someone refers to your genome sequence, they mean the unique combination of DNA sequences from chromosome 1 through your sex chromosomes (XX or XY). Although we have two copies of each chromosome set, we only report one complete sequence. This is because most of the DNA sequence is the same between the two chromosome sets. Whenever there are differences, this is noted in the person's one, combined genome sequence.
One person's genome sequence is very similar to another's. In fact, more than 99% of the human genome sequence is common to all people. This makes sense because we are all the same species-humans-and our bodies tend to have similar features (for example, two arms, two eyes, ten toes) and work in similar manners. Since the genome sequence is more than three billion DNA letters, there is still a lot of room within that 1% difference for variations to exist from person to person. No two people are genetically identical (identical twins being the exception). It is these "differences" that make you, well, you.