To a scientist, each puzzle piece might represent a single gene or a portion of the DNA that makes up the space between genes.

It might seem confusing at first that people have differences in our genes. We are all humans, and all humans must have the same genes, right? To a certain extent, this is true. The structure and position of each gene, like the shape and location of each puzzle piece, is the same for essentially all people.

Individuals may have differences in the sequence of their genes or of the DNA between genes on a chromosome, though. Some differences produce the normal variety of characteristics we see in a person's appearance and behavior (tall, short, fat, thin, etc.). Other sequence differences, if within important gene regions, can result in changes in gene expression, sometimes leading to negative effects on a person's life. Still others may have no effect at all. When scientists use DNA sequence to identify individuals, they pay close attention to the subtle differences in the DNA sequence we all share.

In addition to subtle changes, more significant DNA changes can occur, like the deletion of a portion or all of a chromosome. These changes can alter the expression of many genes simultaneously and for the most part are lethal to the individual. In the jigsaw puzzle, deletion of a whole chromosome would be akin to deleting about two puzzle pieces.