Chromosomes are paired long chains within a cell nucleus that are composed of genes (about 20,000 genes per chromosome pair), which are made up of the chemical substance called DNA. Genes on the chromosomes are made of segments of DNA which contain chemically coded messages resulting in the characteristics of an organism — including humans.
Human bodies are made up of millions of cells. Every cell nucleus — the control center — has the complete coding of a person's genetic makeup. This genetic blueprint is located in the cells' genes. Every gene is a unique slice of genetic data. Each gene (made of DNA strands) on the chromosome has its own specific location and its own unique job. These genes contain the information for how we look: our eye and hair color, our height, the shape of our ears and the curl in our toes. Humans have 46 chromosomes composed of 23 chromosome pairs in every cell nucleus. One half of each pair is contributed by the father, one half by the mother.
Chromosomes can be pictured as strings of beads which are genes. The genes — and therefore chromosomes — are composed of the chemical substance called DNA (DeoxyriboNucleic Acid.) The DNA that makes up the genes on the chromosome string is called "coding DNA" because it is responsible for all genetic characteristics which pass from one generation to the next within a species.
There is also DNA on the chromosome string between the genes. This DNA is called "non-coding DNA." The non-coding DNA has become useful in defining biological relationships and obtaining solid evidence in forensic laboratory evaluations.