The human genome has just over 3 billion DNA base pairs encoding 750 MB of information and containing 20,000-25,000 protein-coding genes. This is substantially less than initial estimates of 100,000 or more. The number of human genes became clear with the completion of the Human Genome Project in 2003. While its goal was to sequence the human genome, another important task of the project was to determine the number of genes, their locations, and give an idea as to their function.
Genes are long stretches of DNA that form the most basic unit of heredity. Variations on genes are called an allele. For instance, it is believed that at least one gene pair dictates human hair color, and depending on the combination of alleles, someone has either black hair, brown hair, red hair, blond hair, or other variations in between. Because the processes of genetic expression are complex, we still have a lot to learn before we know what all 20,000+ genes do and how every feature of human beings relates to their genetics.
The human genome only has about twice the genes of much simpler animals like a nematode (roundworm) or fruit fly. In fact, little correlation between animal complexity and number of genes has been observed. It is thought that the greater biochemical variety in the human body over the fruit fly body occurs because our genes make extensive use of alternative splicing, a method whereby multiple proteins can be synthesized from a single gene. In its most basic form, a gene only synthesizes one protein.
As we continue to analyze the human genome, we can eliminate even more hypothetical genes and narrow in on the true genes, so there may be even less than 20,000. Genes have been found to be associated with many features of the human body and mind, including size, propensity to depression, propensity to becoming overweight, heart conditions, genetic diseases, intelligence, the presence or absence of freckles, and many others. Because the human genome was only sequenced recently, a lot more work is necessary to figure out what all these genes do.