Just as the term “epidermis” refers to the layer of the skin above (or beyond) the dermis, the term “epigenetics” describes nongenetic causes of a phenotype. For example, in genetic imprinting, a different expression of an allele is produced depending on the parent who transmits it. In humans, an example comes from two medical problems—both that result from a deletion on chromosome 15: In one called the Prader-Willi syndrome—originating from a deletion on the paternal chromosome 15—affected children have small hands and feet, short stature, mental retardation, and are obese. In the Angelman syndrome—originating from a deletion on the maternal chromosome 15—affected children have a large mouth and tongue, severe mental and motor retardation, and a happy disposition, accompanied by excessive laughter.

The HGP was begun in 1990 and finally completed in 2003. According to the official HGP website (http://www.ornl.gov/sci/techresources/Human_Genome/home.shtml), the goals are as follows: To identify the number and sequence of genes in human DNA; to store this information in public databases for scientists and the public; to improve the tools used to analyze the genome data and possibly transfer these technologies to the private sector; and finally, and many scientists believe most importantly, to address and understand the ethical, legal, and social issues that may stem from the project.

Overall, the project has been very successful, especially in the analysis of the data. And it’s easy to see why the data will be used for many more years to come: According to the Human Genome Project, humans have about 20,000 to 25,000 genes in their DNA—the goal of the project has been to identify all those genes. In addition, the project goals included determining the sequences of the three billion chemical base pairs that make up human DNA.