These blueprints are contained in a complex chemical called DNA, a long, spiral-shaped molecule that is found inside each body cell. Specific segments of DNA that contain the instructions for making specific body proteins are called genes. Right now, scientists believe that human DNA carries up to 30,000 genes. Sometimes, depending on what a specific gene codes for, even a small genetic error can mean serious problems for the entire body. Sometimes, an error in just one gene can mean the difference between a normal life and one that is shortened or physically difficult.
Genes line up along the length of human DNA, neatly packaged in structures called chromosomes .
Human cells have 46 chromosomes, arranged in 23 pairs, with one member of each pair inherited from each parent at the time of conception. After conception, these original 46 chromosomes duplicate again and again to pass on the same genetic information to each new cell in the developing child. Human chromosomes are large enough to be seen with a light microscope, and the 23 pairs can be identified according to differences in their size, shape, and the way they pick up special laboratory dyes. Genetic Problems Genetic problems can happen for many different reasons. Sometimes a mistake occurs during cell division, causing an error in chromosome number either before or shortly after conception.