An Introduction to Genetics and Genetic Testing

What do you know about your family tree? Have any of your relatives had health problems that tend to run in families? Which of these problems affected your parents or grandparents? Which ones affect you or your brothers or sisters now? Which problems might you pass on to your children?

Thanks to advances in genetics, doctors now have the tools to understand how certain illnesses, or increased risks for certain illnesses, pass from generation to generation. According to some health experts, the definition of an inherited or genetic illness should be expanded beyond the classic inherited disorders (like hemophilia and sickle cell anemia) to include many types of cancer, Alzheimer's disease, and other illnesses. They look toward a future where genetic test results are an important part of every healthy person's medical file.

Genes and Chromosomes

Each person has a unique set of chemical blueprints that determines how his or her body looks and functions. These blueprints are contained in a complex chemical called deoxy-ribonucleic acid (DNA), a long, spiral-shaped molecule that's found inside each body cell. DNA carries the codes for genetic information and is made of linked subunits called nucleotides. Each nucleotide contains a phosphate molecule, a sugar molecule (deoxyribose), and one of four coding molecules called bases (adenine, guanine, cytosine, or thymine). The sequence of these four bases determines the genetic code.

Specific segments of DNA that contain the instructions for making specific body proteins are called genes. Right now, scientists believe that human DNA carries from 25,000 to 35,000 genes. Some genes direct the formation of structural proteins that eventually determine physical features such as brown eyes or curly hair. Others provide the instructions for the body to produce important chemicals called enzymes (these are proteins that act as natural catalysts and help to direct and control the chemical reactions that happen within the body). Sometimes, depending on the codes of a specific gene, even a small error within the DNA structure can mean serious problems for the entire body. Sometimes, an error in just one gene can result in a life that's shortened or physically difficult.

Genes are found in specific segments along the length of human DNA, neatly packaged within structures called chromosomes. Every human cell contains 46 chromosomes, arranged as 23 pairs, with one member of each pair inherited from each parent at the time of conception. After conception, these 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 high-powered 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

Abnormal Numbers of Chromosomes (Trisomies and Monosomies)

Genetic problems can happen for many different reasons. Sometimes, a mistake occurs during cell division, causing an error in the chromosome number either before or shortly after conception. The developing embryo then grows from cells that have either too many chromosomes or too few.

In trisomy, for example, there are three copies of one particular chromosome instead of the normal two (one from each parent). Down syndrome, trisomy 18 (Edwards) syndrome, and trisomy 13 (Patau) syndrome are all examples of this type of genetic problem.

Trisomy 18 syndrome affects one out of every 8,000 newborns. Children with this syndrome have a low birth weight and a small head, mouth, and jaw. Their hands typically form closed fists with abnormal finger positioning. They may also have malformations involving the hips and feet, heart and kidney problems, and mental retardation. Only about 5% of these children live longer than 1 year.

Trisomy 13 syndrome affects one out of every 20,000 newborns. This syndrome causes cleft lip, flexed fingers with extra digits, hemangiomas (blood vessel malformations) of the face and neck, and many different structural abnormalities of the skull and face. It can also cause malformations of the ribs, heart, abdominal organs, and sex organs. Long-term survival is unlikely but possible.

In monosomy, another form of number error, one member of a chromosome pair is missing. There are too few chromosomes rather than too many.