Genetics: A Primer
Below, we present some genetics terms and principles to help you understand how NF is inherited.
What is a Gene?
Our body is made up of millions of cells. Each cell contains a set of chemical structures known as chromosomes. There are 46 chromosomes, arranged in 23 pairs, in each cell in the body. One chromosome of each pair was contributed by the father, and the other by the mother. A gene is a small section of a chromosome. Genes also come in pairs. About 100,000 genes are arranged in a very specific order on the 23 chromosome pairs. One of these pairs, called the sex chromosomes, differs in males and females; the other 22 pairs, called autosomes, are the same in both sexes.
What Do Genes Do?
Genes direct cell behavior. When a gene is activated, a variety of events can occur in the cell, depending on the particular function of that gene. Some genes are responsible for obvious traits such as eye color; others control the production of substances essential to chemical processes inside our bodies. Certain genes simply act as on-off switches for other genes. These reactions are like orders to the cell. They are all the instructions needed for the first cell to develop into a human being and for the body to carry on all the functions of life.
What Is A Gene Mutation?
A mutation is a change. Gene mutations have occurred since the beginning of time and continue to do so. Most mutations are not detectable, and some are not harmful. When a "mutagen" (an agent that causes a mutation) alters the structure of a gene, the gene's "instructions" to the cell are changed or even stopped completely. An alteration of this kind can have serious effects, and may result in a genetic disorder.
NF is the result of such a changed gene.
NF is an "autosomal dominant" disorder. "Autosomal" means that the NF gene is located on one of the 22 pairs of chromosomes called "autosomes." (NF1 is located on chromosome 17; NF2 on chromosome 22.) Since these chromosomes are the same in males and females, the gene can be present in either sex, and it can be passed on from either a mother or a father to a son or a daughter. The term "dominant" means that the presence of only one changed or affected gene causes the disorder to appear; the action of the unaffected gene which is paired with the dominant gene cannot prevent the disorder. Because one gene is enough to cause the disorder, NF can be passed from one generation to the next when only one parent has the gene. "Recessive" disorders, on the other hand, generally occur only in the presence of a pair of affected genes, each inherited from one of the affected individual's parents.
Why are the odds always 50-50 for a child to inherit NF from an affected parent?
The explanation for this lies in the process that brings egg cells and sperm cells to maturity. These cells carry our genetic heritage from one generation to the next. Before reaching maturity each of these cells contains 23 pairs of chromosomes, the full complement of genetic material just like any other body cell. As they approach maturity, however, these cells go through a special process (called "meiosis") that results in each egg or sperm having a single chromosome from each pair half of its original genetic material.
