A genetic disorder is one that involves a difference in a chromosome or a single gene.
Chromosomes are small structures found in every cell in the body. In human beings, each cell has 23 pairs of chromosomes. Each pair consists of one chromosome from the mother, carried in the ovum or egg, and one chromosome from the father, carried in the sperm that fertilized the egg.
The chromosomes carry even smaller structures called genes. Genes contain the information that determines all of an individual's characteristics — everything from height and eye color to predisposition to chronic illness.
Occasionally, a baby is conceived with extra or missing chromosomal and genetic material. Since the chromosomes and the genes they contain orchestrate the body's growth and development, a range of physical and behavioral problems may result. In the case of a chromosome 22q11.2 deletion, genetic material is missing (or deleted) from chromosome pair number 22. The "q" in the name of the disorder refers to the precise location of this missing material — on the q arm (long arm) of the chromosome.
Chromosomes are so small that they can only be examined under the microscope. Genes are even smaller, requiring sophisticated molecular testing.
Most cases occur at random and aren't inherited or related to any identifiable cause. However, approximately 5 to 10 percent of children with a chromosome 22q11.2 deletion inherit it from a parent who has a mild, usually undiagnosed, form. Because the disorder is so variable, even people from the same family, with the same deletion, may have very different kinds of problems.
When one parent has a chromosome 22q11.2 deletion, the chance of passing this disorder on to a child is one in two, or 50 percent, with each pregnancy.
Genetic counseling is recommended for:
Reviewed by: Donna M. McDonald-McGinn, MS, CGC
Date: May 2004