There are approximately 25,000 genes contained on the 46 chromosomes in each cell of the human body. This means that one chromosome contains thousands of genes. A person can have normal chromosomes in number and structure, but still have a disease or condition caused by a mutation in one or more of the genes on the chromosomes. A single gene defect usually does not cause the chromosome structure or number to be abnormal.
Similarly, a person can have normal genes; however, if the person has extra copies of genes due to a chromosome abnormality, then those extra copies can cause the genes to not work properly.
This is an important distinction to make. Consult your health care provider or genetic counselor for more information.