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.