The terms anomaly, abnormality, disorder, defect, disease, and
syndrome are not used consistently, and do not have precise
definitions. Human beings have cells with 46 chromosomes -- 2
sex chromosomes and 22 pairs of nonsex (autosomal) chromosomes.
Males are "46, XY" and females are "46, XX." The chromosomes are
made up of strands of genetic information called DNA. Genes are
sections of DNA. The location of the gene is called the locus.
Most genes carry information that is necessary to make a
protein. The pairs of autosomal chromosomes (one from the mother
and one from the father) carry basically the same information.
That is, each has the same genes. However, there may be slight
variations of these genes. These slight differences occur in
less than 1% of the DNA sequence and produce variants of a
particular gene called alleles. If a gene is abnormal, it may
lead to an abnormal protein or an abnormal amount of a normal
protein. Since the autosomal chromosomes are paired, there are
two copies of each gene, one