Looking at chromosomes
A chromosome is a single, very long strand of DNA, tightly coiled up. The largest, chromosome 1, contains about 8000 genes while the smallest, chromosome 21, contains less than 300. In all, you have 23 pairs of chromosomes in most cells of your body. You can see them under an ordinary microscope, but you would not be able to distinguish individual genes. Scientists can treat chromosomes to reveal characteristic 'banding patterns'. These patterns help them to identify each chromosome, and to spot any missing or duplicated regions.