Each chromosome is different from other chromosomes.  Importantly, each chromosome carries different genes.  For example, chromosome 1 carries a different set of genes than chromosome 5.

Under a microscope, chromosomes even look different from one another.  As you can see in the above karyotype, some chromosomes are larger than other chromosomes.  In addition, each of the chromosomes has a unique banding pattern.  These   bands are used to identify the different sections of each chromosome.  Near the center of each chromosome is a constricted region that divides the chromosome into two arms.  The constricted region is called the centromere.  The shorter arm of each chromosome is called the “p" arm, and the longer arm of the chromosome is called the “q" arm. 

Chromosome Markings and Structure

The markings on each of the chromosomes are very important in identifying which chromosome is which.  If you look closely at the karyotype below, you will see that there are subtle differences between the chromosomes.

Each chromosome has a characteristic black and white banding pattern and has a constriction (called a centromere) in a characteristic location somewhere along its length. These two things, the banding pattern and the centromere, make each chromosome recognizable to a trained eye.

As you can see, the centromere of each chromosome is not exactly in the middle of the chromosome.  This makes the chromosome appear as if it had two distinct segments of unequal length. These segments are called arms.  The shorter arm (called "p" for petit) is always shown on top.  The longer arm is called the “q” arm and is shown below the p arm.

The bands on each chromosome arm divide it up into regions. The regions are numbered starting at the centromere and progressing to the end of the chromosome arm.   Below is a diagram that shows how the bands of chromosome 1 are labeled.

“The above information is from the Chromosome 18 Registry & Research Society and was reproduced with permission.” Some minor changes may have been made to make the information as applicable as possible to 1p36 Deletion Syndrome.