Chromosomes are found in nearly every cell of your body in the nucleus of each cell. Your red blood cells do not have a nucleus and therefore do not have chromosomes in them. The male sperm cells and female egg cells contain only 23 chromosomes each because when they join they normally give one of each chromosome to the developing embryo. The other cells in your body have 23 pairs of chromosomes which makes a total of 46 chromosomes in each cell (to see the male and female normal human karyotypes, please see below). Our chromosomes contain our DNA which makes us who we are.
The picture of the chromosome shows a cell with the nucleus and the chromosomes contained in it. Notice that the middle of the chromosome is referred to as the centromere and the ends of the chromosome are referred to as the telomere.
Each person typically has a pair of number 3 chromosomes. There are two segments in the chromosome, the shorter portion above the centromere is called the “p” arm and the longer portion below the centromere is called the “q” arm (see image of chromosome). The karyotype name will specify which segment of the chromosome is affected. Within each segment are bands to label specific areas of the chromosome.
View the segments and bands of chromosome 3 below. A deletion in a chromosome means that a part or segment of one of the pair of number 3 chromosomes is missing. It can also be called a partial monosomy. A monosomy means that there is the presence of only one chromosome instead of the typical pair.
If you see “del” in a karyotype name it means deletion.
Many of our children have what is called an intersititial deletion meaning “within” the chromosome. This would entail two breaks in the chromosome with a segment missing and the remaining segments joining up.
If a segment near the centromere is missing it is called a proximal deletion.
If a segment near the telomere is missing it is called a distal deletion. You may see a “tel” in the karyotype to describe this.
Sometimes there is only one break in the chromosome which is called a terminal deletion meaning that it goes to the end of the chromosome thus the terminology “ter” in the karyotype name (this does not mean the deletion is any worse than the others).
A very small deletion is called a microdeletion.
For more information on chromosome deletions, translocations, inversions, rings and additions, please see this excellent guide provided by Unique called The Little Yellow Book.