Genetics of 15q Duplications

Basic Genetics

There is no way around it: In order to understand chromosome 15q11.2-13.1 duplication syndrome, you will need to have a basic understanding of heredity, chromosomes, and genes.  Understanding this information will also help you translate the chromosome or DNA report that was used to diagnose the duplication.

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The genetic information that makes us both similar to other people and unique individuals is carried in our DNA.  Each of us inherited half of our DNA from each of our parents and in turn, pass half of our DNA on to our children. 

What is a gene?

A gene is a segment of DNA that carries the instructions that tell a cell how to make a particular protein.  There are about 21,000 different genes in humans and they are strung together on the chromosomes, sometimes close together and sometimes very far apart.  There are two copies of most genes - one located on the chromosome that came from each parent.   Each cell carries a full complement of all the genes, but only activates the specific ones it needs to perform its functions in different cells at different times.

What is a Chromosome? 

In the cells in our bodies, the DNA is packaged up into structures called chromosomes.  There are typically 23 pairs of chromosomes, for a total of 46.  One copy of a chromosome in each pair is inherited from the mother, and one is inherited from the father.  Chromosome 15 is one of the 22 pairs that are called autosomes, which carry most of this information about how our bodies form and function. The last pair of chromosomes determines if we are male or female. They are called the sex chromosomes; females have two X chromosomes and males have one X chromosome and one Y chromosome.

These are normal human chromosomes. There are 23 pairs, including an X and a Y chromosome. Chromosome 15 is highlighted by the box. The chromosomes have been treated with a chemical staining process that causes light and dark bands to appear. Geneticists use these bands as coordinates, like addresses, for a particular position on a chromosomal arm.

What does chromosome 15 look like?

The anatomy of a chromosome includes the centromere (a narrow point on the chromosome which is engaged during the process of cell division), which divides the chromosome into the small (or “p”, for petite) arm and the long (or “q”... for letter after “p”) arm.   There are several types of duplications of chromosome 15q that can occur, involving different, sometimes overlapping, regions of the q arm. Of particular interest to us is the stretch of genes marked by coordinates 15q11.2-13.2, also known as the Prader-Willi syndrome/Angelman syndrome (PWS/AS) critical region, due to the serious phenotypes that result from deletions of certain key genes that reside there. We can  distinguish the core 15q duplications, those that span most or all of the PWS/AS critical region, from - for want of a better term - the edge duplications at q11.2 or q 13.3, in which only the genes that are flanking the PWS/AS critical region on each side are duplicated.

This schematic drawing shows the basic features of chromosome 15.  The p arm at the top is short and contains little genetic information.  The region that is involved in the duplications is on the q arm with the coordinates indicated by numbers.  To the right, the PWS/AS range spanned by the core duplications is marked in the figure as “Core”, whereas the 11.2 region is marked “E” and the 13.3 region is marked “e”.