What causes a 15q11. A 15q11. 2 microdeletion may occur randomly for the first time in an affected person (a de novo mutation), or it may be inherited from a parent with the microdeletion. A blood test to look at the parents chromosomes is needed to find out how the microdeletion occurred. When a 15q11. 2 microdeletion occurs as a de novo mutation, it is due to a random error - either during the formation of a parents egg or sperm cell, or very soon after conception (fertilization of the egg). A parent with the microdeletion has a 50% chance with each pregnancy to pass on the microdeletion. The features of 15q11. 2 microdeletion occur because the deleted region of the chromosome contains several genes that are important for normal growth and development. It is not yet clear why there is a large range of features and severity among people with a 15q11. 2 microdeletion, or why some people are unaffected. 
What causes 15q11.2 microdeletion ?