Williams Syndrome – Genetic Condition – Chromosomal Deletion?!! What??

In a person affected by WS, somewhere along the line, possibly in the formation of the egg or the sperm cell or very early after conception, a random event deletes a small piece from one copy of chromosome 7. The size of the missing piece is slightly different in different people but it often contains 20 to 30 genes and the affected person then has only one copy of these genes rather than the normal two copies.