You are a first year medical student assessing a pediatric patient in the clinic. The patient exhibits all of the clinical symptoms of Down syndrome, which your attending later confirms.
The following summer you are working in a genetics lab examining genetic disease and remember this patient. You remember that Down syndrome, also known as trisomy 21, is characterized by an extra copy of chromosome 21.
You obtain a small skin sample from your patient and examine the karyotype under the microscope. To your surprise, you find that the patient has 23 pairs of chromosomes with no evidence of trisomy.
What is a possible explanation?