You Are a Doctor Examining a Patient Whose Thymus Never

You are a doctor examining a patient whose thymus never developed because of a medical condition called DiGeorge syndrome. You know that this condition usually results from the deletion of part of chromosome 22 and that a single copy of the shortened chromosome is sufficient to cause the syndrome. The patient's parents are both phenotypically normal with normal thymuses. This suggests that:

A) both parents are heterozygous, but one of their alleles is normal so they do not express the disease.
B) both parents are homozygous recessive, so they do not express the disease.
C) the patient inherited the trait from his grandparents, so it skipped a generation and the disease presented only in the patient and not the parent.
D) the deletion occurred during gamete formation and thus was not part of the parents' genomes.