Final answer:
Most cases of uniparental disomy stem from an initial trisomic embryo that loses one extra chromosome, resulting in both copies of a chromosome coming from one parent.
Step-by-step explanation:
Most cases of uniparental disomy originate as an initial trisomic embryo that loses one chromosome. This means the correct answer to the student's question is option a) initial trisomic embryo that loses one chromosome. Normally, embryonic development begins with a euploid set of chromosomes, which in humans consists of 22 pairs of autosomes and one pair of sex chromosomes. However, in the case of aneuploidy such as trisomy, the embryo has an extraneous chromosome. If the extra chromosome is subsequently lost, the individual can end up with a uniparental disomy, which is a condition where both copies of a chromosome come from one parent. An example of a trisomy that can lead to a viable offspring is trisomy 21, which causes Down syndrome.