Which relative is most likely to share one or more haplotypes with a patient?

The correct answer is that a parent is most likely to share one or more haplotypes with a patient because haplotypes are inherited directly from parents. Each individual receives half of their genetic material from each parent, which includes a combination of both maternal and paternal haplotypes.

In the context of histocompatibility, which examines the compatibility of tissue types, particularly for organ transplants, siblings also have a high likelihood of sharing haplotypes due to shared genetic material, but parents are the direct sources of half of an individual's genetic information. First cousins, on the other hand, share approximately 12.5% of their genetic material, while second cousins share even less, making it much less likely for them to have matching haplotypes. Therefore, in terms of genetic inheritance, the parent is the most likely relative to share haplotypes with the patient, confirming its role as the source of direct genetic input.