Final answer:
A parent-child pairing shares a greater number of alleles than a sibling-sibling pairing due to the direct inheritance of a complete chromosome set from each parent, compared to the genetic recombination and diversity present between siblings.
Step-by-step explanation:
When comparing genetic similarities on autosomes, a parent-child pairing will have a greater number of shared alleles than a sibling-sibling pairing. This is because a child inherits a complete set of chromosomes from each parent, which includes one copy of every gene situated on those chromosomes.
On the other hand, siblings inherit different combinations of their parents' alleles, which means that while they share approximately 50% of their DNA, there is variability in the specific alleles they inherit. Additionally, recombination events during meiosis can shuffle the alleles in novel combinations, contributing to genetic diversity amongst siblings much more than between parent and child.