Answer:
The best answer to the question: Why is it that two full siblings can be equally related to a given grandparent, even though both of them have received equal amounts of DNA from their mother and father? Would be: Genetic recombination.
Step-by-step explanation:
When an offspring receives chromosomes from the father´s sperm and the mother´s egg, a process takes place to ensure that these chromosomes appropriately combine and the balance is maintained on the total number of chromosomes and also on which gene will be phenotypically expressed on one offspring, and which in another. This process is called genetic recombination and basically it is a shuffling of genes. This means that the parents of the two full siblings each received a copy of chromosomes from both their own parents, one of which would be the grandparent of the full siblings. But as meiosis takes place and the chromosomes fuse, shuffling, or recombination takes place, and while one offspring will receive a given set of genes, the other may receive the same, or another set. This means that as generations are born, genetic material from the grandparent will further recombine with those of others present and the percentage of genetic material directly linked to that grandparent will diminish, but will not completely disappear. Thus, genetically, we will be linked to the progenitor line, but the percentage of genes directly received will lessen.