Not necessary but that is the direction that nature programs species to do. In fact, it is the final goal of each generation. If one individual didn’t get the memo, the species will continue but the individual - for whatever reason - is effectively removed from the gene pool by genetic design.
Not entirely. Say a couple...a quaint old concept...has four offspring. One homosexual offspring and one that dies in infancy do not reproduce. The two remaining offspring do. All four offspring were variations of two people's DNA. The survivors have a great deal of DNA in common with the non-reproducers, and pass it along.
Each of us is composed, genetically, of an eighth of each of our great-grandparents DNA, a quarter of each of our grandparents. A first cousin shares one eighth of our DNA.