When a man makes sperm or a woman makes an egg, the DNA they got from mom and dad (soon to be grandma and grandpa) that in their body are separated into mom's chromosome #1-23 and dad's chromosomes #1-23 - are shuffled randomly together.
If there are two genes close to each other on the same chromosome there will be a genetic “distance” calculated in centi-Morgans that will reflect how likely there is to be a “crossing over” event swapping the DNA between the two.
If, for example, there is a 70 centi-Morgan distance between two linked genes, then in 70% of the offspring the two genes will not go together and in 30% of the offspring the genes will still be associated together.
Obviously nature loves randomness. Random shuffling of the variable region of an antibody enables the body to “search” the design space of 3-D structure such that it can create an antibody that can bind to every conceivable 3-D structure.
Obviously random changes with an associated selection process (whereby self binding antibodies are eliminated) is a very POWERFUL mechanism.
On the subject you raise (not at all what I was talking about) Geneticists have LONG known that you cannot assume that two people that are mated/married are going to be as genetically distinct as two random people.
People mate assortively for a host of traits that include the obvious (height, intelligence, education) but also things like mid digital hair and incisor length.
I have checked out many a fine woman over the years, and yet I have NEVER said “did you check out the incisor length on THAT babe?”
Who or what is doing the "selecting?"