28. 'Rare' vs 'Impossible'

This complaint is more geared toward genetic traits, but the same basic idea applies to pretty much anything.  Basically, just because something is rare, doesn't mean that it's impossible to have.

As far as genetic traits, mutations, and diseases go, review your biology text book and learn about basic statistics.  Let's say that you and your partner each have a gene that may cause your children to become blind, but it's about a twenty-five percent chance.  Now let's say that this couple does have four children, but none of them are blind, how is that possible?  Well, in this example, the odds and statistics starts over after every child is born.

To explain this, let's say you have a bag of marbles.  Four of the marbles are blue, two is red, and one is yellow.  If you were to reach in the bag without peaking inside, you would have a four-in-seven chance of getting a blue marble, two-in-seven chance of getting a red marble, and one-in seven chance of getting the yellow marble.  Let's say you got the blue marble, leaving six marbles left in the bag—three blue, two red and one yellow.  Then you reached into the bag again for another marble with a fifty percent chance of getting a blue marble, one-third chance of getting a red marble and one-in-six chance of getting a yellow marble.  Each time you reach into the bag to grab a marble, your odds of getting the yellow marble gets higher and higher.  

This is not how genetics works.

Now with the same marbles in the bag example, instead of keep reaching into the bag to get another marble, each time you choose a marble, you put that marble back into the bag and reach again.  Each time you reach into the bag, the odds of getting the yellow marble stays the same—one-in-seven.  So the odds that a child will be born blind is still one-in-four, but the odds don't increase with every child born, the odds stay the same after each child, like putting the marble back in the bag.  This is also the reason why the same couple could have had all four children be blind.  The only reason the odds would increase, is if during each pregnancy something happens or deteriorates which would also cause blindness.

Let's say that in a fandom, one of the canon characters has an extremely rare genetic disorder that affects the immune system and there were only two other documented cases in the world some decades earlier.  So, basically, it's basically one-in-seven billion chance, or even one-in-a million or more chance.  Would it be possible for another character or an OC to have the same disorder?  Yes, because everyone has those statistical odds, in addition that there really could already be people who have that disorder but never went to the hospital or it just was never documented.

For something non-genetically related, let's say there's only one mystical weapon in the world.  If a canon character already possesses it, then it really is impossible for another character to have unless the fan fiction is Alternate Universe or slightly changed in order to accommodate for the extra mystical weapon.