Have you come to any points in your family tree where the branches start to cross? That is, where relatives have actually married one another. I’m sure you have, and if not, you will eventually. Almost all family trees have a few cousin-marriages in them. Marrying close relatives is not as socially acceptable as it once was, though that depends on the culture and time frame that you’re researching. The term “consanguinity” is the official way to describe marriage between close relations.
Through history, there have been many periods where marrying your own cousin (even a first cousin) was acceptable and even considered a good idea. This would keep assets and estates within the family, and was thought to strengthen the bloodlines. And of course, travel was pretty limited in the past and it was hard to get away from your family even if you wanted to.
It was found as a custom more often in royal families, specifically to keep the royal lines free of “commoners”. The royal houses of England and Britain are particularly known for this. If your tree dips into any of these lines, you’re almost certain to get some cousin marriages sooner or later.
So why all the fuss? The main reason why marrying close relations has fallen out of favor was the eventual understanding of genetics, and how certain diseases can become magnified in children of closely related parents. Without turning this into a biology lesson, I can give a little explanation:
Certain disease are caused by recessive genes, which means someone needs to get a gene from both parents in order to actually become ill. Depending on the condition, that can happen pretty rarely. But if one of these recessive genes is showing up in a family where relatives are marrying, the odds of their kids getting these diseases is far greater. One famous example of this is how hemophilia in the royal family of England ended up killing a number of people and spread to several other royal houses in the process.
But the bottom line is that when you start finding consanguinity or interfamily marriages in your family tree, there is nothing worry about. Besides, there is nothing you can do about it now. The only problem can be with your record-keeping and charts. Closely related people will have the same ancestors, which can create several duplicate sections to a family tree. Some software programs are able to take care of this and weed out duplications, otherwise, you can get a very cumbersome tree.