In genetics, we have Queen Victoria family and Tzar and Tzarina family to illustrate the inheritance patterns, but genetics does not provide any american family as an example. What is the reason behind it?
Interesting. I'm not sure where or when your textbook was written, or where you live - but there are other factors at play, no doubt.
What is included in a textbook as a case-example has more to do with social-beliefs, culture, politics and other things that are considered by the book authors, teachers and others - instead of "who is the *best* example for this case?"
An example is the debate in the US over how to write in Obama's name (in history text books, etc) - they argue on whether to initialize his middle name or if they should include the entire middle name.
Instead of following general rules that are applied to essays, thesis papers and other things that students are suppose to write they had several meetings with people just giving their opinions on the issue.
(If I recall you are not in the US)
In the US - from my years of schooling - we don't use American-citizens as case-studies at all. We use leaders from Europe in the past (Mary, the Queen of Scots. The Tudor family Dynasty.) and even further back in history in other countries like Egypt (Pharaoh Ahkenaten and his wife Nefertiti) as example of such things.
I think - in this 'avoid USA examples' is that USA history is *very* recent - and it's likely that modern-day decedents would be offended if their Great Grandparent became the focus for scientific studies in a classroom.
It is very common and accepted to use very prominent and important figures (such as Queens, Kings) because their lineage is very well documented, everyone is notable and easy to track through the generations, and they're public figures so discussing them isn't likely to be taboo.
Good luck in your class!