Correct answers given in Genitive section are missing apostrophes in English.
There are German sentences like the following that are to be translated into English: Das ist das Haus meiner Großeltern. Ich bin der Sohn meiner Eltern.
The 'correct' answers given by Duo to these type of sentences are wrong in that they do not include an apostrophe to indicate possession: That is my grandparents house. I am my parents son.
Both of these sentences are grammatically incorrect and should be: That is my grandparents' house. I am my parents' son.
Please see point 1C on this page: http://www.grammarbook.com/punctuation/apostro.asp
In another thread (https://www.duolingo.com/comment/223373) we've been told this is a software bug. If enough people report it as a bug, maybe it might get fixed. You can report software bugs here: https://support.duolingo.com/hc/en-us/requests/new
This is a point where I totally agree with you (and Heike). If this site is about language then it should be correct in it's usage. At least there should be a hint about the correct spelling, even if it might accept incorrect spelling - as long as the correct sense is being transmitted.