Loganism

I was browsing through some new blogs I found last night (while Laney was napping) and Logan came in here...this is the conversation that followed...

Logan: "Moma, who is that little kid?"
Me: "Just a little boy on a blog I found. He has Down syndrome like Laney."
Logan: "Like in that book we read?"
Me: "Yes, like that ."
Logan: "He kinda looks like Ben." (ben is the boy in the book we read.) "What is Down syndrome again?"
Me:"Down syndrome means that they have an extra chromosome."
Logan: "What's a chromosome?"
Me: "It's what makes us who we are."

Anyone out there reading this have any suggestions on how to explain to my very inquisitive almost 6 yr old son what a genetic disorder is? We have read books about Ds but he ask more difficult questions...aka...the chromosome question. How do I explain something I didn't learn about until high school to my 1st grader?