Down's syndrome is caused by trisomy of chromosome 21. Trisomy refers to the presence of three copies of one chromosome in the genome. Since David has extra genetic material in chromosome 21, he is suffering from Down's syndrome.
Down's syndrome would affect the mental development and learning process of David. He is likely to have a broad skull with a small mouth.
No one cares how smart you are and the questions you ask why would ask this on a good website for answers when your so smart you can ask good questions but not answer it.
