Not true among the following is
Isolated finding of simian crease in Down's in seen in almost 1% of Down's children. So it is not a significant finding in a Down's child.
Most common cause of Down's syndrome is Maternal meiotic disjunction.
Most Common inherited condition causing Intellectual disability in Fragile X Syndrome
The mandible in Down's child is smaller than usual
Isolated finding of simian crease in Down's in seen in almost 20% of Down's children.