If one was to conduct an internet search to find out what are the clinical manifestations of Down Syndrome, it would turn up many sites describing the physical features of Down Syndrome. Typically, you would see the features that are characteristic of a diagnosis of Down Syndrome.
People assume that just because a person is physically different than the typical or normal person, that the difference seen, is abnormal in nature. Further, that this abnormal feature in nature must be incompatible with life or cause difficulty in life. So much so, that when individuals hear that their child has a diagnosis of Down Syndrome, little thought is given to what does that difference actually mean because people are still grasping with the diagnosis itself.