Gene point mutation is when a base that describes how to make a protein is changed. It is responsible for sickle cell because there is a replacement of an A by a T. The consequence is that the 6th amino acid in the chain becomes valine instead of glutamic acid. This results in abnormal hemoglobin and affects how oxygen is carried in the body. The mutated hemoglobin changes the shape of the red blood cells.
A chromosome mutation is when there is a change in the number of copies of chromosomes or an alteration in the chromosome structure. It is responsible for Down Syndrome because there is a trisomy (extra chromosome) in Chromosome 21. This is due to nondisjunction. This means that during meiosis, the chromosome pair did not separate.