Why do you think switching the hemoglobin gene's sixth amino acid from glutamic acid to valine would affect the hemoglobin protein?

The amino acids that make up a protein form its primary structure. A hindrance in the primary structure will affect how the protein molecule folds. The function of protein molecules is highly dependent on their shapes, so if the hemoglobin molecule's shape is distorted, it will not be able to carry out its function of transporting oxygen efficiently.

Switching the hemoglobin gene amino acid from glutamic acid to valine will affect the folding of the hemoglobin and this in turn will affect the function of this protein. The change will affect the way haemoglobin functions and leads to mutation.