How to determine if this blood disorder is recessive or dominant?
Given that this is a text book question about classical genetics, you can safely disregard any reality about the disorder.

I'd argue that the two alleles show incomplete dominance. The mild anemia, which is intermediate between healthy individuals and those with severe anemia, can be attributed to heterozygous individuals. In this case, the expression of the functional allele is not enough to compensate for the nonfunctional allele, producing the mutant phenotype.