That is precisely the point: it's the exact same defect in the genes of related species. See, for discussion and references, crevo debate
What you're thinking of is cases like sickle cell anemia and thallesemia (sp?) which are different diseases, caused by different mutations. Both mutations are positively selected for in areas with malaria.
It seems to me that the presence of a nonfunctional gene for a vital nutrient is evidence against intelligent design and any form of creationism that attempts to 'explain' anything more recent than the initial creation of life. Are there any orthopedic surgeons who are creationists ?
Please list some testable predictions made by intelligent design theory or creation theory. They *are* scientific, aren't they?