Ted makes a good point which, to my knowledge, has *never* been adequately explained to the layman. Genes are instructions for construction. That's it. Two recipes for mayonnaise may appear almost identical, but the one which reads "slowly beat eggs into oil" will fail; while the one which reads "slowly beat oil into eggs" will succeed. The recipes may be identical to 99.99 percent, but if this crucial instruction sequence is mixed up, you ain't getting mayonnaise, no way, no how.