Aseries of enzymes catalyze the reaction x → y → z → a. product a binds to the enzyme that converts x to y at a position remote from its active site. this binding decreases the activity of the enzyme. with respect to the enzyme that converts x to y, substance a functions as?

The correct answer is: An allosteric inhibitor.

An allosteric inhibitor is the regulator of an enzyme that binds at a site other than the enzyme's active site. That site is called the allosteric site or regulatory site.

When the regulator binds enzyme usually goes through conformational change. Since the activity of the enzyme in example is reduced, we call it inhibitor, otherwise it would be activator.

The inhibition shown in example is negative feedback (final product turns off the reaction in the initial site).  

this is true, assuming that the allele is completely recessive and incomplete dominance is not a factor.