Relationship of Genotype to Phenotype
The Xase gene locus specifies the enzyme Xase, which converts the precursor substrate X to the product substance Y. The production of Y is a phenotypic consequence of the genotypic expression of the Xase gene.
A second locus Yase produces an enzyme Yase that converts Y to Z. Because the production of Z is dependent on the proper function of Xase, it is also a phenotype of the Xase gene. The final phenotype may be another intermediate in the metabolic pathway (see the discussion of arginine metabolism in Neurospora), an endpoint product (see the discussion of hemoglobin in humans), or a trait resulting from the expression of such a product (see the discussion of sickle-cell anemia).