Consider patterns of gene expression at three different loci:

    The standard + allele produces a functional Xase enzyme; the a allele produces a non-functional enzyme.  The +a heterozygote produces half the typical amount of enzyme made by an ++ homozygote. This is sufficient for standard phenotypic expression ("green") [haplosufficiency], and the phenotype of the +a heterozygote resembles that of the ++ homozygote. The variant allele a is therefore described as recessive to the wild-type + allele (or, + is dominant to a; this may also be written a⁺ is dominant to a).

    The standard + allele produces a functional Yase enzyme; the B allele produces a non-functional enzyme. The +B heterozygote produces half the typical amount of enzyme. So far this is exactly the same pattern as at the Xase locus. However, the amount of enzyme produced by the +B heterozygote is not engough to produce standard phenotypic expression [haploinsufficiency], and the phenotype of the +B heterozygote resembles that of the BB homozygote ("pink"). The variant allele B is therefore described as dominant to the wild-type + allele  (or, + is recessive to B, or B is dominant to b⁺). Note that, even though the recessive ++ homozygote produces more enzyme, this does not make the + allele "dominant" to the B allele.

    The standard + allele produces a functional Zase enzyme; the d allele results in overproduction of the enzyme, which results in an exaggerated phenotype ("dark blue"). The phenotype of the +d heterozygote is the same as the ++ homozygote ("light blue"): the variant d allele is therefore described as recessive to the wild type + allele (or, d⁺ is dominant to d). Note that the higher level of enzyme production and the exaggerated phenotype produced by the d allele do not make it dominant to the + allele.

    At all three loci, the molecular expression of the heterozygote is exactly intermediate between the two homozygotes, and would be described as semi-dominant.  However, in each case the pattern of resultant phenotypic expression is different. The variant alleles are described as dominant or recessive, depending on the phenotype of the heterozygote relative to that of the two homozygotes.

Based on the phenotypic expression patterns for the genotypes involving alleles ¢ and $, which of the two alleles is dominant to the other? Explain.
How do you explain the phenotypes, given this definition of dominant?