Consider patterns
of gene expression at three different loci, A, B,
and D:
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The standard + allele produces a
functional Xase enzyme;
the a allele produces a non-functional enzyme. The
+a heterozygote produces half the typical concentration
of enzyme made by a ++
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).
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The standard + allele produces a
functional Yase enzyme;
the B allele produces a non-functional enzyme. The +B
heterozygote produces half the typical concentration 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 enough 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+). Even though the recessive ++
homozygote produces more enzyme, this does not make
the + allele "dominant" to the B allele.
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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.
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Based on the phenotypic expression patterns of the genotypic combinations of alleles ¢ and $, which of the two alleles is dominant to the other? Explain.
Given
your decision, explain the genotypic and molecular
basis of the phenotypes.