Multiple Choice
In the context of variations of dominance, which term describes an allele that is phenotypically expressed only when present in the homozygous state?
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3. Extensions to Mendelian Inheritance
Variations of Dominance
Multiple Choice
In the context of variations of dominance, the human ABO blood group system is best described as which type of inheritance?
A
Polygenic inheritance controlled by many genes with additive effects
B
Simple complete dominance with only two alleles
C
Multiple alleles with codominance (I^A and I^B codominant, both dominant over i)
D
Incomplete dominance producing an intermediate phenotype in heterozygotes
Verified step by step guidance1
Understand the ABO blood group system involves three alleles: \(I^A\), \(I^B\), and \(i\).
Recognize that \(I^A\) and \(I^B\) alleles are both dominant over the \(i\) allele but are codominant with each other, meaning both \(I^A\) and \(I^B\) express their traits equally when present together.
Recall that codominance occurs when two different alleles at a locus are both fully expressed in the heterozygote, unlike incomplete dominance where the heterozygote shows an intermediate phenotype.
Note that this system is not polygenic because it is controlled by a single gene with multiple alleles, not many genes with additive effects.
Conclude that the ABO blood group system exemplifies multiple alleles with codominance, where \(I^A\) and \(I^B\) are codominant and both dominant over \(i\).
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