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How many distinct genotypes are possible for the human ABO blood group system, which is controlled by three alleles (I^A, I^B and i)?
AFour
BSix
CThree
DNine
Answer & Solution
Correct answer: A. Four
With three alleles and a diploid genotype, the combinations are I^A I^A, I^A i, I^B I^B, I^B i, I^A I^B and ii, giving six genotypes (and four phenotypes A, B, AB, O).
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