The H substance is needed for the proper phenotypic expression of the A, B, and AB blood types in the ABO blood typing system in humans. Individuals who are recessive for the H substance (hh) will have a type O phenotype, even if they have the A and/or B alleles. Two individuals, each with type AB blood and heterozygous at the H locus have children. What is the probability they will have a child who is phenotypically type O?

The correct answer is - 4/16.

Explanation:

In this question, it is stated that in ABO blood system the proper phenotypic expression of A, B, AB blood types needs H substance. If an individual has recessive for the H substance (hh) will have a type O phenotype.

The cross: ABHh and ABHh

gametes: two set of AH, BH, Ah, Bh

Punnett:

         AH         BH          Ah          Bh

AH    AAHH   ABHH     AAHh     ABHh

BH    ABHH   BBHH     ABHh     BBHh

Ah    AAHh    ABHh     AAhh      ABhh

Bh    ABHh    BBHh     ABhh      BBhh

As shown there is only 4 out of 16 offspring are recessive for H substance therefore, only 4/16 will have O blood group