ABO Blood Group Allele Inheritance

Steps:

• Genotypes: Type A is I^A I^A or I^A i; type B is I^B I^B or I^B i.
• Siblings with A and B require parents to supply I^A to one child and I^B to the other.
• Both parents homozygous (e.g., I^A I^A with I^B I^B yields only AB; I^A I^A with i i yields only A) cannot produce both A and B phenotypes.
• At least one parent must be heterozygous (e.g., I^A I^B with i i yields A or B) to provide the necessary alleles.

Why C is correct:

• At least one parent must be heterozygous to carry and transmit both I^A and I^B alleles, per Mendelian inheritance rules.

Why the others are wrong:

• A: Not required; one parent can be homozygous (e.g., i i) while the other is I^A I^B.
• B: Impossible; homozygous pairs cannot generate both I^A and I^B in separate offspring.
• D: Not required; both parents can be heterozygous (e.g., I^A i and I^B i).

Final answer: C