, the ABO blood grouping is controlled by the gene I, which has three alleles: and . alleles and are dominant over .

1. Child's Genotype: The child has blood group O, which is a recessive trait. This is only possible if the genotype is homozygous recessive (ii). The child must inherit one allele from each parent .
2. Parents' Genotypes:

• The father is blood group B. To pass on an allele, he must be heterozygous ().
• The mother is blood group A. To pass on an allele, she must be heterozygous ().
• If either parent were homozygous dominant ( or ), all offspring would receive a dominant allele, making blood group O impossible . Therefore, the respective genotypes are and .