A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offspring.
Since the child has blood group O, the child's genotype must be ii. This means the child inherited one i allele from each parent. Therefore, both parents must be heterozygous.
Genotypes of Parents:
- Father (Blood group A): i (must carry 'i' to pass it to child)
- Mother (Blood group B): i (must carry 'i' to pass it to child)
Possible Genotypes of Other Offspring: When i × i cross occurs, the possible genotypes are:
- – Blood group AB (co-dominance)
- i – Blood group A
- i – Blood group B
- ii – Blood group O
Explanation
According to NCERT Class 12 Biology, the ABO blood grouping is controlled by gene I with three alleles: , , and i. Since blood group O has genotype ii (homozygous recessive), the child received one 'i' allele from each parent. This confirms both parents are heterozygous carriers. The father (blood group A) has genotype i, and the mother (blood group B) has genotype i. A cross between these genotypes produces four possible offspring genotypes in a 1:1:1:1 ratio: (AB), i (A), i (B), and ii (O).
Solution Steps
Step 1: Identify child's genotype as ii since blood group O is homozygous recessive
Step 2: Deduce parents must be heterozygous since they each passed one 'i' allele to child
Step 3: Determine Father's genotype = i (Blood group A but carries i)
Step 4: Determine Mother's genotype = i (Blood group B but carries i)
Step 5: Cross i × i to get possible offspring genotypes: , i, i, ii