Question

A woman with blood type A has two children. One has type O blood and the other has type B blood. What is the genotype of the mother? What are the genotype and phenotype of the father? What is the genotype of each child?

Accepted Answer

Step 1: Understand the blood type inheritance system. Blood types A, B, AB, and O are determined by the ABO gene, which has three alleles: $$ I^A $$, $$ I^B $$, and $$ i . $$The alleles $$ I^A $$ and $$ I^B $$ are codominant, and $$ i  is $$recessive. Blood type A can be either $$ I^A I^A  or$$ $$ I^A i $$, blood type B can be $$ I^B I^B  or$$ $$ I^B i $$, blood type AB is $$ I^A I^B $$, and blood type O is $$ ii . $$Step 2: Analyze the mother's genotype based on her blood type A and the children's blood types. Since one child has type O blood (genotype $$ ii $$), the mother must carry the recessive $$ i $$ allele to pass it on. Therefore, the mother's genotype must be $$ I^A i . $$Step 3: Determine the father's genotype and phenotype. The children have blood types O and B. The child with blood type O must have received an $$ i $$ allele from both parents, so the father must also carry at least one $$ i $$ allele. The child with blood type B could have genotype $$ I^B I^B  or$$ $$ I^B i . $$Since the father must pass an $$ I^B $$ allele to produce a child with blood type B, the father’s genotype must be $$ I^B i . $$The father's phenotype is blood type B. Step 4: Assign genotypes to each child. The child with blood type O has genotype $$ ii  ($$received $$ i $$ from both parents). The child with blood type B has genotype $$ I^B i  ($$received $$ I^B $$ from the father and $$ i $$ from the mother). Step 5: Summarize the genotypes and phenotypes: Mother is $$ I^A i  ($$blood type A), father is $$ I^B i  ($$blood type B), one child is $$ ii  ($$blood type O), and the other child is $$ I^B i  ($$blood type B).

A woman with blood type A has two children. One has type O blood and the other has type B blood. What is the genotype of the mother? What are the genotype and phenotype of the father? What is the genotype of each child?

Key Concepts

ABO Blood Group Inheritance

Genotype vs. Phenotype

Punnett Square and Genetic Crosses