Question

A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits - blood group A or O - is dominant? Explain.

Answer 1

The blood type of the ABO blood grouping system is based on the presence or absence of our red blood cells with A and B antigens.

Complete answer:
It is not possible to assume from the above-mentioned data since we do not know the blood groups of other offspring, these are the dominant characteristics between the blood groups A and O. In 1901, Karl Landsteiner discovered the ABO blood grouping system. The single genes which are the A, B, and O gene, regulates the ABO blood type. There are three types of alleles in it: $I$, $iA$, and $iB$. Here I stand for isoagglutinogen, another term for a type of antigen. The antigens are present on the RBC cell membrane. Blood type A consists of only antigen A, while the blood type B includes only antigen B. There are both A and B antigens present in the AB blood type and there are no A or B antigens in the O blood type.
The O blood group is the 'Universal donor' and the $'AB+ve'$ blood group is the 'Universal recipient' throughout the blood transfusion process. The immune system produces antibodies against any ABO blood group antigens that are not present on the RBCs of the body. Thus, an individual in Group A has anti-B antibodies, and an individual in Group B has anti-A antibodies. Blood group O has both anti-A and anti-B in its serum. The blood group AB does not contain anti-A or anti-B in its serum.

Note: Individuals with $iAiA$ or $iAi$ have blood type A and those with $iBiB$ or $iBi$ have blood type B. Since both $iA$ and $iB$ are the dominant alleles over allele I, an individual with O blood group only has ii alleles. Here the blood group of the father is A, it can either be homozygous ($AA$) or heterozygous ($AA$)($AO$). The blood group of the mother is O and it is only possible if it is heterozygous ($OO$). The father must be heterozygous ($AO$) for the daughter to have blood group O, and the mother must be heterozygous ($AO$) ($OO$).