Introduction:
Blood group inheritance is determined by the presence or absence of certain antigens on the surface of red blood cells. There are four main blood groups: A, B, AB, and O. The presence of certain antigens determines the blood group, and the absence of all antigens leads to the O blood group. In this scenario, a man with blood group A marries a woman with an unknown blood group, and their daughter has blood group O. Let's analyze if this information is enough to determine whether blood group A or O is dominant.

Explanation:
Blood group inheritance follows certain rules. Each individual receives one blood group allele from each parent, resulting in two alleles that determine their blood group. The possible alleles for blood groups are A, B, and O.

Dominant and recessive alleles:
Dominant alleles are expressed when present, even if there is only one copy, while recessive alleles are only expressed when there are two copies.

Possible genotypes:
Based on the given information, let's consider the possible genotypes for each family member:
- The man (blood group A) could have genotype AA or AO.
- The woman's blood group is unknown, so her genotype could be AA, AO, or OO.
- The daughter (blood group O) would have genotype OO.

Observations:
1. Since the daughter has blood group O, she must have received an allele for blood group O from both parents.
2. If the woman had blood group A (genotype AO), she could have passed on the O allele to her daughter. However, if the woman had blood group O (genotype OO), she could only pass on the O allele to her daughter.
3. If the man had blood group A (genotype AA), he could not have passed on the O allele to his daughter.

Conclusion:
Based on the observations, it can be concluded that blood group O is dominant over blood group A. This is because the daughter has blood group O, indicating that she must have received an O allele from both parents. If blood group A was dominant, the daughter would have inherited at least one A allele from her father, resulting in blood group A or AB.

Therefore, the given information of a man with blood group A marrying a woman with an unknown blood group, and their daughter having blood group O, is enough to determine that blood group O is dominant over blood group A.