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? Why or why not?
No, this information is not enough to tell which trait – blood group A or O – is dominant.
According to the rules of inheritance, each trait has two versions in each child, one from each parent. A dominant trait appears when at least one copy is present, while a recessive trait appears only when both copies are present.
If blood group A is dominant, the man could have genotype AA or AO, and the woman would be OO. If the man is AO, the daughter could inherit O from both parents, showing blood group O. If blood group O is dominant, the man would be AA, and the woman could be OO or OA. The daughter could inherit A from father and O from mother, showing blood group O. Since both scenarios can produce the observed result, this single case cannot determine which trait is dominant.
Explanation
The textbook explains that each trait has two versions in each child, and dominant traits appear with at least one copy while recessive traits need both copies. The question tests whether students can apply these principles. The answer is no because the observed outcome (daughter with blood group O) is possible under both scenarios - whether A is dominant or O is dominant. The phenotype alone doesn't reveal the genotype, and multiple genetic combinations can produce the same observable result.
Solution Steps
Step 1: Understand that each trait has two versions (alleles) in each child, one from each parent.
Step 2: Apply the principle that dominant traits appear with at least one copy, while recessive traits need both copies.
Step 3: Analyze both possibilities - if A is dominant or if O is dominant.
Step 4: Recognize that in both cases, a daughter with blood group O is possible.
Step 5: Conclude that this single case cannot determine which trait is dominant.