Which of the following determines a person's ABO blood type?

• A. The specific alleles inherited at the ABO gene locus
• B. The number of red blood cells in circulation
• C. The amount of hemoglobin in the blood
• D. The presence of antibodies against Rh factor

Answer: (A)

Verified step by step guidance

1. Understand that ABO blood type is determined by the specific alleles inherited at the ABO gene locus, which is located on chromosome 9. These alleles encode for enzymes that modify carbohydrate molecules on the surface of red blood cells.
2. Recognize that there are three main alleles for the ABO gene: IA, IB, and i. IA and IB are codominant, meaning both can be expressed if inherited together, while i is recessive and does not produce any surface antigens.
3. Learn that the presence of specific antigens (A, B, or none) on the surface of red blood cells determines the blood type (A, B, AB, or O). For example, IAIA or IAi results in type A blood, IBIB or IBi results in type B blood, IAIB results in type AB blood, and ii results in type O blood.
4. Understand that the number of red blood cells, the amount of hemoglobin, and the presence of antibodies against Rh factor are unrelated to determining ABO blood type. These factors are associated with other aspects of blood physiology and compatibility.
5. Conclude that the correct answer is 'The specific alleles inherited at the ABO gene locus,' as this directly determines the ABO blood type by encoding the antigens present on red blood cells.