The ABO blood group system is a standard example of codominance and multiple alleles and always appears in genetics textbooks. Together with the Rh blood group system, let us understand the inheritance of blood types, blood transfusions, and blood-type incompatibility between mother and child.
Basic Knowledge
Inheritance of the ABO blood group system: This is an example of multiple alleles, with three alternative alleles (I ᴬ, I ᴮ, i) at one locus.
・I ᴬ is dominant over i: I ᴬI ᴬ or I ᴬi → type A.
・I ᴮ is dominant over i: I ᴮI ᴮ or I ᴮi → type B.
・I ᴬ and I ᴮ are codominant: I ᴬI ᴮ → type AB.
・ii → type O.
Type A red blood cells = A antigen + anti-B serum antibody (IgM). Type B = B antigen + anti-A antibody. Type AB = both antigens and no antibodies. Type O = no antigens and both antibodies.
Principles of ABO blood types and transfusion: When red-blood-cell antigens react with antibodies in the recipient, agglutination (hemolysis) occurs. Type O = universal donor (red blood cells only); type AB = universal recipient. However, modern transfusions generally use the same blood type.
Rh blood group system: People are classified as Rh-positive (Rh⁺) or Rh-negative (Rh⁻) according to the presence or absence of the D antigen. Having the D-antigen gene (RHD) = Rh⁺; not having it = Rh⁻. In an Rh⁻ person, an incompatible transfusion or pregnancy does not produce anti-Rh antibodies during the first exposure (only sensitization occurs), but a strong response occurs during the second exposure, causing hemolytic disease of the newborn (Rh-incompatible pregnancy). If an Rh⁻ mother gives birth to an Rh⁺ child, problems may occur in the next pregnancy. Administration of anti-D immunoglobulin (Rhogam) prevents this.
Applications of blood-type inheritance: Problems that calculate the probabilities of a child's blood type from the parents' blood types frequently appear on entrance examinations. Example: The children of type A (I ᴬi) × type B (I ᴮi) are type A: type B: type AB: type O = 1:1:1:1.
A type O child was born to a type A mother and a type B father. Identify the genotypes of both parents and find the probability that this child's sibling will be type AB.
Answer: The allele combination of a type O child is ii, so both parents must carry the i allele. The type A mother = I ᴬi, and the type B father = I ᴮi. The children of I ᴬi × I ᴮi are I ᴬI ᴮ (AB): I ᴬi (A): I ᴮi (B): ii (O) = 1:1:1:1. Therefore, the probability of type AB is 1/4 = 25%.
- ABO type = multiple alleles (Iᴬ, Iᴮ, i) and codominance (Iᴬ and Iᴮ)
- Type O = ii; has both antibodies (anti-A and anti-B)
- Type AB = Iᴬ Iᴮ; has neither antibody
- Rh⁺ = D antigen present; Rh⁻ = D antigen absent
- Rh-incompatible pregnancy = Rh⁻ mother × Rh⁺ child → risk of hemolytic disease of the newborn
- Rhogam = anti-D immunoglobulin that prevents Rh sensitization
- With a type A mother and type B father, all four types—O, AB, A, and B—can be born
Points to Note
① Type O (ii) is recessive homozygous, while type AB (I ᴬI ᴮ) has two codominant alleles. ② Rh incompatibility is more likely to cause problems from the second child onward (sensitization during the first pregnancy, followed by antibody attack in later pregnancies). ③ “Type O is the universal donor” applies only to red blood cells. Different principles apply to plasma and platelets.
Practice
- List all correspondences between ABO blood group genotypes and phenotypes using the concept of multiple alleles.
- Find the blood-type ratio among the children of an AB-type father and an O-type mother.
- Explain why Rh-incompatible pregnancy is more likely to cause problems from the second child onward, in terms of sensitization and immune memory.