17.2 Multiple Alleles and Blood Groups
Key Takeaways
Multiple alleles means the population has more than two alleles of a gene, but each person carries only two.
The ABO alleles are IA, IB, and i. IA and IB are codominant, and both are dominant to i.
Type A is IAIA or IAi, type B is IBIB or IBi, type AB is IAIB, and type O is ii.
In introductory red-cell teaching, type O is the universal donor and type AB is the universal recipient, which is not a full transfusion protocol.
Rh-negative is dd at a separate locus, and sensitization of an Rh-negative mother can cause hemolytic disease in a later Rh-positive pregnancy.
17.2 Multiple Alleles and Blood Groups
Multiple alleles means that more than two alleles of one gene exist in the population. Human ABO blood type is the standard example, and Rh is a second blood-type locus. A person is diploid, so that person still carries only two ABO alleles, one from each parent. A child inherits two ABO alleles, never three.
Three alleles, two per person
The alleles are IA, IB, and i. IA places A antigen on red cells, IB places B antigen, and i places neither. IA and IB are codominant: when both are inherited, both antigens are fully expressed. Each is dominant to i, so only ii lacks both antigens.
| Blood type | Genotypes | Antigens on red cells | Antibodies in plasma |
|---|---|---|---|
| A | IAIA or IAi | A | anti-B |
| B | IBIB or IBi | B | anti-A |
| AB | IAIB | A and B | neither anti-A nor anti-B |
| O | ii | neither A nor B | anti-A and anti-B |
Type A is IAIA or IAi, type B is IBIB or IBi, type AB is IAIB, and type O is ii. If the stem says only type A, both IAIA and IAi remain possible. IAIA has no i gamete, while IAi does. Genotypes ii and IAIB fix both the phenotype and the alleles.
Codominance is not a blend
Type AB is codominance, not incomplete dominance. Incomplete dominance would be an intermediate phenotype that is neither full A nor full B. AB cells carry both antigens in full form, and typing detects both.
Antibodies and the classic red-cell rule
A person makes antibodies against the ABO antigens they lack. Type A plasma has anti-B. Type B plasma has anti-A. Type O plasma has both anti-A and anti-B. Type AB plasma has neither, because both antigens are self. These ABO antibodies are already in the plasma in this model, unlike anti-D, which usually appears only after exposure to D antigen.
The antibodies give the red-cell rule taught in introductory biology. Type O is the universal donor of red cells because the donated cells lack A and B antigens. The recipient's anti-A or anti-B finds no ABO antigen of that kind on those cells. Type AB is the universal recipient of red cells because the recipient's plasma lacks anti-A and anti-B, so A antigen, B antigen, or both on donated cells are not attacked by those antibodies.
That rule is not a complete modern transfusion protocol. Type O plasma contains anti-A and anti-B, so type O is not a universal donor of plasma or of whole blood. Real transfusions also check Rh and crossmatch the donor to the recipient.
Under that same rule, type A, which has anti-B, can receive A or O red cells but not B or AB cells. Type B, which has anti-A, can receive B or O red cells. Type AB can receive any ABO red cells. Type O can receive only type O red cells.
Two family problems
Each parent contributes one ABO allele in the gamete. Write those gametes before you name the child.
A type A parent and a type B parent
One parent has blood type A with genotype IAi. That parent produces gametes IA and i, half of each. The other parent has blood type B with genotype IBi. That parent produces gametes IB and i, half of each. Four combinations are equally likely, so each is expected in one-fourth of the children.
IA joined with IB produces IAIB, type AB. IA joined with i produces IAi, type A. i joined with IB produces IBi, type B. i joined with i produces ii, type O.
This couple can have a type O child (ii) and a type AB child (IAIB), and they can also have type A and type B children. The type O child received i from both parents, not a third allele. The type AB child received IA and IB and still has only two alleles.
Two type O parents
Both parents are ii, so every gamete carries i and every child is ii, type O. They cannot have a type A child, because neither parent has an IA allele to give. They cannot have a type AB child, which needs both IA and IB. The child cannot inherit a third allele to supply a missing antigen.
A type AB parent is IAIB and has no i to give, so that parent cannot have a type O child. Phenotypes A and B can hide i, but AB is IAIB and O is ii.
Rh is a separate locus
Rh type is a separate locus, not a fourth ABO allele. In the introductory model, allele D is dominant to allele d. DD and Dd are Rh-positive. dd is Rh-negative. A person can be type O and Rh-positive, or type AB and Rh-negative.
An Rh-negative mother is dd, so her red cells have no D antigen. An Rh-positive fetus inherited D from the father, so the fetal cells carry D. If those cells enter her blood, often around birth, she can become sensitized and make antibodies against D.
In a later pregnancy, those antibodies can cross the placenta. If that fetus is Rh-positive, they bind D and destroy fetal red cells. That illness is hemolytic disease of the newborn, also called erythroblastosis fetalis in older books. Hemolysis can cause anemia, and bilirubin from the broken cells can cause serious jaundice in the newborn. An Rh-negative fetus has no D antigen for the antibodies to bind. The risk appears in a later Rh-positive pregnancy, after an Rh-negative mother has been sensitized.
If the father is dd, every child is dd. If the father is DD, every child of an Rh-negative mother is Dd and Rh-positive. If the father is Dd, half the children are expected to be Rh-positive and half Rh-negative.
Important
Type AB is codominance because both A and B antigens are fully present. It is not an incomplete-dominance blend. A child inherits two ABO alleles, never three. Multiple alleles is a description of the population, not a third copy inside one person.
Which description of the ABO blood group is correct?
The population has only two ABO alleles, and type AB is incomplete dominance, a blend that is neither full A nor full B antigen.
Allele i is codominant with IA and IB, so genotype ii places both A and B antigens on red cells.
Each person carries three ABO alleles at the same time, one for A, one for B, and one for O.
The population has alleles IA, IB, and i, but each person carries two. IA and IB are codominant, and both are dominant to i.
One parent has genotype IAi and the other has genotype IBi. Which result can occur among their children?
Every child must be type O, and a type AB child is impossible because IA and IB cannot meet in one child.
Every child must be type AB, and a type O child is impossible because i cannot come from both parents.
A type O child with genotype ii and a type AB child with genotype IAIB are both possible.
The child inherits IA, IB, and i together, so the genotype contains three ABO alleles.
Which statement agrees with introductory ABO and Rh teaching?
Type O people are the universal recipients of red cells because their plasma has no antibodies, and type AB red cells are the universal donor cells because they lack antigens.
Two parents with genotype ii can have a type A or type AB child if that child inherits a third ABO allele.
Type AB blood is incomplete dominance, and an Rh-negative person has genotype DD.
Two type O parents cannot have a type A or type AB child. Rh-negative is dd at a separate locus, and sensitization of an Rh-negative mother can lead to hemolytic disease in a later Rh-positive pregnancy.
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