Multiple Alleles and Blood Groups
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You should be comfortable with the concepts of dominant and recessive alleles, codominance, and how to calculate genotypic and phenotypic ratios from simple monohybrid crosses.
Why this matters
Human blood groups provide the classic real-world example of multiple alleles and codominance in action.
While Mendel’s original experiments involved traits with only two alleles (e.g., purple vs. white flowers), many biological systems are more complex. The ABO blood group system is the most famous example of multiple alleles, where three distinct alleles exist in the population. Understanding this system is crucial not only for genetics but also for medicine, particularly in blood transfusions and organ transplants, where compatibility depends on precise antigen-antibody interactions. This lecture bridges the gap between simple Mendelian ratios and the more complex inheritance patterns seen in humans.
The ABO System in Plain Language
Imagine the ABO blood group system as a factory that produces specific surface markers, or antigens, on red blood cells. There are three main workers in this factory, known as alleles: IA, IB, and i. The IA allele instructs the cell to produce Antigen A, while the IB allele instructs it to produce Antigen B. The i allele is essentially a non-functional version; it produces no antigen at all. Because humans are diploid, we carry two of these alleles, one from each parent. If you inherit IA and IB, your factory produces both Antigen A and Antigen B simultaneously. This is codominance: neither allele masks the other, so both products are visible on the cell surface. If you inherit IA and i, you only produce Antigen A, because the i allele contributes nothing. The IA and IB alleles are both dominant over i. This results in four possible phenotypes (observable blood types): A, B, AB, and O. Type O individuals have inherited two i alleles and thus display no A or B antigens. This system is a perfect illustration of how a single gene can have more than two variants in a population, creating a richer diversity of outcomes than simple dominant-recessive models allow.
Genotypes and Phenotypes
To predict inheritance, we must distinguish between genotype (the genetic makeup) and phenotype (the observable blood type). There are six possible genotypes for the ABO system, which map to the four phenotypes. The genotype IAIA results in Type A blood, as does the heterozygous genotype IAi. Similarly, the genotype IBIB results in Type B blood, as does IBi. The genotype IAIB results in Type AB blood, and the genotype ii results in Type O blood. This mapping is critical because it means that observing a person’s blood type does not always reveal their exact genotype. For example, a person with Type A blood could be homozygous (IAIA) or heterozygous (IAi). However, a person with Type AB or Type O blood has a known genotype: IAIB or ii, respectively. This ambiguity is central to many genetic problems involving blood types, particularly when determining paternity or predicting the blood types of offspring from parents with known phenotypes but unknown genotypes.
Transfusion Compatibility
Blood transfusion safety relies on the principle that antibodies in the plasma will attack foreign antigens on donor red blood cells. The immune system produces antibodies against antigens it does not possess. Therefore, a person with Type A blood (genotype IAIA or IAi) has Antigen A on their cells but lacks Antigen B. Consequently, their plasma contains anti-B antibodies. If they receive Type B blood, their anti-B antibodies will attack the donor’s Antigen B, causing agglutination and hemolysis. Similarly, Type B individuals have anti-A antibodies. Type AB individuals have both Antigen A and Antigen B, so they produce no anti-A or anti-B antibodies; they are universal recipients for red blood cells. Type O individuals have neither Antigen A nor Antigen B, so they produce both anti-A and anti-B antibodies; they are universal donors for red blood cells (specifically, their red cells can be given to anyone, though plasma compatibility is a separate issue). This logical deduction of antibody presence based on antigen absence is a direct application of the multiple allele model.
Predicting Offspring Blood Types
When crossing parents with known blood types, we must consider all possible genotypes. For instance, if a Type A parent (genotype IAi) mates with a Type B parent (genotype IBi), we can set up a Punnett square. The gametes from the Type A parent are IA and i. The gametes from the Type B parent are IB and i. The possible offspring genotypes are IAIB (Type AB), IAi (Type A), IBi (Type B), and ii (Type O). The phenotypic ratio is 1:1:1:1. This demonstrates that even if both parents have "common" blood types, they can produce offspring with any of the four blood types. This flexibility is a hallmark of multiple allele systems and contrasts sharply with simple two-allele systems where only two phenotypes are possible. It also highlights the importance of considering heterozygosity; if either parent were homozygous (e.g., IAIA), the range of possible offspring blood types would be restricted.
A common misconception is that Type O blood is "dominant" because it appears in the offspring of two non-O parents. In reality, Type O is recessive. The i allele is recessive to both IA and IB. Type O only appears when an individual inherits two i alleles. The fact that two Type A parents can have a Type O child is evidence that both parents are heterozygous (IAi), not that O is dominant. This is a classic example of how phenotypic ratios can reveal hidden genotypes. C
Worked example
Problem. A woman with Type A blood (genotype unknown) and a man with Type B blood (genotype unknown) have a child with Type O blood. What is the probability that their next child will have Type AB blood?
- Determine the genotypes of the parents. Since they have a Type O child (ii), each parent must have contributed an i allele. Therefore, the mother must be IAi and the father must be IBi.
- List the possible gametes from the mother: IA and i, each with a probability of 0.5.
- List the possible gametes from the father: IB and i, each with a probability of 0.5.
- Identify the genotype required for Type AB blood: IAIB. This occurs when the mother contributes IA and the father contributes IB.
- Calculate the probability: \( P(\text{Mother } I^A) \times P(\text{Father } I^B) = 0.5 \times 0.5 = 0.25 \).
Units check. Probability is a dimensionless ratio, so no units are required.
Three quick tests
- What is the genotype of a person with Type AB blood?
Answer
IAIB - If a Type A person (genotype IAIA) mates with a Type O person (genotype ii), what are the possible blood types of their children?
Answer
All children will be Type A (genotype IAi). - Why can a Type O person donate red blood cells to a Type A person?
Answer
Type O red cells lack Antigen A and B, so they are not attacked by the anti-A antibodies in the Type A recipient's plasma.
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Every lecture and every concept in this unit, with prerequisites and threads.
Lecture notes drafted with a local model and checked line by line by the editor; every number in the worked example was recomputed.
Concepts cited: — · Threads: information, evolution