What Are Multiple Alleles?

In many introductory genetics examples, a gene has two possible:

alleles.

For example:

B

and:

b.

However, some genes have more than two possible alleles within a:

population.

This situation is called:

multiple alleles.

A gene with multiple alleles has:

three or more alternative forms of the gene in the population.

---

Individuals Still Have Only Two Alleles

This is an important distinction.

A gene may have several possible alleles within the:

population.

However, a typical diploid individual still possesses only:

two alleles for that gene.

Why?

Because one allele is inherited from:

one biological parent

and the other from:

the other biological parent.

---

Population vs Individual

Imagine a gene has four possible alleles:

A¹, A², A³, A⁴.

All four alleles might exist within the population.

However, one individual might have:

A¹A³.

Another might have:

A²A⁴.

Each individual has two alleles, even though the population contains:

four possible alleles.

---

The ABO Blood Group System

One of the best-known examples of multiple alleles in humans is the:

ABO blood group system.

The ABO gene has three commonly discussed alleles:

Iᴬ

Iᴮ

i.

Therefore, ABO blood type demonstrates:

multiple alleles.

---

The Three ABO Alleles

The three alleles are:

Iᴬ

Iᴮ

i.

Iᴬ is associated with production of:

A antigen.

Iᴮ is associated with production of:

B antigen.

The i allele does not produce functional A or B antigen.

These allele combinations determine a person's:

ABO blood group.

---

What Are Antigens?

An antigen is a molecule that can be recognized by the immune:

system.

In the ABO system, A and B antigens can be present on the surface of:

red blood cells.

Blood group A has:

A antigen.

Blood group B has:

B antigen.

Blood group AB has:

A and B antigens.

Blood group O has:

neither A nor B antigen.

---

ABO Genotypes and Phenotypes

There are six possible common genotypes but only four ABO:

phenotypes.

Therefore:

6 genotypes → 4 blood types.

---

Blood Type A

A person with blood type A can have either genotype:

IᴬIᴬ

or:

Iᴬi.

This means knowing that someone has blood type A does not necessarily tell us their exact:

genotype.

---

Blood Type B

A person with blood type B can have:

IᴮIᴮ

or:

Iᴮi.

Again, the phenotype alone does not reveal the exact genotype.

---

Blood Type AB

A person with blood type AB has genotype:

IᴬIᴮ.

In this case, the phenotype does reveal the:

genotype.

Both alleles are expressed.

---

Blood Type O

A person with blood type O has genotype:

ii.

Therefore, the O phenotype also reveals the:

genotype.

Both alleles are i.

---

Dominance in the ABO System

The relationships among the three alleles are important.

Iᴬ is dominant over i.

Iᴮ is dominant over i.

But:

Iᴬ and Iᴮ are codominant with each other.

Therefore:

Iᴬi → Type A

Iᴮi → Type B

IᴬIᴮ → Type AB.

---

What Does Codominance Mean Here?

In codominance, both alleles are expressed in a heterozygous:

individual.

For genotype:

IᴬIᴮ

the Iᴬ allele leads to production of A antigen.

The Iᴮ allele leads to production of B antigen.

Therefore, both:

A and B antigens

are present.

The phenotype is:

AB.

This is codominance.

---

AB Is Not an Intermediate Blood Type

Codominance does not mean that A and B blend together to create an intermediate:

antigen.

A person with AB blood has:

A antigen AND B antigen.

Both are expressed separately.

This is why ABO inheritance is an excellent example of:

codominance.

---

ABO Inheritance

Each biological parent contributes:

one ABO allele.

The offspring therefore receives:

one allele from each parent.

If a parent has genotype:

Iᴬi

they can produce gametes containing:

Iᴬ

or:

i.

If another parent has genotype:

Iᴮi

they can produce:

Iᴮ

or:

i.

We can use a Punnett square to predict the possible offspring.

---

Cross: Iᴬi × Iᴮi

Possible genotypes:

IᴬIᴮ

Iᴬi

Iᴮi

ii.

---

Converting Genotypes to Blood Types

Now translate each genotype into its phenotype:

IᴬIᴮ → AB

Iᴬi → A

Iᴮi → B

ii → O.

Therefore, this cross predicts:

25% A

25% B

25% AB

25% O.

---

A Powerful Result

The cross:

Iᴬi × Iᴮi

can produce offspring with all four ABO blood types:

A

B

AB

O.

This is possible because each parent carries:

two different alleles.

---

Cross: Type O × Type O

Type O has only one possible genotype:

ii.

Therefore:

ii × ii.

Each parent can contribute only:

i.

All offspring will be:

ii.

Therefore:

100% Type O.

---

Cross: Type AB × Type O

The genotypes are:

IᴬIᴮ × ii.

The AB parent can produce:

Iᴬ or Iᴮ.

The O parent can produce only:

i.

Therefore:

50% Type A

50% Type B.

No Type AB or Type O offspring are expected from this cross.

---

Why Can't AB × O Produce O?

For an offspring to have Type O blood, the genotype must be:

ii.

That requires an i allele from:

both parents.

A Type AB parent has genotype:

IᴬIᴮ.

They do not possess an:

i allele.

Therefore, they cannot contribute i.

So Type O is not possible from:

IᴬIᴮ × ii.

---

Why Can't AB × O Produce AB?

An AB offspring requires:

IᴬIᴮ.

The Type O parent can contribute only:

i.

Therefore, the offspring cannot receive both Iᴬ and Iᴮ.

So Type AB is also impossible in this:

cross.

---

Cross: Type AB × Type AB

The genotypes are:

IᴬIᴮ × IᴬIᴮ.

Possible blood types:

Type A = 25%

Type AB = 50%

Type B = 25%.

Type O is:

not possible.

---

Why Can't Two AB Parents Produce Type O?

Type O requires:

ii.

Neither AB parent possesses an:

i allele.

Therefore, neither parent can contribute i.

Without two i alleles, Type O cannot:

occur.

---

Cross: Type A × Type O

This problem is more complicated because Type A can have two possible:

genotypes.

The Type A parent could be:

IᴬIᴬ

or:

Iᴬi.

The Type O parent must be:

ii.

Therefore, we need to consider:

two possible crosses.

---

Possibility 1: IᴬIᴬ × ii

The Type A parent can contribute only:

Iᴬ.

The Type O parent contributes only:

i.

Therefore:

100% Iᴬi

and:

100% Type A.

---

Possibility 2: Iᴬi × ii

The Type A parent can contribute:

Iᴬ

or:

i.

The Type O parent contributes only:

i.

Therefore:

50% Iᴬi → Type A

50% ii → Type O.

This demonstrates why knowing the parents' phenotypes may not always be enough to make one exact prediction.

---

Using Offspring to Determine a Parent's Genotype

Suppose a Type A parent and a Type O parent have a Type O:

child.

The Type O child must be:

ii.

One i came from the Type O parent.

The other i must have come from the Type A:

parent.

Therefore, the Type A parent cannot be:

IᴬIᴬ.

They must be:

Iᴬi.

This is an example of using offspring evidence to determine a:

genotype.

---

Cross: Type B × Type O

The same reasoning applies to Type B.

The Type B parent could be:

IᴮIᴮ

or:

Iᴮi.

If:

IᴮIᴮ × ii

then all offspring are:

Iᴮi → Type B.

If:

Iᴮi × ii

then:

50% Type B

50% Type O.

---

Can Two Type A Parents Have a Type O Child?

Yes, under the standard ABO model.

Both Type A parents would need to be:

Iᴬi.

Cross:

Iᴬi × Iᴬi.

Possible blood types:

75% Type A

25% Type O.

Therefore, two Type A parents can have a Type O child if both carry:

i.

---

Can Two Type B Parents Have a Type O Child?

Yes.

Both would need genotype:

Iᴮi.

Cross:

Iᴮi × Iᴮi.

Possible genotypes:

IᴮIᴮ

Iᴮi

Iᴮi

ii.

Therefore:

75% Type B

25% Type O.

---

Can a Type A Parent and Type B Parent Have a Type O Child?

Yes, if their genotypes are:

Iᴬi

and:

Iᴮi.

Each parent can contribute:

i.

The child can therefore receive:

ii

and have:

Type O blood.

---

Can Type O Parents Have a Type AB Child?

Under the standard ABO inheritance model:

No.

Type O parents have genotype:

ii × ii.

They can contribute only:

i alleles.

A Type AB child requires:

IᴬIᴮ.

Neither Iᴬ nor Iᴮ is available from Type O parents.

---

Solving Blood Group Problems

A useful method is:

Step 1: Write the possible genotype for each blood type.

A:

IᴬIᴬ or Iᴬi

B:

IᴮIᴮ or Iᴮi

AB:

IᴬIᴮ

O:

ii

Step 2: Identify which genotypes are possible for the parents.

Step 3: Determine the possible gametes.

Step 4: Construct the Punnett square.

Step 5: Determine offspring genotypes.

Step 6: Convert genotypes into blood types.

Step 7: Compare your prediction with the evidence.

---

Using Blood Groups to Exclude Possibilities

Blood group information can sometimes show that a particular genetic relationship is:

not possible under the standard ABO model.

For example:

Two Type O parents:

ii × ii

can produce only:

ii offspring.

Therefore, an AB child would not fit that simple inheritance:

combination.

---

But Blood Type Has Limitations

ABO blood type can sometimes exclude a proposed biological relationship under the standard model, but it usually cannot prove:

parentage.

Why?

Because millions of people share each blood:

type.

Modern genetic relationship testing uses many DNA markers rather than relying only on:

ABO blood groups.

---

Blood Type Evidence Problem

Suppose:

Parent 1 has Type A blood.

Parent 2 has Type B blood.

Their child has Type O blood.

What can we determine?

The child must be:

ii.

Therefore, each parent must have contributed:

i.

So Parent 1 must be:

Iᴬi.

Parent 2 must be:

Iᴮi.

The evidence allows us to infer both parental:

genotypes.

---

Another Evidence Problem

Suppose:

Parent 1 = Type AB

Parent 2 = Type O

Possible children are:

Type A

or:

Type B.

If a child were reported as Type O, that result would not match the expected inheritance from:

IᴬIᴮ × ii

under the standard ABO model.

---

Determining an Unknown Parent

Suppose one parent has:

Type O blood (ii).

Their child has:

Type AB blood (IᴬIᴮ).

Could this parent supply either allele needed for the child's AB genotype?

The Type O parent can supply only:

i.

The AB child requires:

Iᴬ and Iᴮ.

Therefore, this combination does not fit the standard ABO inheritance:

model.

---

Blood Groups and Probability

Suppose:

Iᴬi × Iᴮi.

The probability of Type O is:

25%.

If the first child has Type O, does that change the probability that the next child will have Type O?

No.

Each conception is an independent:

event.

For the same parental genotypes, the probability remains:

25%.

---

Probability Does Not Guarantee Outcomes

Suppose a cross predicts:

25% Type A

25% Type B

25% Type AB

25% Type O.

Four children are not guaranteed to include one child of each blood:

type.

The family could have four Type A children.

That would be less likely, but it is still:

possible.

Punnett squares describe probability, not a fixed sequence of outcomes.

---

ABO Blood Groups and Transfusion

ABO blood groups are medically important because incompatible red blood cells can trigger dangerous immune:

reactions.

A person's immune system may contain antibodies that react with A or B antigens not normally present on their own red blood:

cells.

For this reason, ABO compatibility is carefully checked before blood transfusion.

---

ABO Antigens and Antibodies

In the simplified ABO model:

Type A

• A antigen
• anti-B antibodies

Type B

• B antigen
• anti-A antibodies

Type AB

• A and B antigens
• neither anti-A nor anti-B antibodies

Type O

• neither A nor B antigen
• anti-A and anti-B antibodies

This helps explain why blood-group matching is important during:

transfusions.

---

ABO Is Not the Only Blood Group System

Human blood type is more complicated than simply:

A, B, AB, or O.

There are many blood group systems.

Another medically important system is the:

Rh blood group system.

This is why a person's blood type may be written as:

A+

O−

AB+

and so on.

The + or − is not part of the ABO gene itself.

---

ABO and Rh Are Different Systems

For example:

A+

means that the person has:

ABO Type A

and is:

Rh positive.

Similarly:

O−

means:

ABO Type O

and:

Rh negative.

The ABO and Rh systems involve different genes and should not be confused.

---

Multiple Alleles Create More Genetic Diversity

If a gene has only two alleles:

A and a

there are three possible diploid genotypes:

AA

Aa

aa.

But with three alleles, such as:

Iᴬ, Iᴮ, and i,

there are six common genotype combinations:

IᴬIᴬ

IᴬIᴮ

Iᴬi

IᴮIᴮ

Iᴮi

ii.

Multiple alleles therefore increase the number of possible:

genotypes.

---

Multiple Alleles and Codominance Are Different Concepts

These terms describe two different aspects of:

inheritance.

Multiple alleles means:

More than two forms of a gene exist within the population.

Codominance means:

Two different alleles are both expressed in a heterozygous individual.

The ABO system demonstrates:

both concepts.

---

Why ABO Demonstrates Multiple Alleles

The ABO gene has:

three commonly discussed alleles.

These are:

Iᴬ

Iᴮ

i.

Therefore:

ABO demonstrates multiple alleles.

---

Why ABO Demonstrates Codominance

An individual with genotype:

IᴬIᴮ

expresses both:

A antigen

and:

B antigen.

Therefore:

Iᴬ and Iᴮ are codominant.

The resulting phenotype is:

Type AB.

---

Why ABO Also Demonstrates Dominance

The ABO system contains another allele relationship.

Iᴬ is dominant over:

i.

Therefore:

Iᴬi → Type A.

Iᴮ is dominant over:

i.

Therefore:

Iᴮi → Type B.

So the ABO system demonstrates:

multiple alleles

codominance

and:

complete dominance over i.

---

Common Mistake: Thinking a Person Has Three ABO Alleles

The population has three commonly discussed ABO:

alleles.

An individual normally has only:

two.

For example:

Iᴬi

contains two alleles.

The third allele may exist elsewhere in the population, but it is not part of that individual's genotype.

---

Common Mistake: Saying AB Is a Blend

Type AB is not an intermediate blend between Type A and Type:

B.

Both A and B antigens are present.

Therefore, the relationship is:

codominance,

not incomplete dominance.

---

Common Mistake: Assuming Type A Means IᴬIᴬ

Type A has two possible genotypes:

IᴬIᴬ

or:

Iᴬi.

Similarly, Type B can be:

IᴮIᴮ

or:

Iᴮi.

Always consider both possibilities unless additional evidence identifies the genotype.

---

Common Mistake: Treating i as Codominant

The i allele is not codominant with Iᴬ or:

Iᴮ.

Instead:

Iᴬ > i

and:

Iᴮ > i.

Codominance occurs between:

Iᴬ and Iᴮ.

---

Common Mistake: Using Blood Type Alone to Prove Parentage

ABO blood types can sometimes exclude a proposed relationship under the standard model.

However, they generally cannot uniquely identify a biological:

parent.

DNA analysis provides much stronger evidence.

---

Check Your Understanding

1. What does the term multiple alleles mean?

2. Can one diploid individual normally have three alleles for the same gene? Explain.

3. Name the three commonly discussed alleles of the ABO blood group gene.

4. Which allele produces A antigen?

5. Which allele produces B antigen?

6. What is the relationship between Iᴬ and Iᴮ?

7. What is the relationship between Iᴬ and i?

8. What is the relationship between Iᴮ and i?

9. List the two possible genotypes for Type A blood.

10. List the two possible genotypes for Type B blood.

11. What genotype produces Type AB?

12. What genotype produces Type O?

13. Explain why Type AB demonstrates codominance.

14. Explain why the ABO system demonstrates multiple alleles.

15. Why is Type AB not an example of incomplete dominance?

16. Construct a Punnett square for Iᴬi × Iᴮi.

17. What blood types are possible from this cross?

18. What is the probability of Type O?

19. What is the probability of Type AB?

20. Determine the possible offspring from IᴬIᴮ × ii.

21. Why can't IᴬIᴮ × ii produce a Type O child under the standard model?

22. Can two Type O parents normally produce a Type A child? Explain.

23. Can two Type A parents produce a Type O child? Under what condition?

24. Can two Type B parents produce a Type O child? Under what condition?

25. A Type A parent and Type O parent have a Type O child. What must the Type A parent's genotype be?

26. A Type A parent and Type B parent have a Type O child. What can you determine about both parental genotypes?

27. Why can blood group evidence sometimes exclude a proposed biological relationship?

28. Why can ABO blood type usually not prove parentage?

29. What is the difference between ABO and Rh blood groups?

30. Explain how multiple alleles, dominance, and codominance all operate within the ABO blood group system.

---

Key Terms

• Multiple alleles: Three or more alternative forms of a gene existing within a population.
• ABO blood group: Human blood-group system involving the Iᴬ, Iᴮ, and i alleles.
• Iᴬ: ABO allele associated with production of A antigen.
• Iᴮ: ABO allele associated with production of B antigen.
• i: ABO allele that does not produce functional A or B antigen.
• Antigen: Molecule that can be recognized by the immune system.
• Antibody: Immune protein capable of recognizing and binding specific antigens.
• Codominance: Inheritance pattern in which both alleles are expressed in a heterozygote.
• Genotype: Allele combination possessed by an individual.
• Phenotype: Observable characteristic resulting from a genotype and other influences.
• Blood Type A: Phenotype produced by IᴬIᴬ or Iᴬi.
• Blood Type B: Phenotype produced by IᴮIᴮ or Iᴮi.
• Blood Type AB: Phenotype produced by IᴬIᴮ.
• Blood Type O: Phenotype produced by ii.
• Rh system: Blood-group system separate from the ABO system.
• Punnett square: Diagram used to predict possible offspring genotypes.

---

Key Takeaways

• Multiple alleles means that more than two alleles for a gene exist within a population.
• A diploid individual still normally possesses only two alleles for that gene.
• The ABO blood group system is a classic example of multiple alleles.
• The three commonly discussed ABO alleles are Iᴬ, Iᴮ, and i.
• Iᴬ produces A antigen.
• Iᴮ produces B antigen.
• Iᴬ and Iᴮ are codominant.
• Both Iᴬ and Iᴮ are dominant over i.
• IᴬIᴬ and Iᴬi produce Type A.
• IᴮIᴮ and Iᴮi produce Type B.
• IᴬIᴮ produces Type AB.
• ii produces Type O.
• Type AB demonstrates codominance because both A and B antigens are expressed.
• AB is not an intermediate blend between A and B.
• There are six common ABO genotypes but four ABO phenotypes.
• Each biological parent contributes one ABO allele to an offspring.
• Punnett squares can be used to predict possible blood types.
• Iᴬi × Iᴮi can produce A, B, AB, or O offspring.
• IᴬIᴮ × ii can produce Type A or Type B offspring.
• Two Type O parents normally produce only Type O offspring under the standard ABO model.
• Two Type A parents can produce Type O if both are Iᴬi.
• Two Type B parents can produce Type O if both are Iᴮi.
• Offspring blood types can sometimes provide evidence about unknown parental genotypes.
• ABO blood-group information can sometimes exclude a proposed relationship, but it generally cannot prove parentage.
• The Rh system is separate from the ABO system.
• The ABO system demonstrates multiple alleles, codominance, and dominance within the same genetic system.