Blood Typing

Blood Typing

High School

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Learning Objectives

• Understand how blood type is determined by genes inherited from parents.

• Distinguish between genotype and phenotype in the context of blood typing.

• Describe the roles of dominant, recessive, and co-dominant alleles.

• Identify the characteristics of universal donors and recipients.

• Differentiate between class and individual evidence as it relates to blood.

Key Vocabulary

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Allele

One of three versions of the blood type gene, which are A, B, or O.

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Genotype

A person's specific combination of inherited alleles, like AA, AO, or BO, from their parents.

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Phenotype

The observable, physical blood type of a person, such as Type A or Type B.

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Dominant Gene

A gene that is expressed, like A or B, overpowering a recessive gene in the process.

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Recessive Gene

A gene, like O, that is only expressed when two copies of it are present.

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Co-dominant Genes

Genes, like A and B, that are both fully expressed when they are inherited together.

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The Genetics of Blood

• The gene for ABO blood type is located on chromosome 9.

• You inherit one copy of this gene from each of your parents.

• There are three different alleles for the blood type gene: A, B, and O.

• Your genotype is your inherited alleles; your phenotype is your actual blood type.

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Multiple Choice

A person's observable, physical blood group is described by which term?

• Allele

• Genotype

• Chromosome 9

• Phenotype

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How Alleles Determine Blood Type

• The 'A' and 'B' alleles are dominant, while the 'O' allele is recessive.

• Alleles 'A' and 'B' are co-dominant, expressing both in Type AB blood.

• Genotypes AA or AO give Type A, while BB or BO give Type B.

• The recessive O allele is only expressed in the genotype OO for Type O.

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Solved Example 1
A mother with AO genotype and a father with BO genotype have a child. What is the probability of this child having blood type O?

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Step 1: Analyze and Sketch the Problem

• Goal: Determine the probability of the child having blood type O.

• Knowns: Mother's genotype is AO, Father's genotype is BO.

• Unknown: The probability of an OO genotype.

• Formula: Use a Punnett square to determine the possible genotypes of the offspring.

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Solved Example 1
A mother with AO genotype and a father with BO genotype have a child. What is the probability of this child having blood type O?

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Step 2: Solve for the Unknown

• Draw a Punnett square, placing the mother's alleles (A, O) on one axis and the father's alleles (B, O) on the other.

• Fill in the squares with the resulting genotypes: AB, AO, BO, OO.

• Count the number of possible genotypes: There are 4.

• Count the number of OO genotypes: There is 1.

• The probability is the number of OO genotypes divided by the total number of genotypes: 1/4 or 25%.

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Solved Example 1
A mother with AO genotype and a father with BO genotype have a child. What is the probability of this child having blood type O?

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Step 3: Evaluate the Answer

• The Punnett square shows four equally likely outcomes: AB, AO, BO, and OO.

• The OO genotype, which results in Type O blood, is one of these four possibilities.

• Therefore, a 25% probability is the correct calculation based on Mendelian genetics.

Multiple Choice

If a person has the genotype AO, what is their blood type (phenotype)?

• Type AB

• Type A

• Type AO

• Type O

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Universal Donors and Recipients

Universal Donor

• People with Type O blood are known as the ‘universal donors’ in blood transfusions.

• Their red blood cells lack A and B antigens, preventing immune reactions in recipients.

• This means their blood can be safely given to a person with any blood type.

Universal Recipient

• Individuals with Type AB blood are called the ‘universal recipients’ of blood donations.

• Their bodies do not produce A or B antibodies, which attack foreign blood cells.

• This allows them to safely receive blood from any other ABO blood type.

Multiple Choice

Why is a person with Type AB blood known as the 'universal recipient'?

• They do not produce A or B antibodies.

• They can donate blood to anyone.

• Their red blood cells have no antigens.

• They have the recessive O allele.

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Blood Evidence: Class vs. Individual

Class Evidence

• ​Blood type is considered class evidence because it is shared by many people.

• ​​This can help to narrow the number of potential suspects down to a group.

• ​However, it cannot be used to pinpoint one specific individual as the source.

Individual Evidence

• ​If DNA can be extracted, blood becomes individual evidence that identifies one person.

• ​​Except for identical twins, a person's DNA profile is completely unique to them.

• ​This provides a definitive link between the blood evidence and a specific suspect.

Multiple Choice

Why is blood type considered class evidence?

• Because it contains DNA.

• Because many people in a group can share the same blood type.

• Because it can only be used to exclude a suspect.

• Because it provides a unique link to one individual.

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Common Misconceptions About Blood Types

Misconception	Correction
Genotype 'AO' results in Type O blood.	The 'O' allele is recessive; the dominant 'A' allele determines the blood type.
Alleles A and B are always dominant over other alleles.	A and B are co-dominant, resulting in Type AB blood if inherited together.
Blood type is definitive proof of guilt in a crime.	Blood type is class evidence, not individual evidence like DNA.

Multiple Choice

If a parent with genotype AB and a parent with genotype OO have children, what blood type is impossible for their children to have?

• Type A

• Type B

• All are possible

• Type O

Multiple Choice

How does the concept of co-dominance apply to a person with Type AB blood?

• The B allele is dominant over the A allele.

• The A allele is dominant over the B allele.

• Both the A and B alleles are expressed, so they have characteristics of both blood types.

• Neither the A nor the B allele is expressed in the phenotype.

Multiple Choice

At a crime scene, a blood sample is found to be Type B. Suspect 1 has blood type O, and Suspect 2 has blood type B. What is the most accurate conclusion a forensic scientist can draw from this evidence alone?

• Suspect 1 is eliminated, and Suspect 2 remains a possibility.

• Suspect 2 is guilty.

• Both suspects are eliminated.

• Suspect 1 is guilty.

Multiple Choice

A person's phenotype is Type A blood. Which of the following statements correctly describes their possible genotypes?

• The genotype must be AO, as this is the only way to get Type A blood.

• The genotype is OO, as the O allele is recessive.

• The genotype must be AA, because A is a dominant allele.

• The genotype could be AA or AO, as the dominant A allele masks the recessive O allele.

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Summary

• Your genotype (A, B, O alleles) from your parents determines your blood type phenotype.

• Alleles A and B are co-dominant with each other and are dominant over the O allele.

• Type O is the universal blood donor, while Type AB is the universal recipient.

• In forensics, blood type is class evidence, while DNA from blood is individual evidence.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

• 1

• 2

• 3

• 4