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THE GENETICS OF BLOOD

Introduction The Human Genetics Tutorial with problem solving exercises concerning the inheritance of the ABO blood group alleles has resulted in a steady stream of inquiries to the Biology Project from mothers, grandmothers, and children inquiring about the possible blood type of the father of a given child. Here is a typical inquiry: I have been reading your info about inheritance of blood types and I am getting very confused! I am trying to figure out what blood type the father of my son could have since my son and I are both type A+. Also, my brother is type 0 and my mom is A+. We can't find anything that explains how this can be. Could you please help??? --From a concerned Mom in Alberta, Canada" Rick Hallick, Professor of Biochemistry at The University of Arizona, replied to each question received. We created a problem set with edited versions of the questions. Could you have helped these people? Compare your answers with Dr. Hallick's. We recommend going through his explanation of the inheritance of blood types and Rh factors before attempting to answer the questions. A review of the tutorials of blood types problem 11 and problem 13 of the Monohybrid Cross Problem Set of the Mendelian Genetics might also be helpful.

The Human ABO markers: The A, B, and O alleles Human blood type is determined by co-dominant alleles. An allele is one of several different forms of genetic information that is present in our DNA at a specific location on a specific chromosome. Blood Types

There are three different alleles for human blood type:

BLOOD TYPES FOR SIMPLICITY WE CALL THESE

IA A IB B i O

2

Genotypes Each of us has two ABO blood type alleles, because we each inherit one blood type allele from our biological mother and one from our biological father. A description of the pair of alleles in our DNA is called the genotype. Since there are three different alleles, there are a total of six different genotypes at the human ABO genetic locus.

Allele from

Parent 1

Allele from

Parent 2

Genotype of offspring

Blood types of offspring

A A AA A

A B AB* AB

A O AO A

B A AB* AB

B BB B B

B O BO B

O O OO O

How are blood types related to the six genotypes?

A blood test is used to determine whether the A and/or B characteristics are present in a blood sample. It is not possible to determine the exact genotype from a blood test result of either type A or type B. If someone has blood type A, they must have at least one copy of the A allele, but they could have two copies. Their genotype is either AA or AO. Similarly, someone who is blood type B could have a genotype of either BB or BO.

BLOOD TYPE POSSIBLE GENOTYPES

A AA or AO

B BB or BO

A blood test of either type AB or type O is more informative. Someone with blood type AB must have both the A and B alleles. The genotype must be AB. Someone with blood type O has neither the A nor the B allele. The genotype must be OO.

BLOOD TYPE POSSIBLE GENOTYPES

AB AB

O OO

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How are ABO alleles inherited by our children?

Each biological parent donates one of their two ABO alleles to their child. A mother who is blood type O can only pass an O allele to her son or daughter. A father who is blood type AB could pass either an A or a B allele to his son or daughter. This couple could have children of either blood type A (O from mother and A from father) or blood type B (O from mother and B from father).

INHERITED BLOOD TYPE OF A CHILD

O from the mother A from the father

A

O from the mother B from the father

B

Since there are 4 different maternal blood types and 4 different paternal blood types possible, there are 16 different combinations to consider when predicting the blood type of children.

What about the Rh factor?

The Rh factor genetic information is also inherited from our parents, but it is inherited independently of the ABO blood type alleles. There are 2 different alleles for the Rh factor known as Rh+ and Rh-.

Someone who is "Rh positive" or "Rh+" has at least one Rh+ allele, but could have two. Their genotype could be either Rh+/Rh+ or Rh+/Rh-. Someone who Rh- has a genotype of Rh-/Rh-.

Just like the ABO alleles, each biological parent donates one of their two Rh alleles to their child. A mother who is Rh- can only pass an Rh- allele to her son or daughter. A father who is Rh+ could pass either an Rh+ or Rh- allele to his son or daughter. This couple could have Rh+ children (Rh- from mother and Rh+ from father) or Rh- children (Rh- from mother and Rh- from father).

RH FACTOR POSSIBLE GENOTYPES

Rh+ Rh+/Rh+

Rh-/Rh+

Rh- Rh-/Rh-

MOTHER FATHER CHILD

Rh- Rh+ Rh+

Rh- Rh- Rh--

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Blood Types Problem Set

Problem 1: Is a type O possible in this situation?

Could a man with type B blood and a woman with type AB produce a child with type O blood?" a. Possible b. Not Possible Problem 2: Possible blood types of offspring. What if the mother is type O+ and the father is A-? What would the offspring's blood type be? The offspring could be: a. A+, or O+ b. A-, or O- c. A+, A-, O+, or O- d. A+, or O-

Problem 3: What blood type would the father be?

If the mother of a child is blood type O+ and the child is A-, what blood type would the father be? Does the Rh factor of the child being - mean that one of the parents has to be negative? Both of my parents are Rh - all of us siblings are Rh- as well. Could two Rh- parents give birth to a Rh+ child? and vice versa? The father could be: a. A+, A-, or O- b. A+, A -, AB+, or AB- c. B+, B-, or O- Problem 4: What are the potential blood types of the father of my son? I have been reading your info about inheritance of blood types and I am getting very confused! I am trying to figure out what blood type the father of my son could have since my son and I are both type A+. Also, my brother is type 0 and my mom is A+. We can't find anything that explains how this can be. Could you please help? The father could be: a. A, AB, B, or O b. Either A or B c. Either A or O

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Answers to the Problems 1. B Explanation: The possible genotypes of a man with blood type B are BB or BO and the genotype of a woman with blood type AB is AB. The child would receive an A allele or a B allele from the mother and a B allele or an O allele from the father. Therefore, the child could not possibly be of blood type O. The following diagrams are Punnett squares for the two possible combinations, i.e. AB x BB or AB x BO.

2. C

Explanation:

Inheritance

Both the ABO blood type alleles and the Rh factor genetic information are inherited from our

parents, but they are inherited separately.

Blood types

The mother of the child is blood type 0. We know that she is homozygous. The father could be

homozygous with two alleles of type A or he may be heterozygous with one allele of type A and

one allele of type O. If the father is heterozygous, there is a 50% probability for a blood type A

child and a 50% probability for a blood type O child. If the father is homozygous, every

offspring will be blood type A.

Mother's Possible

Genotype

Father's Possible

Genotypes

Child's Possible

Genotypes

OO

AA AO

OO

AO AO , OO

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Rh factor

The mother of the child is Rh+. She may be homozygous or she may be heterozygous with one

Rh+ allele and one Rh- allele. If the mother is heterozygous, because the father is Rh-, there is a

50% probability for an Rh+ child and a 50% probability for an Rh- child. If the mother is

homozygous, every offspring will be Rh+.

Mother's Possible

Rh factor

Father's Possible

Rh factor

Child's Possible

Rh factor

Rh+Rh+

Rh-Rh- Rh+Rh-, Rh+Rh-

Rh+Rh-

Rh-Rh- Rh+Rh-, Rh-Rh-

3. B

Explanation:

Blood types

The father has to be the source of the A allele of a child with blood type A, when the mother is

of blood type O. The father could be type A or type AB. The types that can be ruled out for the

father are type O and type B.

Mother's Possible

Genotype

Child's Possible

Genotypes

Father's Possible

Genotypes

OO AO AA, AB

Rh factor

The Rh factor information doesn't discriminate potential fathers of the child. Even though

the child is Rh-, it is not necessary that either parent be Rh-. The father of the child could be

either Rh+ (heterozygous with one Rh+ allele and one Rh- allele) or Rh- (homozygous with two

recessive Rh- alleles.)

When the mother is Rh- and the father is Rh+ (heterozygous with one Rh+ allele and one

Rh- allele), there is a 50% probability for an Rh+ child and a 50% probability for an Rh- child.

When the mother is Rh- and the father is Rh-, as in the case of your own family, then all children

(all of your siblings) will be Rh-.

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4. A

Explanation:

Potential blood types of father

The mother in question is blood type A+. Her genotype at the ABO location is either AA

or AO. Her Rh genotype is either Rh+/Rh+ or Rh+/Rh-. The information that the maternal

grandmother is also blood type A+ and a brother is blood type O tells us that the maternal

grandmother of the child has genotype AO, since she is type A but donated an O allele to one of

her children.

The mother wants to know the potential blood types of the father of her son. The son is

blood type A+. Unfortunately for this particular case, the mother cannot distinguish between any

potential fathers from blood type alone. Note from the table that this mother could have created

a child with type A blood with a father of any of the four possible blood types, type A, type AB,

type B, or type O. Likewise, the father of the child could be either Rh+ or Rh-.

Mother's Blood Type

Possible Mother's Genotype

Father's Blood Type

Possible Father's

Genotype

Possible Child Blood Type

A AA, AO A AA, AO A, O A AA, AO A AA, AO A, O A AA, AO AB AB A, AB, B A AA, AO B BB, BO A, AB, B,

O A AA, AO O OO A, O

Paternity issues

It should be apparent from this discussion that blood type is not a very good test for

paternity. In some cases, unambiguous information can be obtained, i.e. a type AB male cannot

father a type O child. However in most cases, the results are uncertain.

If determining the paternity of a child is important, there are very sensitive DNA test

currently available that can establish paternity to a certainty in excess of 99.99%, or exclude

someone as the biological father with absolute certainty. Elsewhere in the Biology Project is an

exercise to follow the inheritance of DNA markers in a paternity study.