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BLOOD TYPE INHERITANCE
Name: _____________________
Inheritance of Human ABO Blood Types: An Example of Multiple Alleles
Reading: CMR p. 169
Introduction
Until now, you have considered that in every gene pair, there could only be two possible alleles:
A and a, for instance. For many traits, however, there may exist more than two alleles that affect
the trait. This is called multiple alleles.
In any case, only two alleles will be present in the same genotype at the same time, one
contributed by the male parent and one contributed by the female parent.
Human blood types are an example of a trait for which there are multiple alleles. There are four
possible human blood types (phenotypes): type A, type B, type O, or type AB. Blood types
depend on the presence of specific proteins (called Immunoglobulins) on the surface of red blood
cells and another type of protein (antibodies) in the plasma (liquid part of blood).
Genetics of ABO Blood Types
There are three possible alleles that control blood type. They are:
an allele for type A: symbolized as IA
an allele for type B: symbolized as IB
an allele for type O: symbolized as i
Remember that in any one person there will be only two of these alleles present. There may be
two of the same allele (such as IAIA or ii) or there may be two different alleles (such as IAIB or
IAi).
Which allele is dominant? This is where the fun begins! Remember these simple rules:
• The allele for type O is ALWAYS recessive to alleles for type A or type B.
• Alleles for type A and type B are codominant to each other – when they are both present,
they are both expressed.
Problems in Blood Type Genetics
1. What is the genotype of a person with type O blood? __________
2. What is the genotype of a person with type AB blood? __________
3. One possible genotype for a person with type A blood is IAi. What is the other possible
genotype? __________
4. One possible genotype for a person with type B blood is IBIB. What is the other possible
genotype? __________
Genetics
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Blood Type Inheritance
Answers:
1. ii – remember the allele for type O is always recessive, so the genotype must be
homozygous to be expressed.
2. IAIB – remember that the alleles for A and B type blood are codominant, so they are both
expressed.
3. IAIA – this genotype is homozygous.
4. IBi – remember that the allele for type O blood is recessive to the allele for type B or type
A blood.
If you made any mistakes on these genotypes, review the section on the “Genetics of Blood
Types” before you continue.
5. A blood inheritance problem:
A man who is blood type O marries a woman who is blood type AB. What could the
blood type of their children be?
Man’s genotype is ii.
Man’s gametes are i and i.
Punnett square:
i
Woman’s genotype is IAIB.
Woman’s gametes are IA and IB.
(fill in the second column)
IA
I Ai
IB
IBi
i
a. What percent of the offspring from this marriage are likely to be type A blood?
b. What percent of the offspring of this marriage are likely to be type B blood?
c. Is it possible for this couple to have a child of type O blood? Why or why not?
Answers:
a. 50%
b. 50%
c. No, because type O blood must have a genotype of ii and this is not possible here.
Genetics
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Blood Type Inheritance
What about the blood Rh factor?
The ABO blood groups you have been working with are not the only means of categorizing red
blood cells. There is another protein factor on the surface of these cells that can vary from one
individual to another. This protein is called the Rh factor and an individual is said to be “Rh
positive” if they do have the factor and “Rh negative” if they do not. These phenotypes can be
abbreviated “Rh+” and “Rh-” respectively.
Unlike ABO blood types, Rh is an example of simple monohybrid dominant/recessive
inheritance. The allele for having an Rh factor (being Rh+) is dominant, and the allele for not
having the Rh factor (Rh-) is recessive. So the only way a person could be Rh- is to have a
homozygous recessive genotype.
Problem: What are the expected offspring of a woman who is Rh negative and a man who is Rh
positive? The man is heterozygous.
Let
R = Rh+
r = Rh -
Woman’s genotype is ________.
Woman’s gametes are = ____ and ____.
Man’s genotype is = ________.
Man’s gamete’s are = ____ and ____.
_______ % of offspring will be Rh+
_______ % of offspring will be Rh-
Answer:
The woman’s genotype is rr. Her gamete are r and r.
The man’s genotype is Rr; his gametes are R and r.
R
r
r
Rr
rr
r
Rr
rr
50% are Rh+
50% are Rh-
If you were unable to do this simple monohybrid problem, go back to that section of you
worksheet and review. Do NOT proceed unless you understand.
If you do proceed without fully understanding the problem, you will get mixed up when several
different types of problems are given to you at one time!
Genetics
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Blood Type Inheritance