Download Complete Versus Incomplete Dominance

Complete Versus
Incomplete Dominance
Versus Co-dominance
Pre-AP Biology
Bellwork

Speculate on how Type AB blood is
inherited.
Be ready to share.
Objectives





Contrast complete dominance and
incomplete dominance
Contrast complete dominance and
codominance
Explain how Blood Types are inherited
Construct Punnett Squares for blood type
Explain why a Type A person cannot get a
transfusion of Type B blood
Key Terminology
Complete dominance
 Incomplete dominance
 Codominance
 Antibodies
 Agglutination

Complete Dominance

Definition


Dominant allele fully expressed in
heterozygote and homozygous
dominant
Ex. Flower color of pea plants
Testcross
By looking at a dominant phenotype
cannot determine if genotype is
homozygous or heterozygous
 Do a Testcross
 Cross unknown with homozygous
recessive

Heterozygous phenotypic ratio of 1:1
 Homozygous phenotypic ratio of 4:0

Incomplete Dominance

Definition


One allele not completely dominant over
another allele
heterozygote will be intermediate to
homozygous dominant and homozygous
recessive
Ex. Flower Color in Snapdragons
Think-Pair-Share
What is blending theory?
Why doesn’t incomplete
dominance support
blending theory?
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Co-dominance
Codominance

Definition


Both alleles are fully
expressed in the
heterozygote at the
same time
Ex. Human Blood
Type AB
ABO Blood Typing



One system used to
type blood is the
ABO Blood Group
The 4 blood types
are A, B, AB and O
These are the 4
different
phenotypes of the
same trait
Human Blood Types
(Cont’d)
The ABO blood types are determined
by paired combinations of three
alleles IA, IB, and i
 The letters A and B refer to two
genetically determined carbohydrates
on the surface of red blood cells

Human Blood Types
Human Blood Types
(Cont’d)
Neither IA and IB is dominant over
the other
 IA and IB are both dominant over i
(i=recessive)
 When IA and IB are both present
they are codominant


because both are expressed in
heterozygotes as Type AB
Human Blood Types
(cont’d)

With three alleles, six possible
genotypes
Blood Type
A
B
AB
O
Possible Genotypes
I A I A, I A i
IB IB, IB i
IA IB
ii
Human Blood Types
(Cont’d)
Even for traits with
multiple alleles, an
individual can have
only 2 of the
possible alleles for
that gene
Dad
I
Mom

A
A
A A
I
A
A A
I I I I I
B
A B A B
I I I I I
Think-Pair-Share
Explain why blood types
are an example of
codominance.
Get ready to share.
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Blood Transfusion
Foreign antigens cause the immune
system to produce antibodies
 Antibodies react with a foreign
antigen to inactivate that antigen
 A and B will be antigens if you do
not already have those surface
carbohydrates

Blood Agglutination
If you have Type A blood and are given
Type B blood in a transfusion
 The B antigen on the red blood cells in
the transfusion will react with your
antibodies against the B antigen
 Causing it to form a solid (agglutinate)
that clogs up the blood vessels

Think-Pair-Share
Explain what would happen if a Type
B person gets a transfusion from a
Type AB person.
Explain what would happen if a Type
AB person gets a transfusion from
a Type B person.
Donation of Blood
Table 1. Human Blood Types
Blood
type
A
B
AB
O
Can receive
blood from
O, A
O, B
O, A, B ,AB
O
Can donate
blood to
A, AB
B, AB
AB
O, A, B, AB
Sample Problem #1

Use a Punnett Square to predict the
offspring of a cross between a
heterozygous Type A and a Type O
person.
Get ready to share.
Sample Problem #2


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Suppose that you are a geneticist and are
asked to testify in a paternity case in
which the mother is A type blood, the
child has O type blood and the alleged
father has B type blood
Attorney claims that because the child
has O type blood and the father has B
type blood, his client cannot be the
father. Is the attorney’s claim true?
Use a Punnett Square to support your
answer
Sample Problem #3


SpongeBob loves growing flowers for his
pal Sandy. Her favorite flowers , Poofkins
are found in red (RR), purple (Rr) and
Blue (rr)
What would happen if SpongeBob crossed
two Poofkins with purple flowers


What percentage would have red, purple, blue flowers?
Does this cross demonstrate complete or incomplete
dominance? How do you know?
Problem #4

In humans, pointed eyebrows (B) are
dominant over smooth (b). Bob is
heterozygous for pointed eyebrows and is
married to Mary who has smooth.



What is the phenotypic ratio, genotypic ratio
of this cross?
Does this cross demonstrate complete
dominance or incomplete dominance?
What is the probability that the children will
have smooth eyebrows?
Problem #5

A fruit fly with a gray body (B) and red
eyes (P) (genotype BbPp) is mated with a
fly having a black body (b) and purple (p)
eyes (genotype bbpp).


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Is this a monohybrid or dihybrid cross?
What are the different gamete combinations
for the two flies?
What is the phenotypic ratio, genotypic ratio?
Problem # 6

A couple are both phenotypically normal
(H) but their son suffers from hemophilia
(h), a sex linked disorder.




What are the genotypes of the parents?
What fraction of the children are likely to
suffer from hemophilia?
What fraction of the children are carriers?
What fraction of girls versus boys will have
hemophilia? Why is one gender more prone
to have the disease?
3 Minute Paper
You have three minutes to write
down the difference between
complete dominance, incomplete
dominance, and co-dominance
Be prepared to share
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