Name _______________________
Codominance and Multiple Alleles
There are four blood types: A, B, AB, and O. However there are three alleles that control blood types:
IA, IB, and i. Alleles IA and IB are codominant, and the allele “i” is recessive.
1
2
3
What are the two genotypes for type A blood?
AA or AO
What are the two genotypes for type B blood?
BO or BB
What is the one genotype for type AB blood?
AB
What is the one genotype for type O blood?
OO
A man has type A blood and his wife has type B blood. A physician types the blood of their four children
and finds that each of the children has a different blood type. What are the genotypes of the parents?
How would you explain that all four blood types could be represented by these four children?
The blood types A and B have two genotypes. For Blood type A, the genotypes are AA or AO. For blood
type B, the genotypes are BB or BO.
A couple preparing for marriage have their blood typed along with the other blood tests. Both are AB.
They ask what blood types their children may have. What would you tell them?
A
B
A
B
AA
AB
BA
BB
AA 1/4
AB 2/4
BB 1/4
AA blood 25%
AB blood 50%
BB blood 25%
4
A woman sues a man for the support of her child. She has type A blood, her child has type O, and the
man has type B. Could the man be the father? Explain.
Yes, The man with a type B blood group could be the father of her child. The child must have either of
the blood groups of the parents or O blood group, except for the case of either of the parents having an
AB group.
5
A young woman in Hollywood sued a famous movie actor for support of her child, claiming that he was
the father of her child. By typing the blood of the three persons involved, the following results were
obtained: The child is type B; the Mother is type A; the Actor is type O. Could the actor have been
the father? Explain.
The father is type O, which means he is OO. The child is B, so the father has to have a B in
one of his 2 alleles. The actor is not the father.
6
A wealthy, elderly couple dies together in an accident. Soon after a man shows up to claim their fortune,
claiming that he is really their only son who ran away from home when a boy. Hospital records show that
the deceased couple were blood types AB and O. The claimant to the fortune was type O. Was he an
imposter? Explain.
No, because the parents have AB and no O, but
the kid has OO. There is no way for the parent with
AB to give a O to the kid to make him OO.
7
(This is a 2 trait dihybrid cross) In blood groups, Rh positive blood is dominant (R) over the gene for Rh
negative blood (r). What types of offspring could be expected for an Rh positive AB woman, (whose
father was Rh negative), who marries a man with heterozygous A positive blood?
ia ib Rr
__________ Genotype of the woman?
ia R
ia R
ia r
ia ia RR
ib R
ia ia Rr
ia io Rr
__________Genotype of the man?
ib r
ia ib RR
ib ia Rr
ib ia rr
ib io Rr
1/16 ia ib RR
1/16 ib ia rr
1/8 ia ib Rr
1/8 ia io Rr
ia r
ia ia Rr
ia ia rr
ia ib Rr
io R
ia io RR
ia io Rr
ib io RR
io r
io ia Rr
ia io rr
ib io Rr
ib io rr
8
1/16 io ia rr
1/8 ib io Rr
1/16 ib io RR
1/16 ib io rr
1/16 ia io RR
1/8 ia ia Rr
1/16 ia ia RR
1/16 ia ia rr
18.75% AB positive
6.25 AB negative
37.5% A positive
12.5% A negative
18.75 B positive
6.25% B negative
The best example for multiple alleles involves coat color in rabbits. Coat color in rabbits is determined by
a single gene that has at least four different alleles. These four alleles demonstrate a dominance
hierarchy in which some alleles are dominant over others. The four alleles for coat color in rabbits in
order of dominance are as follows:
C – Full color (often called wild type or agouti)
cch - light gray or chinchilla
ch - albino with black extremities or Himalayan
c - albino
What offspring would be expected from the following crosses:
a) Cch x cc
c
c
C
c^h
Cc
cc^h
Cc
cc^h
1/2 Cc
1/2 cc^h 50% Full Color
50% Himalayan
b) Cc x Cc
C
C
CC
Cc
c
c
Cc
1/4 CC
2/4 Cc
1/4 cc
75% Full color
25% albino
1/4 CC
2/4 Cc^ch
1/4 cc^chch
25% Full color
50% Full color/Light gray
25% Light gray
cc
c) Ccch x Ccch
C
C
c^ch
CC
Cc^ch
c^ch
Cc^ch
cc^ch
ch
d) cchc x cc
c
c
^ch c
cc
c^ch c
c
cc
2/4 cc
2/4 c^ch c
50% Albino
50% Light gray/Albino
1/4 CC
1/4 Cc
1/4 Cc^h
1/4 cc^h
50% Full Color
25% Full Color/Himalayan
25% Himalayan
c^ch c
e) Cch x Cc
C
C
c^h
f) chc x cchc
c^h
c
CC
Cc^h
c^ch
cc^chh
cc^ch
c
Cc
cc^h
c
cc^h
cc
1/4 cc^chh
1/4 cc^ch
1/4 cc^h
1/4 cc
25% Light gray/Himalayan
25% Light Gray
25% Himalayan
25% Albino