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Investigating different
patterns of inheritance:
Codominance and
Incomplete Dominance
Simple Mendelian Genetics
Codominance


Both alleles of a gene pair are equally
dominant.
Neither allele is dominant or recessive to
the other and as a result both alleles are
expressed in a heterozygous offspring.
Codominance
The heterozygous offspring will express (show) both phenotypes
Punnett Square for Codominance
X
RR
WW
R
R
W
RW
RW
W
RW
RW
RW
100% mixed color carnations (RW)
Punnett Square for Codominance
X
RW
RW
R
W
R
RR
RW
W
RW
WW
25% red carnations (RR)
50% mixed color carnations (RW)
25% white carnations (WW)
Incomplete Dominance



One allele of a gene pair is only partially
dominant over the other.
In these cases an intermediate form of the
trait is expressed in heterozygous
organisms.
Often seen in flowers.
Incomplete Dominance
The heterozygous offspring will express (show) a blend of the
Parent phenotypes
Punnett Square for incomplete dominance
X
RR
rr
R
Rr
R
r
Rr
Rr
r
Rr
Rr
100% pink snapdragons (Rr)
Punnett Square for incomplete dominance
X
Rr
Rr
R
r
R
RR
Rr
r
Rr
rr
25% red snapdragons (RR)
50% pink snapdragons (Rr)
25% white snapdragons (rr)
Investigating different
patterns of inheritance
Sex Linked Traits
Sex Linked Traits
• Traits that are controlled by genes located
on the sex chromosomes (X or Y)
• These traits are more common in males
than females.
• Why???
Sex Linked Traits
X Chromosome
Larger and contains
multiple genes
Y Chromosome
Smaller and
contains no genes
Girls = XX
Boys = XY
Sex Linked Traits
Girls = XX
Boys = XY
Color blindness is a recessive condition inherited on the X chromosome:
BB = Individual has normal vision
Bb = Individual has normal vision (carries the colorblind gene)
bb = Individual is colorblind
XB XB - Normal
XB Xb – Normal (carrier)
Xb Xb – Color Blind
XB YØ - Normal
Xb YØ – Color Blind
Are you colorblind?
http://colorvisiontesting.com/ishihara.htm
Sex Linked Example
Girls = XX
Boys = XY
Consider a case where a mother is color blind and the father has normal
vision. What is the probability that this couple’s children will be colorblind?
Xb
Xb
XB
XBXb
XBXb
YØ
XbYØ
XbYØ
• All Daughters are carriers (XBXb )
with normal vision
• All Sons are color blind (XbYØ)
Sex Linked Example
Girls = XX
Boys = XY
Consider a case where a mother is a carrier and the father has normal
vision. What is the probability that this couple’s children will be colorblind?
XB
YØ
XB
Xb
XBXB
XBXb
XBYØ
XbYØ
• 50% the daughters will be carriers
(XBXb ) but all will have normal vision.
• 50% of the sons will be color blind
(XbYØ) and 50% will have normal
vision (XBYØ).
Sex Linked Traits are more common in boys
Girls = XX
Boys = XY
How can a female can inherit colorblindness?
XB
Xb
Xb
XBXb
XbXb
YØ
XBYØ
XbYØ
Dad has to be color blind and
Mom has to at least be a
carrier of the trait!
Investigating different
patterns of inheritance
Multiple Alleles
Multiple Alleles
• Traits that are controlled by three or more
alleles.
• Even though more than two alleles exist
for a particular trait, humans only carry
two at a time.
• ABO Blood Type is a good example!
Blood Type - ABO
• Three alleles exist in the general
population – A-B-O
• Humans only carry two of these alleles
• A and B are co-dominant
• O is recessive
• Let’s see how this works!
Blood Type - ABO
A and B are Co-Dominant and O is recessive
What allele combinations (Genotypes) are possible for
A blood type?
AA or AO
What allele combinations (Genotypes) are possible for
B blood type?
BB or BO
Blood Type - ABO
A and B are Co-Dominant and O is recessive
What allele combination (Genotype) is possible for AB
blood type?
AB
What allele combination (Genotype) is possible for O
blood type?
OO
ABO Blood Type
Consider the following genotypes:
Father genotype = AO
A
Father’s Blood Type = ________
Mother Genotype = OO
Mother’s Blood Type = ________
A
O
AO
O
O
OO
What are the possible blood types
for the children? What are the
genotypes?
50% chance of blood type A (AO)
O
AO
OO
50% chance of blood type O (OO)
ABO Blood Type
Consider a family where the father is blood type B, the Mother
is blood type O, and their daughter is blood type O. What is
the possibility that future children will have the following blood
types?
Why not BB?
O
B
O
BO
OO
0
Blood type A _____%
50
Blood type B _____%
50
O
BO
OO
Blood type O _____%
0
Blood type AB _____%
Try the Punnett Squares for
your families blood types.
Most Common Blood types in
the United States
Unviversal Donors and
Recipients