Mendelian Exceptions
 Mendel got lucky – all 7 traits he studied showed
complete (simple) dominance.
Mendelian Exceptions
 Mendel got lucky – all 7 traits he studied showed
complete (simple) dominance.
 One allele is completely dominant over the other allele
 Homozygous dominant and heterozygous = same
phenotype
 1 allele is enough for full expression of dominant trait
Incomplete Dominance =
the norm
 Heterozygote = intermediate phenotype
 1 allele is not enough for full expression
Incomplete Dominance
 Ex: flower color
 R = red
 r = white
RR x rr
Incomplete Dominance
 F2: Rr x Rr
Incomplete Dominance

F2: Rr x Rr
1 red
2 pink
1 white
RR
Rr

Alleles not blended – still able to separate.
Rr
rr
Codominance – Both alleles
are expressed equally
Codominance – Both alleles
are expressed equally
 Ex: Cows
 B = black
 W = white
 BB = Black
 WW = White
 BW = ???
Codominance – Both alleles
are expressed equally
 Ex: Cows
 B = black
 W = white
 BB = Black
 WW = White
 BW = Black and white
 Other examples: calico cats, streaked flowers…
Multiple alleles
 Allele: Alternate forms of a gene
 Only 2 possible alleles in an individual, BUT any # of
alleles may be present in a population due to mutations
Blood Types (ABO blood
types)
 Example of simple dominance, codominance, and
multiple alleles
Blood Types (ABO blood
types)
 Example of simple dominance, codominance, and
multiple alleles
 Alelles = A, B, O
 A & B = Dominant
 O = Recessive
Blood Types (ABO blood
types)
 Example of simple dominance, codominance, and
multiple alleles
Genotype
Phenotype
 Alelles = A, B, O
 A & B = Dominant
 O = Recessive
AA
Blood Types (ABO blood
types)
 Example of simple dominance, codominance, and
multiple alleles
Genotype
Phenotype
 Alelles = A, B, O
 A & B = Dominant
 O = Recessive
AA
A
Blood Types (ABO blood
types)
 Example of simple dominance, codominance, and
multiple alleles
Genotype
Phenotype
 Alelles = A, B, O
 A & B = Dominant
 O = Recessive
AA
AO
A
Blood Types (ABO blood
types)
 Example of simple dominance, codominance, and
multiple alleles
Genotype
Phenotype
 Alelles = A, B, O
 A & B = Dominant
 O = Recessive
AA
AO
A
A
Blood Types (ABO blood
types)
 Example of simple dominance, codominance, and
multiple alleles
Genotype
Phenotype
 Alelles = A, B, O
 A & B = Dominant
 O = Recessive
AA
AO
BB
A
A
Blood Types (ABO blood
types)
 Example of simple dominance, codominance, and
multiple alleles
Genotype
Phenotype
 Alelles = A, B, O
 A & B = Dominant
 O = Recessive
AA
AO
BB
A
A
B
Blood Types (ABO blood
types)
 Example of simple dominance, codominance, and
multiple alleles
Genotype
Phenotype
 Alelles = A, B, O
 A & B = Dominant
 O = Recessive
AA
AO
BB
BO
A
A
B
Blood Types (ABO blood
types)
 Example of simple dominance, codominance, and
multiple alleles
Genotype
Phenotype
 Alelles = A, B, O
 A & B = Dominant
 O = Recessive
AA
AO
BB
BO
A
A
B
B
Blood Types (ABO blood
types)
 Example of simple dominance, codominance, and
multiple alleles
Genotype
Phenotype
 Alelles = A, B, O
 A & B = Dominant
 O = Recessive
AA
AO
BB
BO
AB
A
A
B
B
Blood Types (ABO blood
types)
 Example of simple dominance, codominance, and
multiple alleles
Genotype
Phenotype
 Alelles = A, B, O
 A & B = Dominant
 O = Recessive
AA
AO
BB
BO
AB
A
A
B
B
AB (codom.)
Blood Types (ABO blood
types)
 Example of simple dominance, codominance, and
multiple alleles
Genotype
Phenotype
 Alelles = A, B, O
 A & B = Dominant
 O = Recessive
AA
AO
BB
BO
AB
OO
A
A
B
B
AB (codom.)
Blood Types (ABO blood
types)
 Example of simple dominance, codominance, and
multiple alleles
Genotype
Phenotype
 Alelles = A, B, O
 A & B = Dominant
 O = Recessive
AA
AO
BB
BO
AB
OO
A
A
B
B
AB (codom.)
O
 Q: What combination of parents have an equal chance of
having any of the 4 blood types?
 Q: What combination of parents have an equal chance of
having any of the 4 blood types?
Mexican Hairless Dogs
 Hairless (H) is completely dominant over hairy (h).
 When two hairless are crossed:
 2/3 hairless
 1/3 hairy
Why???
Mexican Hairless Dogs
 Hairless (H) is completely dominant over hairy (h).
 When two hairless are crossed:
 2/3 hairless
 1/3 hairy
Mexican Hairless Dogs
 Hairless (H) is completely dominant over hairy (h).
 When two hairless are crossed:
 2/3 hairless
 1/3 hairy
HH
Hh
Hh
hh
Mexican Hairless Dogs
 Hairless (H) is completely dominant over hairy (h).
 When two hairless are crossed:
 2/3 hairless
 1/3 hairy
HH
Hh
Hh
hh
Lethal Allele
 2 copies of a lethal allele = death (usually stillborn)
 Hint: if offspring ratio is x/3, think lethal allele
(someone is dying)
Lethal Allele
 Q: How can we breed hairless dogs without stillborns?
3, 4, 5, etc. trait crosses
 If a pea plant that is TTRrPp is crossed with a plant
that is TtrrPp, what percent of the offspring will be
heterozygous for all three traits?
 If a pea plant that is TTRrPp is crossed with a plant that is
TtrrPp, what percent of the offspring will be heterozygous
for all three traits?
Is there an easier way???
Product Rule
 Make a simple 4 square cross for each trait, then multiply
the results for each
TT
TT
Rr
Rr
PP
Pp
Tt
Tt
rr
rr
Pp
pp
Epistasis
 The process of one gene controlling the expression of
another
 An epistatic gene can completely mask the effects of
another gene
 2 genes -> 1 trait
Example: Lab Fur Color
 2 genes are responsible for fur color:
 Black gene
 Brown-blonde gene
Example: Hair Color
 2 genes are responsible for hair color:
 Black gene
 Brown-blonde gene
 Black is epistatic (dominant) over the red-blonde gene
Polygenic Inheritance
 Most traits (especially visible ones) are due to the
interaction of multiple proteins, thus multiple genes
 As such, most traits do not follow simple Mendelian ratios