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INCOMPLETE DOMINANCE
AND
CODOMINANCE
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INCOMPLETE DOMINANCE
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F1 hybrids have an appearance somewhat in
between the phenotypes of the two parental
varieties.
Example: snapdragons (flower)
red (RR) x white (rr)
r
r
RR = red flower
rr = white flower
R
R
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INCOMPLETE DOMINANCE
r
r
R Rr
Rr
R Rr
Rr
produces the
F1 generation
All Rr = pink
(heterozygous pink)
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INCOMPLETE DOMINANCE
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CODOMINANCE
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Two alleles are expressed equally in multiple
alleles in heterozygous individuals.
Example: blood type
1.
2.
3.
4.
type A
type B
type AB
type O
=
=
=
=
IAIA or IAi
IBIB or IBi
IAIB (equally expressed)
ii (recessive alleles)
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CODOMINANCE PROBLEM

Example: homozygous male Type B (IBIB)
X
heterozygous female Type A (IAi)
IA
i
IB
IAIB
IBi
1/2 = IAIB
(A & B = equally
expressed)
IB
IAIB
IBi
1/2 = IBi (B
dominant over i)
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ANOTHER CODOMINANCE PROBLEM
• Example: male Type O (ii)
x
female type AB (IAIB)
IA
IB
i
IAi
IBi
i
IAi
IBi
1/2 = IAi
1/2 = IBi
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CODOMINANCE
Question:
If a boy has a blood type O and
his
sister has blood type AB,
what are
the genotypes and
phenotypes of
their parents?
boy - type O (ii) X girl - type AB (IAIB)
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CODOMINANCE
Answer:
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SEX-LINKED TRAITS
 Traits
(genes) located on the sex
chromosomes
 Sex chromosomes are X and Y
 XX genotype for females
 XY genotype for males
 Many sex-linked traits carried on X
chromosome
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SEX-LINKED TRAITS
Example: Eye colour in fruit
flies
Sex Chromosomes
fruit fly
eye colour
XX chromosome - female
Xy chromosome - male
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SEX-LINKED TRAIT PROBLEM


Example: Eye colour in fruit flies
(red-eyed male) x (white-eyed female)
XRY
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x
XrXr
Remember: the Y chromosome in males does
not carry these traits.
Xr
Xr
RR = red eyed
Rr = red eyed
R
X
rr = white eyed
XY = male
Y
XX = female
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SEX-LINKED TRAIT SOLUTION:
Xr
XR
XR
Xr
Y
Xr Y
Xr
XR
Xr
Xr Y
50% red eyed
female
50% white eyed
male
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FEMALE CARRIERS
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GENETIC PRACTICE
PROBLEMS
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BREED THE P1 GENERATION
tall (TT) x dwarf (tt) pea plants
t
t
T
T
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SOLUTION:
tall (TT) vs. dwarf (tt) pea plants
t
t
T
Tt
Tt
produces the
F1 generation
T
Tt
Tt
All Tt = tall
(heterozygous tall)
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BREED THE F1 GENERATION
tall (Tt) vs. tall (Tt) pea plants
T
t
T
t
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SOLUTION:
tall (Tt) x tall (Tt) pea plants
T
t
T
TT
Tt
t
Tt
tt
produces the
F2 generation
1/4 (25%) = TT
1/2 (50%) = Tt
1/4 (25%) = tt
1:2:1 genotype
3:1 phenotype
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