Genetics Part 2:
and
Classic/Complete Dominance

Classic Dominance: (aka “complete
dominance”) the heterozygote displays the
dominant phenotype.
It doesn’t always work
this way, though …
Observation:
A Litter of Puppies
Which is Dad?

Mom
Dad 2 is the sire
of the litter!

Dad 1?

Dad 2?
How is that possible??

Many traits have complex inheritance.
 They can involve more than two alleles!
(Example: blood types)
May not be as simple as complete
dominance
 Sometimes involves more than 1 gene!

(Examples: eye color, height, skin color)

In the case of the dogs shown in the previous
two slides, there are 2 separate gene locations
that control coat color.


Incomplete Dominance: occurs when
Incomplete
Dominance
offspring
have a phenotype
that is in
between the two parent phenotypes.
Heterozyote displays a blended phenotype
(dominant no longer masks recessive!)
Ex) Four o’clock flowers
 Get ready to write in
the answers to the
example and Square 1!

Incomplete Dominance Example
Four o’clock flowers (This is square 3 on your notes sheet)
Genotype Phenotype
RR
Red Flowers
rr
White Flowers
Rr
Pink Flowers
Show a cross between
two heterozygous pink
flowers.
Rr X Rr
R
r
R
RR
Rr
Genotypic ratio:
1 RR : 2 Rr : 1 rr
r
Rr
rr
Phenotypic ratio:
1 Red : 2 Pink: 1 white
There is only 1 genotype per phenotype!
Now, you try it!

Complete squares 2 and 3
on your fill-in notes sheet
using the “rules” from our
Four ‘O Clock Flowers
example.



Codominance: both alleles are expressed in a
heterozygote.
Neither allele is recessive!
Both show up in the phenotype – there is no
“in between” like with incomplete dominance.

Codominance
Examples:



blood types,
spotted cows,
spotted flower color
SPECIAL CASE: How to write
genotypes for codominant alleles

Notation:
USE WITH CODOMINANCE ONLY!
 Example:


Codominance in cow coat colors
Represents
the gene for
coat color
Genotype written as:
The allele for the color
R
C
CC
R
R
(in this case, it is red)
Phenotype written as: Red
Codominance Example
Roan Coat Color in Cattle
Genotype
Phenotype
C R CR
CWCW
C R CW
Red Coat
White Coat
Roan Coat
Show a cross between a red bull and a roan cow.
CRCR X CRCW
CR
CR
C R CR CR CR CR
C W CR CW CR CW
Genotypic Ratio:
2 CR CR : 2 CR CW
Phenotypic Ratio:
2 Red Coat : 2 Roan Coat




Blood type = Codominance and Multiple Alleles
There are 3 alleles for blood genotypes: IA, IB, i
There are 4 blood phenotypes: A, B, O, AB
IA and IB are codominant; both are expressed for blood
type AB.
Type O, which has genotype ii, is recessive to both A and
B.
Genotypes
Phenotypes
IA IA or IA i
IB IB or IBi
ii
IA IB
Type A
Type B
Type O
Type AB
Multiple alleles!
The only one
that is
Codominant!
Blood Type Example
(write this down!)
A woman has Type A blood and her genotype is IAi
She marries a man who is Type B with genotype IBi.
What are the possible blood types of their children?
IB
IA
i
IAIB
i
IAi
Type AB
Type A
IBi
ii
Type B
Type O
The possible
offspring blood types
are:
Type AB, Type A,
Type B, and Type O
Back to the dog example from yesterday…
There are TWO alleles
for Coat Color
K = Black
k = Not Black
Y = Red
y = Tri-color
 Dam (mom): KkYY  Sire (dad): kkyy
The Pups are:
KkYy = Black
kkYy = Red
To Sum Up

Classic Dominance:
Dominant
Phenotype

Incomplete Dominance:
X

An “in between” of
the dominant and
recessive
Codominance:
X
Both alleles are
displayed