


An “dominant” allele may not be completely
dominant over the recessive allele
This is referred to as “incomplete
dominance”
A “blended” phenotype results



In co-dominance, neither phenotype is
recessive
Heterozygous individuals express BOTH
phenotypes
Common example is the ABO blood type
system
◦
◦
◦
◦
A
B
AB
O

Blood type is coded for by two alleles:

Blood Type A:

Blood Type B:

Blood Type AB:

Blood Type O:
◦A&B
◦ The lack of an allele means O (think “zero”)
◦ AA or AO
◦ BB or BO
◦ AB
◦ OO
◦ Neither A nor B
USA
O+
A+
B+
AB+
O-
A-
B-
AB-
37.4%
35.7%
8.5%
3.4%
6.6%
6.3%
1.5%
0.6%
Incomplete Dominance or Co-Dominance?
Incomplete Dominance or Co-Dominance?
Phlox “peppermint twist”
Incomplete Dominance or Co-Dominance?
Incomplete Dominance or Co-Dominance?
Incomplete Dominance or Co-Dominance?



More dominant

Some organisms may have multiple possible
alleles for one gene
Only one of these genes may be expressed,
but some are more dominant than others
(Example: Dragons can be multiple different,
solid colors)
D2D3
D1d
(Dominance order):
◦
◦
◦
◦
D1 = Red
D2 = Blue
D3 = Green
d = white
D1D2
dd