Incomplete & Codominance
Complete dominance
The traits studied in Mendel’s pea plants
were examples of inheritance (or simple
dominance): One allele completely
masks the other allele. This is not always
the case.
We will compare the
heterozygous genotypes for
complete dominance to the
heterozygous genotypes for two
other types of inheritance.
Incomplete Dominance:
One allele is not completely dominant over the
other. There is a “blending” of phenotypes when
both alleles are present.
A monohybrid cross (of heterozygotes)results
in three separate phenotype possibilities,
instead of just two as in simple or complete
dominance.
Example: Incomplete Dominance
The plant called the “four-o-clocks”
(Mirabilis) inherits flower color through
incompletely dominant alleles.
R = red color
W = white color
A cross between a
red and white flowering plant will yield a
third phenotype
—pink, in the
F1 generation.
If two of the F1 off
-spring are crossed,
we don’t see the
3:1 F2 ratio as in
Mendel’s crosses.
Genotypic Ratio
1RR : 2RW : 1WW
Phenotypic Ratio
1red:2 pink:1white
Genotypic & Phenotypic ratios
are the same
Codominance
Codominance occurs when both alleles for the
trait are expressed simultaneously in the
heterozygous genotype.
Example: In horses and
cattle, there is a coat
that exists of red and
white hairs. It is known
as “roan.”
R may indicate Red hairs
W may indicate White hairs
An animal with roan coat color would have a
genotype of RW. Both alleles are expressed at
the same time (if the animal is heterozygous).
(There are also a “blue roan” horse in which
there are codominant black and white genes)