Lesson #1.7-Incomplete
Dominance
Codominance
Dihybrid Crosses
Exceptions to Mendel’s principles
• So far, offspring have either the phenotype
of one parent or the other.
• Sometimes, there is no dominant or
recessive gene, or the trait is controlled by
many alleles or genes.
Incomplete Dominance
• Sometimes two alleles can be equally
dominant; they interact to produce a new
phenotype
• The new phenotype in the offspring is a mix
of the two parent phenotypes.
Incomplete Dominance
• A third (new) phenotype appears in the
heterozygous condition
Incomplete Dominance
Incomplete Dominance
Sample Problem 1:
• Flower color in snapdragons:
Trait: colour in snapdragons
CR = red
CR CR = red
CW = white
CW CW = white
CR CW = pink
Problem: Incomplete
Dominance
• Show the cross between a red and a white
flower.
F1 PHENOTYPES:
100% pink
F1 GENOTYPES:
100% CRCW
CR
CR
CW
CR CW
CR C W
CW
CR CW
CR C W
Codominance
• The new phenotype in the offspring is a
combination of the two parent
phenotypes (i.e. both alleles are
expressed at the same time)
• The heterozygous condition, both
alleles are expressed equally
Sample Problem 1:
Sickle Cell Anemia in Humans
Trait: Red Blood Cell (RBC) shape
BNBN = normal
BSBS = sickle
BNBS =
cells
cells
normal and
sickle
Problem: Codominance
• Show the cross between an individual with
sickle-cell anemia and another who is a
carrier but not sick.
N
S
B
B
BNBS x BSBS
BS
BNBS
BSBS
F1 PHENOTYPES:
50% normal and sickle
50% sickle
F1 GENOTYPES:
50% BNBS
50% BSBS
BS
BNBS
BSBS
Codominance
• In codominance, neither allele is dominant;
both are expressed. A cross between
organisms with two different phenotypes
produces offspring with has both phenotypes
of the parental traits shown.
• Both alleles contribute to the phenotype
2. Codominance
– Example: In come chickens
Black Chicken x White  Speckled Chicken
Roan Horse
http://search.vadlo.com/b/q?rel=2&keys=Dominance+Incomplete+Dominance+Codominance+PPT
YOU tell me which type of
dominance…
Codominance!
Type of
Dominance?
Incomplete
Dominance!
Type of Dominance?
Incomplete Dominance!
Dihybrid crosses are those
where we consider the
inheritance of two
characteristics at the same
time.
• Mendel had observed that the seeds of his
pea plants varied in several ways – among
them, whether the peas were round or
wrinkled and whether they were green or
yellow.
• He decided to look at how both traits were
inherited together….

It appears that the inheritance of seed
shape has no influence over the
inheritance of seed colour

The two characters are inherited
INDEPENDENTLY
This follows Mendel’s Law of
Independent Assortment

There are four possible combinations of the
two characteristics. Peas can be:

1) round and yellow

2) round and green

3) wrinkled and yellow

4) wrinkled and green

From his monohybrid trials, Mendel had
discovered that the allele for yellow
colour was dominant to that for green.

He had also found that the allele for
round peas was dominant to the allele for
wrinkled peas.
Sample Problem 1:
Two heterozygous pea plants are crossed for two traits:
pea colour & pea texture.
(Yellow is dominant to green and round is dominant to
wrinkled)
Trait:
Pea shape and colour
R = Round
r=
wrinkled
Y = Yellow
y = green
DO
NOT
COPY
PARENT:
Phenotype
Round, yellow
Genotype
RRYY
Gametes
F1
RY, RY, RY, RY
all
X Wrinkled, green
X rryy
X ry, ry, ry, ry
RrYy
All of the F1 generation would be heterozygous for both
characteristics, meaning that they would all be round and
yellow. As with monohybrid crosses, he then crossed two
of the F1 generation together…
Sample Problem 1:
Two heterozygous pea plants are crossed for two traits:
pea colour & pea texture.
(Yellow is dominant to green and round is dominant to
wrinkled)
Phenotype:
Round, yellow
Genotype:
RrYy
Gametes:
RY, Ry, rY, ry
F2
X
Round, yellow
RrYy
RY, Ry, rY, ry
We really need a punnet square for this…
RY
RY
Ry
rY
ry
Ry
rY
ry
RY
Ry
rY
ry
RY
Ry
rY
ry
RRYY
RRYy
RrYY
RrYy
RRYy
RRyy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
9 : 3 : 3 : 1
Round
Round Wrinkled Wrinkled
Yellow
Green Yellow
Green
This is the typical ratio
expected in a dihybrid cross.
This shows that traits are
inherited INDEPENDENTLY!