Download Incomplete Dominance

Incomplete Dominance and
Codominance
• In Mendel’s work, alleles were
either dominant or recessive.
• If yellow (Y) is dominant to green (y),
then:
– YY will be yellow
– Yy will be yellow
– yy will be green
• Notice that we only got 2 different
phenotypes
• However, sometimes we get 3 different
phenotypes…
Incomplete Dominance
• In the case of incomplete
dominance, two alleles are neither
dominant or recessive
• They blend to produce a new
phenotype.
• For example, when a red
snapdragon is crossed with a
white snapdragon, all of the F1
offspring are an intermediate
color – pink.
Example
• A certain type of flower will produce green
petals when a blue flower is crossed with a
yellow flower. Cross two green flowers.
BB = blue YY = yellow BY = green
BY x BY
Genotype:
B
Y
B
Y
BB
BY
BY
YY
25% = BB
50% = BY
25% = YY
Phenotype: 25% = blue
50% = green
25% = yellow
Codominance
• Codominance occurs when both alleles
are equally dominant and both are
expressed in a new phenotype.
• If a red cattle is crossed with a white
one, the offspring are roan.
• A roan coat is a mixture of red and white
hairs intermingled.
• Essentially both alleles are dominant.
Multiple Alleles
• For each of the traits that Mendel
studied, there were only 2 alleles (ex.
round/wrinkled, tall/short, etc.)
• It is possible to have more than 2 alleles
for a trait.
• Human blood type is an example of a
trait that has multiple alleles (A, B, O)
Blood Types
• Type O is recessive
• Types A and B are dominant to O, but
are codominant to each other
Blood Type Chart
Blood Type
Genotype
(phenotype)
O
oo
A
AA, Ao
B
BB, Bo
AB
AB
Example
• A woman whose blood type is pure A marries
a man whose blood type is O. What are the
possible blood types of their children?
AA x oo
Genotype:
o
o
A
Ao
Ao
A
Ao
Ao
100% = Ao
Phenotype: 100% = type A