2. CODOMINANCE,
INCOMPLETE DOMINANCE
AND MULTIPLE ALLELES
Compiled by
Siti Sarah Jumali
Level 3 Room 14
Ext 2123
Lecture outline
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2nd Topic – extention of Mendelian Genetics
Codominance
Incomplete dominance
Multiple alleles
Lethal alleles
Epistasis
Polygenic inheritance
Linked genes
Crossover value and genes mapping
Sex linked genes
TWO ALLELES
CODOMINANCE
• Co- means together
codominant means equal in dominance
• alleles are approximately equal effect in
individuals; alleles are equally detectable in
individuals.
• Phenotypes for both alleles are exhibited in the
heterozygote
• The hybrid shows neither of the parents’ trait,
instead, a third, different phenotype
• Examples of this is blood types; ABO, iAiB.
Pay attention on how to write codominance;
Writing it is with superscript
example
Calico cat
Unlike Law of Segregation..
• Using the calico cat as example, The genotype
for fur can be represented as CB or CO.
• CB is for black color and
• CO denotes orange color
• Therefore the codominance is written as
• CBCB x COCO  100% CBCO
Common situation of Codominance
• Common phenotype used is roan fur in cattle
• Cattle can either be all red RR;
all white WW or;
Roan RW
Roan cattle
Other example
• Human blood type AB
– Two types of protein A and B appear together on
the surface of blood cells
Question:
• What is the probability of a child having type
AB blood if one of the parents is heterozygous
for A blood and the other is heterozygous for B
blood?
• What other genotypes are possible for children
of these parents?
INCOMPLETE DOMINANCE
• A form of intermediate inheritance in which
one allele for a specific trait is not completely
dominant over the other allele.
• The heterozygote has an intermediate
phenotype between that of either homozygote
• This results in a combined phenotype.
Pink Snapdragon
Rosendahl
Example
• In cross-pollination experiments between red
and white snapdragon plants, the resulting
offspring are pink.
• The dominant allele that produces the red color
is not completely expressed over the recessive
allele that produces the white color.
Incomplete dominance eg
• Crossing of organisms that has two different
phenotypes produces offspring with another
different phenotype which is a blend of the
parental traits
• Examples??????
Snakes
Test cross for Incomplete dominance
Punnet Square for Incomplete
dominance
Try this
• 1. Predict the phenotypic ratios of offspring when a
homozygous white cow is crossed with a roan bull.
• 2. What should the genotypes & phenotypes for
parent cattle be if a farmer only wanted a cattle with
white fur
• 3. A cross between a black cat & a tan cat produces a
tabby pattern (black and tan fur together)?
– A) What pattern of inheritance is this?
– B) What percent of kittens would have tan fur if a tabby cat
is crossed with a black cat?
THREE ALLELES
MULTIPLE ALLELES
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More than three alleles
Many genes have multiple alleles
Three or more different alleles
Exclude dominant and recessive effects
All alleles show its own effects in inheritance
Examples: Blood type, hair color
Multiple alleles cont’d
• Multiple alleles - gene has several allelic forms
– Example: blood type is determined by multiple alleles
• IA = A antigen on red blood cells
• IB = B antigen on red blood cells
• i = Neither A nor B antigen on red blood cells
– Possible phenotypes and genotypes for blood type:
• This is an example of codominance because both IA and IB are fully
expressed
Several genes and the
environment can influence a single
multifactorial characteristic
• Polygenic inheritance occurs when a trait is
governed by two or more genes
– Multifactorial traits - controlled by polygenes
subject to environmental influences
9-24
Polygenic inheritance:
Dark dots stand for
dominant alleles; the
shading stands for
environmental
influences
9-25
Iris color
The Human Blood
Skin color
• An interesting example is coat color in rabbits
– Four different alleles
• C (full coat color)
• cch (chinchilla pattern of coat color)
– Partial defect in pigmentation
• ch (himalayan pattern of coat color)
– Pigmentation in only certain parts of the body
• c (albino)
– Lack of pigmentation
– The dominance hierarchy is as follows:
• C > cch > ch > c
– Figure 4 illustrates the relationship between
phenotype and genotype
Phenotype
• Agouti (wild type)
• Chinchilla (mutant)
• Himalayan(mutant)
• Light grey
• Albino (mutant)
FIGURE 4
Genotype
c+c+, c+cch, c+ch, c+c
cchcch
chch,chc
cchch, cchc
cc
Why is that?
• Caused by tyrosinase; producing melanin
• Two types of melanin: eumelanin (black
pigment) and phaeomelanin (orange/yellow
pigment)
• The himalayan pattern of coat color is an example
of a temperature-sensitive conditional allele
– The enzyme encoded by this gene is functional only at
low temperatures
• Therefore, dark fur will only occur in cooler areas of the body
• This is also the case in the Siamese pattern of coat color in cats