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The Post-Mendel Era
Exceptions to Mendel
Problem
• Cross a homozygous red flowered plant
with a homozygous white flowered plant.
(R and r)
• What offspring do you expect?
• After the time of Mendel we have
discovered most genes are more complex
then simply dominant and recessive
• Examples
– Incomplete dominance
– Codominance
– Multiple alleles
Incomplete Dominance
• The blending of two different traits in a
heterozygous genotype
• Example: red and white crossed making
pink
•
WW
RR
RW
Practice Problem
• Incomplete Dominance –
If color exhibits incomplete
dominance in violets, what will be the
phenotypes of the offspring of a
homozygous red and a homozygous
white plant?
Incomplete Dominance
Incomplete Dominance
W
W
R
R
RW
RW
RW
RW
Codominance
• In a heterozygous genotype both genes
are fully expressed.
• Example yellow begonias crossed with red
Codominance
Practice Problem
• Codominance –
Cross a cat with black fur (B) with a
cat that has tan fur (b). If we assume
that fur color exhibits codominance,
what will be the phenotypes of the
offspring. (Hint: black and tan fur
together make a color called “tabby”
in cats).
Practice Problem
• Codominance –
What if we wanted to get some cats
that have tan fur, some with black,
and some tabby? What would have
to be the phenotypes of the parents?
Multiple Alleles
• More then two alleles can exist for one
trait.
• Each individual can only have two. This
leads to more than three possible
genotypes.
• In the example of blood type where 3
alleles exist there are 6 genotypes and 4
phenotypes possible
I
A
I
B
i
• Above are the three possible alleles for blood type. You have
a combination of two of these.
• Phenotype (blood type)
Genotype
A
I I or I i
A A
B
AB
O
Bl
II
A
II
ii
B B
B
A
or
Ii
B
• IA and IB are codominant
• i is recessive
Practice Problem
• Multiple Alleles –
What are the chances of a woman
with Type AB and a man with Type A
having a child with Type O?
Practice Problem
• Multiple Alleles –
Dr. Paul is blood type O. His father
was blood type A and his mother was
blood type B. What were the
genotypes of his parents?
California Court Case
• 1946 the California supreme court ruled
that Charlie Chaplin was the biological
father of child he claimed was not his.
• His defense was that the baby had type B
blood. He had type A and the woman who
sued him had type O
• Was the court correct?
19
AA, AO = Type A
i
i
A
Ai
Ai
i
ii
ii
BB, BO = Type B
AB = Type AB
OO = Type O
The judge should take a course in genetics!
20
Polygenic
• Describes a trait that is controlled by more
than one pair of alleles
• This allows for a variety of intermediates
• Human height, skin and hair color, body
build.
Polygenic traits tend to form a bell curve
Sex linked inheritance
•
•
•
•
A trait that travels on the X chromosome.
Results that men can not be carriers
Women can be carriers
For a woman to be afflicted she has to
have an afflicted father and a mother that
is either afflicted or a carrier