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MCDB 1041 Class 12
Examples of complicated inheritance
patterns
Learning goals:
•  Explain why having multiple alleles for a single gene can result in
multiple distinguishable traits
•  Compare the different outcomes in situations where alleles have
more complicated relationships than simple dominance
•  Distinguish between incomplete dominance and co-dominance
•  Recognize (and calculate probabilities for) situations of codominance and lethality
•  Compare and be able to identify the different examples of
inheritance.
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Most human phenotypes do not result from single
genes with two alleles in a simple dominant vs.
recessive relationship
as we have been discussing
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Alleles, one more time!
•  Any time a gene acquires a mutation anywhere in its
DNA sequence, a new allele is created.
•  Mutations can have a negative effect, no effect, or even
a positive effect on the protein made from the gene: a
change in the DNA sequence is by definition a
MUTATION and a NEW ALLELE.
•  Examples:
–  Phenylketonuria (PKU) inability to break down phenylalanine: can
be caused by 300 possible different mutations (in the same gene)
–  Cystic fibrosis
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Cystic Fibrosis
•  30,000 Americans have CF
•  Disease is NOT caused by a single mutation: 1,400
different mutations have been identified in CFTR gene.
•  Thus there are 1400 different possible mutations that,
if homozygous, will cause CF
•  Different combinations of alleles can produce slightly
different symptoms with different severity:
–  the most commonly found allele produces 70% of the
cases of cystic fibrosis, with respiratory infections
and congested lungs.
–  An individual homozygous for a different allele might
have high incidence of bronchitis and pneumonia.
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Simpler example (same idea):
combinations of different alleles can
yield different phenotypes
Example: 2 alleles of a gene, A and a
If A is dominant over a:
AA and Aa = one phenotype
or aa = different phenotype
Two alleles, 3 combinations, but only two phenotypes
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If there were three alleles for a gene: A, a1, a2,
where:
A is dominant over a1 and a2
a1 is dominant over a2
how many different phenotypes could result from all
possible combinations of allele?
a. two
b. three
c. four
d. five
e. six
In this case, the a1a1 phenotype is different from the
a2a2 phenotype, and both of these are different from
the AA phenotype
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Incomplete Dominance:
Familial Hypercholesterolemia
Die at very early age
Develop atherosclerosis in adulthood
Normal (baseline chance of developing atherosclerosis)
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Atherosclerosis
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You cross a true-breeding white buffalo (white is homozygous recessive)
to a true-breeding black buffalo. All the offspring are brown.
Parents
offspring
A brown buffalo is mated to another brown buffalo. If
they have 8 offspring, how many do you predict will
be black?
A.  0
B.  2
C.  4
D.  8
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Different dominance relationships
Incomplete dominance: the heterozygous phenotype is
a blending of the two homozygous phenotypes; usually
an intermediate phenotype.
In other words, the carrier actually has a phenotype
different from the homozygous dominant individual
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Different dominance relationships:
beyond simple dominant and recessive
Blood type
Universal
recipient
Universal
donor
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Let s try a few problems with blood type:
What is the chance that Bill has type B blood if
his mother is type AB (IAIB) and his father is
type B (IBi)?
a. 1/16
b. 1/8
c. 1/4
d. 1/3
e. 1/2
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Ella and Shannon are skeptical that they have the
same father. Ella has type B blood and
Shannon has type A. Their mother is type O
and their father is type AB. What should
they conclude?
a. He could be the father of both.
b. He could be Ella s father but not Shannon s.
c.  He could be Shannon s father, but not Ella s.
d. He cannot be the father of either.
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The previous example of blood type showed:
a.  co-dominance
b.  incomplete dominance
c.  normal dominance
d.  multiple alleles for single gene
e.  a and d
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Remember the example of Little People Big World from
scenario.
aAnother
few weeks
ago? Matt and Amy both had dwarfism,
but different types.
Remember the Roloff family?
The function of the two genes we
followed in this pedigree are
slightly different, but they are
both involved in growth of bones,
so mutations in either of these
genes can lead to a similar
phenotype
This is called “genetic
heterogeneity”: similar
phenotype caused by
mutations in different
genes
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Ron
Peggy
Matt
Jeremy
Zach
Gordon
Amy
Molly
Jacob
Pat
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Variable expressivity and “penetrance”
•  People with the
same genotype
Polydactyly:
caused can
by ahave
dominant allele
slightly different
PP orphenotypes
Pp
polydactyly
pp
no polydactyly
Expressivity: extra fingernail, extra digits,
sometimes a combination of both in the same
individual.
Penetrance: Sometimes people don’t have the
phenotype at all, despite having the dominant
allele!
•  This is due to proteins interacting with each other,
modifying the final phenotype.
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Handout
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