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Welcome back to IB 150...
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Lecture 7 Complex Patterns of Inheritance
Assigned Readings Ch. 14, 14.3 to end
By the end of this lecture you should be able
to Identify complex patterns of inheritance not
described by Mendel. degrees of dominance
epistasis pleiotropy environmental
effects.  Give examples of degrees of
dominance. complete dominance incomplete
dominance codominance
 Understand that an allele of one gene can mask
an allele of another gene. epistasis in mouse
coat color, and in other cases Understand that a
single allele may have multiple effects on
phenotype. sickle cell allele Define the norm
of reaction. phenotypic range
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Post-Mendelian studies have revealed that
relationships between genotype and phenotype can
be complex

• degrees of dominance
• multiple alleles
• epistasis
• pleiotropy
• polygenic inheritance
• environmental effects

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Snapdragons
www.florifacts.umn.edu/Trials/Annual20Beddin...
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Example of incomplete dominance
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A medical problem - some blood transfusions
produce lethal clumping of cells. Dont worry
about details yet...
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The Human ABO Blood Group System
multiple alleles
codominance
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Human ABO Blood Groups

• Gene I specifies which sugar is found on the
  outside of red blood cells
• 3 alleles are present in the human population
• IA N-acetyl-galactosamine
• IB galactose
• i (also referred to as o) no sugar present
• 6 possible genotypes

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Table 14.2
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• Immunology 101
• Sugar on the blood cell is an antigen (A, B, A
  and B, or none)
• Your immune system thinks your own antigens are
  fine
• Your immune system makes antibodies against
  non-self antigens
• Antibodies recognize and target cells with
  antigens for destruction

something that elicits an immune response
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One of the oddities of the ABO blood group gene
is that we make antibodies to ABO antigens not
present in our genotype. For example, Type A
people make b antibodies. This is not typical,
but an unusual aspect of the blood group.
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A medical problem - some blood transfusions
produce lethal clumping of cells. Dont worry
about details yet...
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Codominance in the Human ABO Blood Group System
Dominance
Dominance
Codominance
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Possible Dominance Relationships between Two
Alleles
Dominance Relationship Heterozygote
Phenotype Complete (Mendelian) same as homozygous
dominant Incomplete intermediate between the
two homozygotes Codominance phenotypes of
the two homozygotes are expressed
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Assessment of dominance depends on the level of
analysis!
A heterozygote may display a dominant phenotype
at the organismal level, but at a biochemical
level may show incomplete dominance.
Tay-Sachs disease caused by absence of an
enzyme, hexosaminidase A (Hex-A)
Homozygous dominant normal levels of Hex-A,
normal development of child Homozygous recessive
no Hex-A, death of child by age
5 Heterozygote1/2 normal levels of Hex-A, normal
development of child
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Epistasis one gene alters or blocks the
expression of another independently inherited
gene
Disclaimer Made-up example
A mutant gene that caused complete baldness in
humans would be epistatic to a gene that
determines hair color.
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A 9331 ratio is the standard result for a
dihybrid cross
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Epistasis Normal dihybrid ratio is altered from
9331 to 9340
C/c gene is epistatic to the B/b gene
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Color genetics in Cape Primrose - an explanation
for how epistatis works.
Enzyme 1
Enzyme 2
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Pleiotropy one gene causes multiple effects on
phenotype
Sickle cell anemia one mutant gene, many symptoms
Single amino acid substitution in the hemoglobin
protein
Pain, stroke, leg ulcers, bone damage, jaundice,
gallstones, lung damage, kidney damage, eye
damage, anemia, delayed growth
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Polygenic inheritance additive effects
(essentially, incomplete dominance) of multiple
genes on a single trait
AA dark Aa less dark aa - light And similarly
for the other two genes - in all cases dominance
is incomplete for each gene. Think of each
capital allele (A, B, C) as adding a dose of
brown paint to white paint.
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• environment often influences phenotype
• the norm of reaction phenotypic range due to
  environmental effects
• norms of reactions are often broadest for
  polygenic characters.

Blue require low pH
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Environmental effects effect of temperature on
pigment expression in Siamese cats
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Lecture 7 Complex Patterns of Inheritance
Assigned Readings Ch. 14, 14.3 to end
By the end of this lecture you should be able
to Identify complex patterns of inheritance not
described by Mendel. degrees of dominance
epistasis pleiotropy environmental
effects.  Give examples of degrees of
dominance. complete dominance incomplete
dominance codominance
 Understand that an allele of one gene can mask
an allele of another gene. epistasis in mouse
coat color Understand that a single allele may
have multiple effects on phenotype. sickle cell
allele Define the norm of reaction. phenotypic
range