Complex patterns of inheritance not described by Mendel

1
Lecture 9 Genetics Complex inherited patterns
variation
Readings Ch. 14, 14.3 to end
You need to be able to explain the following to
yourself and to me (on exams)
Complex patterns of inheritance not described by
Mendel More on partial dominance dominance
depends on analysis Tay-Sacks disease Multiple
alleles ABO blood group locus  Epistasis an
allele of one gene can mask an allele of another
gene epistasis in mouse coat color, and in other
cases
Pleiotropy a single allele may have multiple
effects on phenotype. sickle cell
allele Polygenic inheritance Environmental
effects
2
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

3
Partial dominance basic idea level of
analysis Tay-Sacks disease
Snapdragons
www.florifacts.umn.edu/Trials/Annual20Beddin...
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Partial dominance basic idea level of
analysis Tay-Sacks disease
Example of incomplete dominance
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Partial dominance basic idea level of
analysis Tay-Sacks disease
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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Assessment of dominance depends on the level of
analysis!
Survival
die
live
Complete Dominance
HexA/ HexA
HexA/ HexA-
HexA-/ HexA-
Tay-Sachs
Amount of hexaminidase
die
live
Incomplete Dominance
HexA-/ HexA-
HexA/ HexA
HexA/ HexA-
Tay-Sachs
HexA- codes for a nonfunctional enzyme.
7
Multiple alleles ABO blood group s
A medical problem - some blood transfusions
produce lethal clumping of cells. Dont worry
about details yet...
Type A blood transfused into Type B person
Type B blood transfused into Type B person - OK
8
Multiple alleles ABO blood group s
Table 14.2
There are 3 different alleles, IA, IB, and i
Allele IA makes a cell surface antigen,
symbolized with a triangle IB makes a different
antigen, symbolized as a circle i makes no
antigen
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Multiple alleles ABO blood group s
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

10
Multiple alleles ABO blood group s

• 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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Multiple alleles ABO blood group s
The Human ABO Blood Group System
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Multiple alleles ABO blood group s
The Human ABO Blood Group System
multiple alleles
codominance
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Multiple alleles ABO blood group s
Codominance in the Human ABO Blood Group System
Dominance
Dominance
Codominance
14
Epistasis non-epistasis mouse coat
color explanation
A 9331 ratio is the standard result for a
dihybrid cross
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Epistasis non-epistasis mouse coat
color explanation
Epistasis Normal dihybrid ratio is altered from
9331 to 9340
C/c gene is epistatic to the B/b gene
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Epistasis non-epistasis mouse coat
color explanation
Color genetics in Cape Primrose (Streptanthus) -
an explanation for how epistatis works.
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Color genetics in Cape Primrose - an explanation
for how epistatis works.
Enzyme 1
Enzyme 2
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Color genetics in Cape Primrose - an explanation
for how epistatis works.
Genotype F functons f does not
Enzyme 1
F/F or F/f
Enzyme 2
Could be mauve or blue, but not rose
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Color genetics in Cape Primrose - an explanation
for how epistatis works.
Genotype F functons f does not
X
f/f
Enzyme 1
Enzyme 2
Will be rose regardless of genotype at gene D -
gene O is epistatic to gene D
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Pleiotropy sickle cell disease
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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Polygeny
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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Environmental effects

• 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
23
Environmental effects
Environmental effects effect of temperature on
pigment expression in Siamese cats
24
Lecture 9 Genetics Complex inherited patterns
variation
Readings Ch. 14, 14.3 to end
You need to be able to explain the following to
yourself and to me (on exams)
Complex patterns of inheritance not described by
Mendel More on partial dominance dominance
depends on analysis Tay-Sacks disease Multiple
alleles ABO blood group locus  Epistasis an
allele of one gene can mask an allele of another
gene epistasis in mouse coat color, and in other
cases
Pleiotropy a single allele may have multiple
effects on phenotype. sickle cell
allele Polygenic inheritance Environmental
effects