Download Extensions of Mendelian Inheritance

Extensions of Mendelian Genetics
Outline/Study Guide
Broader course objective
Explain more complex modes of inheritance and how this influences the
inheritance and expression of genes; use this information in predicting
genetic outcomes and the analysis of genetic data
Necessary for Labs--Patterns of Inheritance in Maize, Blood typing.
Lecture outline/study guide
• Other factors that can change ideal Mendelian ratios
– How can lethality affect the ratios of the remaining
genotypes/phenotypes in Mendelian segregation?
– What is the difference between “penetrance” and “expressivity”?
– What is incomplete dominance? Co-dominance? What examples of
each might you be able to give?
• What is an “allelic series”?
• Epistasis--two (or more) genes can interact to affect one phenotype (e.g.
color of peppers).
– When is a gene interaction considered “epistatic”?
– What is the difference between the epistatic gene and the hypostatic
gene?
• What is the difference between “epistatic” vs. “dominant”?
2:1 ratio from cross
between two yellow
mice results from a
lethal allele.
Mm x
(Manx)
Mm
(Manx)
Sperm
M
M
Lethality in
Manx cats
causes
altered ratio
Brooker, Figure 5.13b
m
MM
(early
embryonic
death)
Mm
(Manx)
Mm
(Manx)
mm
(non-Manx)
Egg
m
1:2 ratio
of kittens
that are born
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Phenotype affected by
temperature
Biology: Unity and Diversity of Life, Starr and Taggert
Penetrance—in this group of identical genotypes, how
many actually show the phenotype?
Fig from iGenetics 1st ed., P. Russell
Expressivity—in this individual of particular genotype,
how strongly does he express the phenotype?
Fig from iGenetics 1st ed., P. Russell
Variable expressivity of Neurofibromatosis
(NF1)
Weak phenotype: cafe au lait
spots
Strong phenotype--cutaneous neurofibromas
P0 generation
CWCW
CRCR
Gametes
1:2:1 phenotypic
ratio NOT the 3:1
ratio observed in
simple Mendelian
inheritance
CR
CW
x
Pink
CRCW
F1 generation
Gametes
In this case, 50% of
the CR protein is not
sufficient to produce
the red phenotype
CR
or
CW
Selffertilization
CR
CW
CRCR
CRCW
F2
generation
Incomplete
dominance as seen
in plants
CR
CW
CRCW
Brooker Figure 5.5
CWCW
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Allelic Series—being “dominant” or “recessive” depends
upon which alleles are being expressed together
e.g. flower color gene: C [enzyme necessary for making pigment]
Allele
CR
Phenotype
Red
C50
Dark pink
C20
Light pink
C0
white
Enzyme
activity
100%
50%
20%
0%
Genotype
Phenotype
CR CR
Red
C50 CR
Red
C50 C50
Dark pink
C50 C20
Pink
C50 C0
Light pink
C20 C20
Light pink
C0 C0
white
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Dominant (functional) allele: R (round)
Recessive (defective) allele: r (wrinkled)
Genotype
RR
Rr
rr
Amount of functional
(starch-producing)
protein
100%
50%
0%
Phenotype
Round
Round
Wrinkled
With unaided eye
(simple dominant/
recessive relationship)
With microscope
(incomplete
dominance)
Figure 5.6
Why is this called a
phenotype and not a
genotype?
A
Co-dominance—
both alleles equally
expressed
(example: bloodtypes—
genotype and surface
antigens
A
B
A
B
A
B
A
B
B
ABO Blood Type
•
•
Allele i is recessive to both IA and IB
Alleles IA and IB are co-dominant (in cells with both alleles the trait is a
mixture of both phenotypes seen in the homozygotes)
Antigen
O
RBC
Antigen
A
N-acetylgalactosa
mine
Antigen
B
RBC
RBC
Antigen
A
Galactose
RBC
Blood type:
O
A
B
AB
Genotype:
ii
IAIA or IAi
IBIB or IBi
IAIB
Surface antigen:
neither A nor B
A
B
A and B
Serum antibodies:
against A and B
against B
against A
none
Brooker Figure 5.9a
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Antigen
B
Examples of sex limited inheritance?
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© robert Maier/Animals, Animals
(a) Hen
© robert Maier/Animals, Animals
(b) Rooster
Brooker, fig 5.12
Duplicate Recessive Epistasis
white
When the recessive allele of either
gene masks the expression of the
other gene.
Precursor
1
Gene A
Precursor
2
Gene B
Purple
Pigment
white
x
Duplicate
Recessive
Epistasis
White variety #1
(CCpp)
White variety #2
(ccPP )
F1
All purple
(CcPp)
Complementation: Each recessive allele
(c and p) is complemented by a
wild-type allele (C and P).
This phenomenon indicates that the
recessive alleles are in different genes.
Self-fertilization
F2
CP
Epistasis: Homozygosity for the recessive
allele of either gene results in a white
phenotype, thereby masking the purple
(wild-type) phenotype.
Both gene products encoded by the wildtype alleles (C and P) are needed for a
purple phenotype.
CP
Cp
cP
cp
Brooker, Figure 5.14
Cp
cP
cp
CCPp
Purple
CcPP
Purple
CcPp
Purple
CCpp
White
CcPp
Purple
Ccpp
White
CcPP
Purple
CcPp
Purple
ccPP
White
ccPp
White
CcPp
Purple
Ccpp
White
ccPp
White
ccpp
White
CCPP
Purple
CCPp
Purple
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“dominant” ≠ “epistatic”
“dominant/recessive”
• Alleles of same gene
7q21.1
Gene 1—Hair color
Black
Brown
Auburn
Red
Blond
Epistatic—one gene masking another
Gene 1—Hair color
Black
Brown
Auburn
Red
Blond
Gene 2-Baldness
Full hair
partial hair
bald
“dominant” ≠ “epistatic”
“dominant/recessive”
• Alleles of same gene
7q21.1
Gene 1—Hair color
Black
Brown
Auburn
Red
Blond
Epistatic—one gene masking another
Gene 1—Hair color
Black
Brown
Auburn
Red
Blond
Gene 2-Baldness
Full hair
partial hair
bald
14p1.31
Hypostatic
gene
Epistatic
gene
• A geneticists crossed a red eyed fly with
another red eyed fly. In the next
generation she observed phenotypic
proportions of 263 red-eyed flies : 137
brown-eyed flies. What hypothesis would
best explain the parental cross that gave
rise to these flies? Use chi-square
analysis to support your hypothesis.
• How would you ‘prove’ this without doing
the chi-square test?
Extensions of Mendelian Inheritance : practice
questions
The following comprehension questions (at end of each chapter section)
in Brooker, Concepts of Genetics are recommended:
• Comprehension Questions (at end of each section): 5.2, 5.4, 5.6, 5.8
Answers to Comprehension Questions are at the very end of every chapter.
• Solved Problems at end of chapter (answers included):
• Conceptual questions and Experimental/Application Questions at
end of chapter (answers found by logging into publisher’s website, or
find them in the book):
– Concepts—C1, C2, C3, C4, C5, C6, C8, C9, C10, C11, C15, C20
– A little more challenging—C18, E1, E2, E7, E10