5. Ch3_Oct_5_no_Q - 10/4/09

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Unformatted text preview: 10/4/09 Gene$c
Heterogeneity
&
Complementa$on
Tests
 •  Heterogeneous
trait:
a
muta$on
at
any
one
of
mul$ple
genes
can
result
in
 the
same
phenotype
 •  Complementa$on:

2
different
muta$ons
in
the
heterozygous
state
exhibit
as
 the
wild‐type
phenotype

 
 
 
 

indicates
the
muta$ons
are
at
different
loci
(in
different
genes)
 Individual
1
homozygous
for
 muta$on
in
Gene
B;
 Individual
2
homozygous
for
 muta$on
in
Gene
A
 Offspring
is
heterozygous
for
 muta$ons
at
both
genes
(but
carries
 func$onal
copies
for
each
gene).
 107
 Gene$c
Heterogeneity
&
Complementa$on
Tests
 Individual
1
homozygous
for
 muta$on
in
Gene
B;
 Individual
2
homozygous
for
 muta$on
in
Gene
A.
 Offspring
is
heterozygous
for
 muta$ons
at
both
genes
(but
carries
 func$onal
copies
for
each
gene)
so
 has
func$onal
hearing.
 Each
parent
is
homozygous
for
a
muta$on
 in
the
same
gene
(gene
A).
 Offspring
thus
have
func$onal
copies
for
 gene
A,
but
not
for
gene
B.
Thus
doesn’t
 have
func$onal
hearing
 108
 1 10/4/09 Gene$c
Heterogeneity
&
Complementa$on
Tests
 •  Two
cell
lines
with
independently
acquired
recessive
muta$ons.
Are
the
2
 muta$ons
within
the
same
gene?
(You
know
the
genotype,
but
are
uncertain
as
 to
whether
it
is
for
the
same
gene
or
different
genes…how
do
you
find
out
if
 they
are?)
 •  Cross
the
2
lines,
e.g.,
with
alleles
aa

and
bb.
 •  If
‘a’
and
‘b’
are
of
the
same
gene,
then
the
2
copies
that
the
progeny
 receive
will
s$ll
give
a
mutant
phenotype.
 •  If
they
affect
different
genes,
WT
phenotype
will
be
restored
because
 the
progeny
will
have
only
one
copy
of
the
recessive
alleles
of
each
 gene.
 109
 Gene$c
Heterogeneity
&
Complementa$on
Tests
 •  Muta$ons
in
same
gene
or
different
genes?
 •  3
phenotypically
iden$cal
muta$ons:
cinnabar,
scarlet
&
cinnabar‐2
 •  WT
=
darker
red;
these
muta$ons
are
all
brighter
red
 •  Make
pairwise
crosses
b/w
flies
homozygous
for
different
muta$ons

 •  Phenotypes
of
hybrids
show
cinnabar
&
cinnabar‐2
are
alleles
of
1
colour‐ determining
gene
(same
gene);
scarlet
muta$on
is
in
a
different
gene
 110
 2 10/4/09 Recessive
Epistasis
 •  Effects
of
allele
at
1
gene
hide
effects
of
allele
at
another
gene.
 •  Allele
masking
is
epista$c
to
gene
being
masked
(hyposta$c)
 •  Presence
of
homozygous
recessive
genotype
at
1
gene
masks
any
allelic
 x
 combina$on
at
another
gene
 True‐breeding
 parents
 BBEE










 x
 
 
bbee
 B
=
black
 b
=
brown
(rec.
pheno)
 E
=
no
effect
 ee

masks
B
or
b
&
gives
gold
 F1
 BbEe
 What
are
the
F2
phenotypic
ra$os
if
ee
is
epista$c?
 Recessive
epistasis
 111
 Recessive
Epistasis

 •  Effects
of
allele
at
1
gene
hide
effects
of
allele
at
another
gene.
 •  Allele
masking
is
epista$c
to
gene
being
masked
(hyposta$c)
 Recessive epistasis 9:3:3:1

genotypic
ra$o
 9:3:4




phenotypic
ra$o
 112
 3 10/4/09 Recessive
Epistasis
 •  Recessive
epistasis
in
human
blood
types

Bombay
phenotype
 •  2
genes
involved:
 •  1
for
carbohydrate
(A,
B,
or
 none)
 •  1
for
polymer
that
A,
B
or no
carbohydrates
adached
 to
 •  Bombay
phenotype

epistasis
 •  Type
A
or
B,
so
can
make
 carbohydrate
 •  BUT
no
polymer
to
adach
 it
to
 •  Thus
don’t
see
any
carbohydrates
on
surface
of
RBC

looks
like
type
O
 113
 Epistasis
 •  Effects
of
allele
at
1
gene
hide
effects
of
allele
at
another
gene.
 •  Allele
masking
is
epista$c
to
gene
being
masked
(hyposta$c)
 •  Recessive
Epistasis

homozygous
recessive
alleles
of
1
gene
mask
effects
of
 x 
 
 
 
 
 
another
gene
 •  Dominant
Epistasis

dominant
allele
of
1
gene
masks
effects
of
another
 
 
 
 
 
 
gene
(b/c
dominant,
only
need
one
copy
of
allele)
 •  2
types
 •  1.
masks
effects
of
both
alleles
of
the
other
gene
 •  E.g.,

summer
squash
colour
 •  12:3:1
phenotypic
ra$o
 •  2.
masks
effects
of
dominant
allele
of
another
gene
 •  E.g.,

colour
of
certain
strains
of
chickens
 •  13:3
phenotypic
ra$o
 114
 4 10/4/09 •  Effects
of
allele
at
1
gene
hide
effects
of
allele
at
another
gene.
 •  Allele
masking
is
epista$c
to
gene
being
masked
(hyposta$c)
 Dominant
Epistasis
 Dominant
epistasis
(type
I)
 • F2
characteris$c
12:3:1
 •  Presence
of
A
allele

yellow
 •  Presence
of
B
allele
masks
effects
of
A
or
a
allele
 (B
is
epista$c
to
alleles
at
the
A
gene)
 115
 •  Effects
of
allele
at
1
gene
hide
effects
of
allele
at
another
gene.
 •  Allele
masking
is
epista$c
to
gene
being
masked
(hyposta$c)
 Dominant
Epistasis
 Dominant
epistasis
(type
II)
 • F2
characteris$c
13:3
 •  Presence
of
A
allele
&
absence
of
B
allele

colour
 •  Presence
of
B
allele
masks
effects
of
A
or
a
allele
(B
 is
epista$c
to
alleles
at
the
A
gene)
 116
 5 10/4/09 Mul$factorial
Traits
 •  The
alleles
of
2
or
more
genes
can
interact
to
generate
novel
phenotypes;
 •  E.g.,
len$ls
(green
colour)
(Fig.
3.11)
 •  The
dominant
alleles
of
two
interac$ng
genes
can
both
be
necessary
for
 the
produc$on
of
a
par$cular
phenotype;
 •  E.g.,
purple
flower
colour
(Fig.
3.
13
 •  The
alleles
of
one
gene
can
mask
the
effects
of
another
gene’s
alleles;
 •  Mutant
alleles
at
one
of
two
or
more
different
genes
can
result
in
the
 same
phenotype.
 117
 2
(+)
genes
interac$ng

&
Incomplete
Dominance

 F2
 118
 6 10/4/09 2
or
More
Genes…
 Depending
on
the
nature
of
the
gene
interac$ons,
get
a
 variety
of
phenotypic
ra$os.
 119
 1
Gene,
2
Genes…Using
Breeding
Studies
 •  1
or
2
genes?

 1.  Look
at
phenotypic
ra$os

e.g.,
9:7,
13:3

suggests
2
genes
interac$ng
 
 2.  
Use
breeding
studies
to
determine
if
hypothesis
is
correct
 White
(albino)
 Pure‐breeding
 Brown

 Pure‐breeding
 P
genera$on
 X
 F2
genera$on
(interbreed)
 black
 X
 black
 1
gene?
 Albinos
=
bb
 Black
=
Bb
 Brown
=
BB
 90
black
 2
genes?
 30
brown
 9:3:4

recessive
epistasis?
 40
albino
 120
 7 10/4/09 1
Gene,
2
Genes…Using
Breeding
Studies
 121
 Same
Genotype








Same
Phenotype

 •  Penetrance
 •  Complete
(100%)
 •  Incomplete
(<100%)
 •  Expressivity
 •  E.g.,age
of
onset
for
Hun$ngton
 •  Expressivity
vs.
Penetrance
 Penetrance Expressivity Penetrance & Expressivity 122
 8 10/4/09 Same
Genotype








Same
Phenotype

 •  Penetrance Penetrance Expressivity Penetrance & Expressivity •  Expressivity Why have differences in penetrance & phenotypic expression? •  Modifier Genes •  Environmental Factors 123
 Same
Genotype








Same
Phenotype

 •  Expressivity
 •  Lobe
muta$on
(dominant)
 •  Each
fly
shown
is
 heterozygous
for
the
 muta$on
 •  Variable
expressivity
 •  Phenotypes
vary
from
 complete
absence
of
 eye
to
an
almost
WT
 eye
 124
 9 10/4/09 Quan$ta$ve
Traits
 Covered
in
early
classes
 •  Con$nuous
varia$on
 •  Generally
polygenic
 •  Alleles
are
addi$ve
in
nature
 Normal
distribu$on
=
bell‐shaped
curve
 125
 Pedigrees
 •  Diagram
rela$onships
among
members
of
a
family
 126
 10 10/4/09 Pedigrees
 •  Dominant
Trait
 127
 Pedigrees
 •  Recessive
Trait
 128
 11 10/4/09 •  X‐linked
Trait
‐
Recessive
 Pedigrees
 129
 •  X‐linked
Trait
‐
Dominant
 Pedigrees
 130
 12 10/4/09 •  Y‐linked
Trait
 Pedigrees
 131
 Problem
Solving
 •  If
parents
are
carriers
for
CF
&
have
4
kids;
 •  According
to
monohybrid
cross,
expect
3
unaffected
&
 1
affected

but
human
families
small
sample
sizes,
 so
skewed
data;
 •  5
possible
outcomes
 •  4
unaffected;
none
affected
 •  3
unaffected;
1
affected
 •  2
unaffected;
2
affected
 •  1
unaffected;
3
affected
 •  None
unaffected;
4
affected
 •  For
each
child
–
chance
unaffected
=
¾;
chance
 affected
=
¼

 •  Probability
all
unaffected
=
(¾)4
 132
 13 10/4/09 Problem
Solving
 •  If
parents
are
carriers
for
CF
&
have
4
kids;
 •  5
possible
outcomes
 •  4
unaffected;
none
affected
 •  3
unaffected;
1
affected
 •  2
unaffected;
2
affected
 •  1
unaffected;
3
affected
 •  None
unaffected;
4
affected
 •  For
each
child
–
chance
unaffected
=
¾;
chance
 affected
=
¼

 •  Probability
3
unaffected
&
1
affected
 •  (¾)3
x
¼
…but
4
different
ways…
 •  Let
U
=
unaffected;
A
=
affected
 •  Order
of
kids:
 •  UUUA,
UUAU,
UAUU,
AUUU
 •  Need
to
take
each
possible
order
into
 considera$on

can
have
1st
order
OR
2nd
OR
3rd
 OR
4th

 •  Thus
[(¾)3
x
¼]
+
[(¾)3
x
¼]
+
[(¾)3
x
¼]
+[(¾)3
x
¼]
 =
108/256
 •  For
2
unaffected
&
2
affected

6
different
orders
 133
 Why
are
some
muta$ons
dominant
and
others
are
recessive?
 • Dominant
muta$on
–
phenotypic
 effects
in
heterozygotes
&
 homozygotes
 • Recessive
muta$ons
–
only
have
 phenotypic
effects
in
homozygotes
 – Owen
involve
lose
of
gene
func$on

 • Gene
no
longer
specifies
a
polypep$de;
 • Or
codes
for
a
nonfunc$onal
or
 underfunc$onal
polypep$de
 • May
not
work
at
body
temperature…
 134
 14 ...
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