4. Ch3_Oct_2_with_Q - 10/4/09 Lethal
Alleles
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Unformatted text preview: 10/4/09 Lethal
Alleles
 X Mthfr+/+ Mthfr+/- + + +/- -/- +/+ +/- •  Recessive
lethal
alleles

lethal
only
when
2
copies
of
allele
 •  Carried
by
heterozygotes
 94
 Pleiotropy
&
Lethal
Alleles
 •  Heterozygotes
–
all
yellow
 •  RelaConship
of
coat
colour
 phenotypes?
 •  For
another
trait
–
two
alleles
 produce
phenotypes
with
different
 relaConship
 •  E.g.,
AYAY
recessive
lethal
 •  Recessive
lethal
increased
 occurrence
in
inbred
populaCons
 or
where
small
founder
 populaCons

 •  E.g.,
Tay
Sachs
disease
 2 yellow: 1 agouti 95
 1 10/4/09 Dominant
lethal
alleles
can
be
transmiKed
by
heterozygous
 carriers?
 1.  True
 2.  False
 3.  It
depends.
 96
 1
Gene:
Extensions
to
Mendel’s
Analysis
 97
 2 10/4/09 Why
can’t
phenotypes
of
haploid
organisms
be
dominant
or
 recessive?
 1.  Dominance
&
recessiveness
describe
interacCons
between
the
 phenotypes
of
2
alleles
of
the
same
gene
in
the
same
individual.
 2.  Because
only
one
allele
is
present,
thus
the
associated
 phenotype
is
always
dominant
in
haploid
organisms.
 3.  Alleles
in
haploid
individuals
are
transmiKed
like
mitochondrial
 or
chloroplast
DNA.
 4.  Most
haploid
organisms
are
bacteria,
&
bacterial
geneCcs
is
 completely
different
from
eukaryoCc
geneCcs.

 98
 2
Genes

MulCfactorial
Inheritance

 •  MulCfactorial

phenotype
resulCng
from
acCon
of
2
(or
more)
 genes
 •  OR
can
result
b/w
genes
&
environment…
 •  Products
of
2
genes
can
interact
to
determine
a
trait
 •  Epistasis
–
products
of
allele
at
1
gene
mask
effects
of
alleles
of
 another
gene
 •  MutaCons
in
alleles
of
2
or
more
different
genes
can
result
in
the
 same
phenotype
 99
 Same
nomenclature
as
text.
 3 10/4/09 2
Genes

MulCfactorial
Inheritance

 •  Two
genes
each
affec-ng
one
trait
 •  Genes
A
&
B
‐
alleles
A
(tall),
a
(short),
B
(green),
b
(yellow)
 •  F2

9:3:3:1

genotypic
classes
 
 
 
 
A‐B‐:
A‐bb:
aaB‐:
aabb
 •  F2

9:3:3:1

phenotypic
classes
 
 
 
9
tall,
green:
3
tall,
yellow:
3
short,
green:
1
short,
yellow
 •  Two
genes
together
affec-ng
one
trait

 •  SCll
have
F2
9:3:3:1
genotypic
raCo
 •  BUT
variaCons
on
F2
9:3:3:1
phenotypic
raCo
 •  Very
important
to
understand
this
raCo
of
the
genotypic
classes.
 •  MutaCons
in
alleles
of
2
or
more
different
genes
can
result
in
the
same
 phenotype
 100
 Products
of
2
genes
interacCng…

 •  Products
of
2
genes
can
interact
to
determine
a
trait
 •  Produces
novel
phenotypes
 tan
 •  E.g.,
green
lenCl
colour
 grey
 All
brown
 AaBb 
x 
AaBb


 ¼
AA 








½
Aa









 



¼
aa


 ¼
BB

½
Bb

¼
bb


¼
BB

½
Bb

¼
bb



¼
BB

½
Bb

¼
bb
 Tan
 Tan
 Green
 Brown
 Brown
 

Grey
 101
 4 10/4/09 Products
of
2
genes
interacCng…

 •  Products
of
2
genes
can
interact
to
determine
a
trait
 •  Produces
novel
phenotypes
 4
genotypic
classes;
 4
phenotypic
classes
 9:3:3:1 ratio (both genotype & phenotype) 102
 Products
of
2
genes
interacCng
to
determine
a
trait
 •  Complementary
Gene
AcCon
 •  For
a
parCcular
genotype

1
‘dominant
allele’
needed
at
each
of
the
2
 genes
 If just have dominant allele at A gene, but none at B gene OR vice versa no pigmentation (thus, flower is white) •  What
would
be
the
expected
F2
phenotypic
raCo?
 103
 5 10/4/09 Products
of
2
genes
interacCng
to
determine
a
trait
 •  Complementary
Gene
AcCon
 •  SomeCmes
4
F2
genotypic
classes
don’t
produce
4
phenotypes?
 F2 4 genotypic classes; Only 2 phenotypic classes 9:3:3:1

genotypic
raCo
 9:7








phenotypic
raCo
 104
 2
gene
products…
Complementary
Gene
AcCon
 • 
Products
of
2
genes
can
interact
to
determine
a
trait
 • 
Complementary
Gene
AcCon
 • How
does
this
phenotypic
raCo
 come
about?
 • Gene
A
encodes
for
Enzyme
A
 • Gene
B
encodes
for
Enzyme
B
 • Both
play
a
role
in
purple
 pigment
producCon
 • If
one
is
nonfuncConal
or
 missing,
purple
pigment
is
not
 made.
 105
 6 ...
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