Extensions of Mendel

non-Mendelian geneic
interactions
I. Interactions between the alleles of one
gene
A. Incomplete Dominance
B. Multiple alleles
C. Codominance
D. Lethal alleles
II.
Interactions between the alleles of more
than one gene
A. Gene interaction
B. Epistasis
I. Interactions between the alleles
of one gene
A. Incomplete Dominance
Two alleles (heterozygote) produce an intermediate
phenotype
Wildtype v. Mutant
Symbols for alleles
• At the molecular level, the mutant allele results in a
reduced amount of functional protein
– 2 doses =
– 1 dose =
– 0 dose =
1

incomplete
dominance
•F 1 
hybrids
have
an
appearance
somewhere
in
between
the
phenotypes
of
the
two
parental
varieties
•
F 1 
is
pink,
an
intermediate
color
between
white
and
red,
F 2
1:2:1
Example:

Tay-Sachs
disease
–
Homozygous
recessive
individuals
are
severely
affected
(death
by
age
3),
Heterozygotes
express
only
about
50%
of
hexosaminidase
enzyme
for
lipid
metabolism.

Slightly
affected.
The closer we look, the more we find that heterozygotes are
different from homozygous dominant individuals.
2

Example question:
Palomino horses have golden yellow coats, chestnut horses have a brown
coat, and cremello horses have a coat that is almost white. A series of
crosses between the three different type of horses produce the following
offspring:
Cross
Palomino x Palomino
offspring
13 palomino, 6 chestnut, 5 cremello
Chestnut x Chestnut
16 chestnut
Cremello x Cremello
Palomino x Chestnut
Palomino x Cremello
Chestnut x Cremello
13 cremello
8 palomino, 9 chestnut
11 palomino, 11 cremello
23 palomino
Explain the inheritance of the three phenotypes, list the genotypes of all the
parents and offspring
B. Multiple
alleles
• Some genes are
found in three or
more alleles that
are different from
each other
e.g. white clover,
coat color in rabbits
3

Example:
C
=
full
coat
color;
dominant
to
all
other
alleles
cch
=
chinchilla
coat,
a
partial
defect
in
pigmentation;
dominant
to
ch
and
c
ch
=
himalayan
coat,
color
in
only
certain
parts
of
body;
dominant
to
c
c
=
albino,
no
color;
recessive
to
all
other
alleles
A
rabbit
with
chinchilla
fur
is
mated
to
a
himalayan.
Some
of
their
F 1 
offspring
have
himalyan
fur,
some
have
chinchilla
fur
and
some
are
albino.
Name
the
genotypes
of
the
parents
and
the
genotypic
ratios
of
the
F 1 
offspring.
Codominance
Genotype
Blood Type
Specific type of multiple
alleles, when two alleles
I A /I A or I A /i
I B /I B or I B /i
I A /I B
i/i
A
B
AB
O
4

Example:
The LM and LN alleles at the MN blood group locus exhibit codominance.
Give the expected genotypes and phenotypes and their ratios in progeny
resulting from the following crosses:
a. L M L M x L M L N
b. L N L N x L N L N
c. L M L N x L M L N
d. L M L N x L N L N
5

Allele in an essential gene
that has the potential of
causing the death of an
organism.
loss of function mutation
tolerated in heterozygote, but
recessive lethal allele in the
homozygous state
– Age of onset
– Conditional lethal alleles
– Semilethal alleles
C. Lethal alleles
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II. Interactions between the alleles of
more than one gene
Genes interact in concert with other
genes and with the environment to
influence a particular characteristic.
Final
product
Interactions between genes produce
many different phenotypes
Examples: morphological
characteristics - Height, weight,
growth rate, pigmentation
7

One trait, involving between two genes
• Bateson & Punnett 1906 studied comb
morphology in chickens –
RP
Rp
rP
rp
RP
RRPP
RRPp
RrPP
RrPp
Rp
RRPp
RRpp
RrPp
Rrpp
rP
RrPP
RrPp
rrPP
rrPp
rp
RrPp
Rrpp
rrPp
rrpp
8

9:3:3:1
with
four
different
phenotypes
•R-P-
(walnut),
R-pp
(rose),
rrP-
(pea),

rrpp
(single)
A. Epistatic interactions
Expression
of
one
gene
or
gene
pair
masks
or
modifies
the
expression
of
another
gene
or
gene
pair
Epistatic = gene product that masks another gene. Hypostatic = the second gene
product being masked by another gene.
Often arise because two or more different proteins participate in
an enzymatic pathway leading to the formation of a single
product.
Enzyme C needed to convert the precursor into the intermediate,
Enzyme P converts the colorless intermediate into purple pigment
9

•Sweet
Peas
–
flower
color
P:
White
x
White
F 1 :
F 2 :
CP
Cp
cP
cp
CP
CCPP
CCPp
CcPP
CcPp
Cp
CCPp
CCpp
CcPp
Ccpp
cP
CcPP
CcPp
ccPP
ccPp
cp
CcPp
Ccpp
ccPp
ccpp
C-P-
(purple),
cc
or
pp
masks
C
or
P
(producing
white
flowers)
homozygosity
for
the
white
allele
at
one
gene
masks
the
purple
producing
allele
of
another
gene
[EPISTASIS]
2
genes:
bw + 
&
st + ,
both
necessary
for
red
eye
10

P:
w/w;m + m + 
x
w + w + /mm
F 1 :

w + /w;m + /m
(all
blue)
F 2 :
= Blue: w + /-; m + /-
= Magenta: w + /-; m/m
= White: w/w; m + /- & w/w; m/m
=
If
the
first
step
in
the
pathway
is
blocked
due
to
a
homozygous
mutant
(w/w)
or
both
steps
are
blocked,
then
the
flower
will
be
white.
=
B/B,
E/E
B/B,
e/e
b/b,
E/E
11

BE
BbEe

x

BbEe
Be
bE
be
BE
BBEE
BBEe
BbEE
BbEe
Be
BBEe
BBee
BbEe
Bbee
bE
BbEE
BbEe
bbEe
bbEe
be
BbEe
Bbee
bbEe
bbee
Precursor
Molecule
(golden)
Complementation analysis
• If
two
recessive
mutations
are
alleles
of
the
same
gene,
then
the
phenotype
of
an
organism
that
contains
one
copy
of
each
mutation
is
mutant;
if
they
are
alleles
of
different
genes,
then
the
phenotype
of
an
organism
that
contains
one
copy
of
each
mutation
is
wildtype.
• Complementation
analysis
may
be
used
to
screen
any
number
of
individual
mutations
that
result
in
the
same
phenotype.
• All
mutations
determined
to
be
present
in
any
single
gene
are
said
to
fall
into
the
same
complementation
group.
• Many
disorders
have
mutations
in
several
genes
that
lead
to
the
mutant
phenotype.
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