1 Dihybrid Cross Dihybrid Cross Incomplete Dominance

Dihybrid
Cross
Dihybrid
Cross
• Cross
involving
two
traits
• Mendel
• Determine
P
generaDon
– Diploid
– Observed
that
the
genes
for
the
different
characters
he
studied
were
passed
on
independently
of
one
another
– Genes
for
these
characters
resided
on
separate,
non‐homologous
chromosomes
– Found
the
physical
basis
for
independent
assortment
of
chromosome
pairs
during
meiosis.
• Determine
gametes
– Haploid
• Cross
gametes
in
square
to
yield
offspring
– Now
need
16
squares
instead
of
four
– Use
FOIL
for
calculaDng
gametes
• First,
outer,
inner,
last
• Summarize
Hypothesis: Dependent assortment
P
generation
F1
generation
Gametes RY
RrYy
Sperm
Sperm
1
–
2
RY
1
–
2
1
–
2
1
–
4
ry
1
– RY
4
RY
Eggs
1
–
4
ry
rY
Eggs
1
–
4
Hypothesized
(not actually seen)
• Not
all
inheritance
works
through
the
principles
Mendel
perceived
in
his
peas.
• Incomplete
dominance
ry
RrYy
1
–
2
F2
generation
rryy
RRYY
ry
Gametes RY
Incomplete
Dominance
Hypothesis: Independent assortment
rryy
RRYY
1
–
4
Ry
RY
1
–
4
rY
1
–
4
Ry
1
–
4
ry
RRYY
RrYY
RRYy
RrYy
RrYY
rrYY
RrYy
rrYy
RRYy
RrYy
RRyy
Rryy
9
––
16
ry
RrYy
rrYy
Rryy
rryy
Actual results
(support hypothesis)
3
––
16
3
––
16
1
––
16
Yellow
round
Green
round
Yellow
wrinkled
Green
wrinkled
– neither
allele
for
a
given
gene
is
completely
dominant
– heterozygous
genotypes
can
yield
an
intermediate
phenotype
• Pink
snapdragons
• Wavy
hair
1
Incomplete
Dominance
Codominance
P generation
RR
red
rr
white
• Codominance
1. The starting plants are a
snapdragon homozygous for
red color (RR) and snapdragon
homozygous for white color (rr).
– Neither
allele
is
recessive
– In
some
instances,
differing
alleles
of
the
same
gene
will
have
independent
effects
in
a
single
organism.
F1 generation
2. When these plants are crossed,
the resulting Rr genotype yields
only enough pigment to produce
a flower that is pink—the only
phenotype in the F1 generation.
Rr
100% pink
R
sperm
• ABO
blood
groups
r
F2 generation
– Such
is
the
case
with
the
gene
that
codes
for
the
type
A
and
B
glycolipids
that
extend
from
the
surface
of
human
red
blood
cells.
3. In the F2 generation, alleles
combine to produce red, pink,
and white phenotypes.
R
RR
Rr
Rr
rr
egg
r
1 :
2 : 1
red
pink white
Figure 11.10
Codominance
• ABO
blood
groups
Codominance
Blood type
(phenotype) . . .
– MulDple
alleles
• An
individual
who
has
one
A
and
one
B
allele
will
have
type
AB
blood
– In
such
a
situaDon,
neither
allele
is
dominant;
rather,
each
is
having
a
separate
phenotypic
effect
– But
the
O
allele
is
recessive
to
both
A
and
B
•
•
•
•
•
AA
=
type
A
BB
=
type
B
AO
=
type
A
BO
=
type
B
OO
=
type
O
. . . has these surface
glycolipids . . .
. . . and is
produced by
these genotypes
Surface glycolipids
on red blood cells
A
AA or AO
B
BB or BO
AB
AB
O
OO
no surface glycolipids
Figure 11.11
2
Polygenic
Inheritance
• Human
beings
and
many
other
species
can
have
no
more
than
two
alleles
for
a
given
gene,
each
allele
residing
on
a
separate,
homologous
chromosome.
Polygenic
Inheritance
• Most
traits
in
living
things
are
governed
by
many
genes.
• These
genes
oXen
have
several
allelic
variants.
– Because
we’re
diploid
• BUT,
many
allelic
variants
of
a
gene
can
exist
in
a
populaDon
– With
only
two
of
those
possessed
by
any
one
individual
Polygenic
Inheritance
• Polygenic
inheritance
– AddiDve
effect
of
2
or
more
genes
on
a
single
trait
– Skin/hair/eye
colors
and
height
in
humans
Polygenic
Inheritance
• Polygenic
inheritance
– tends
to
produce
conDnuous
variaDon
in
phenotypes
• there
are
no
fixed
increments
of
difference
between
individuals.
• Human
skin
– comes
in
a
range
of
colors
in
which
one
color
shades
impercepDbly
into
the
next.
3
Polygenic
Inheritance
P generation
aabbcc
AABBCC
(very light) (very dark)
• Polygenic
traits
tend
to
manifest
in
bell‐curve
distribuDons
F1 generation
AaBbCc
– most
individuals
display
near
average
trait
values
1
–
8
• rather
than
extreme
trait
values
1
–
8
1
–
8
AaBbCc
Sperm
1
–
8
1
–
8
1
–
8
1
–
8
1
–
8
F2 generation
1
–
8
1
–
8
1
–
8
20
––
64
1
–
8
1
–
8
Fraction of population
Eggs
1
–
8
1
–
8
1
–
8
1
––
64
Polygenic
Inheritance
80
15
––
64
20
––
64
15
––
64
6
––
64
1
––
64
6
––
64
1
––
64
Skin color
Polygenic
Inheritance
• Gene
interacDons
and
gene–environment
interacDons
are
so
complex
in
polygenic
inheritance
that
predicDons
about
phenotypes
are
a
ma\er
of
probability,
not
certainty.
(b) The bell curve
Number of individuals
(a) Continuous variation in human height
6
––
64
15
––
64
beak depth
60
40
20
0
5
6
7
8
9
10 11
Beak depth (mm)
12
13
14
Figure 11.13
4
Genes
and
Environment
Genes
and
Environment
• Nature
versus
nurture
– The
effects
of
genes
can
vary
greatly
in
accordance
with
the
environment
in
which
the
genes
are
expressed
• Genotype
and
environment
interact
– produce
that
organism’s
phenotype
Figure 11.14
Pleiotropy
Individual homozygous
for sickle-cell allele
Sickle-cell (abnormal) hemoglobin
• Pleiotropy
is
a
phenomenon
in
which
one
gene
has
many
effects.
• Genes
work
in
an
interrelated
fashion,
such
that
a
single
gene
is
likely
to
have
mulDple
effects.
– PKU
‐
phenolketonuria
– Sickel
cell
anemia
Abnormal hemoglobin crystallizes,
causing red blood cells to become sickle-shaped
Sickle cells
Clumping of cells
and clogging of
small blood vessels
Breakdown of
red blood cells
Physical
weakness
Impaired
mental
function
Anemia
Heart
failure
Paralysis
Pain and
fever
Pneumonia
and other
infections
Accumulation of
sickled cells in spleen
Brain
damage
Damage to
other organs
Rheumatism
Spleen
damage
Kidney
failure
5
Genes
affec;ng
genes
BbCc
• Epistasis
Sperm
1/
– a
gene
at
one
locus
alters
the
phenotypic
expression
of
a
gene
at
a
second
locus
1/
4 bC
BbCc
1/
1/
4 Bc
4 bc
Eggs
1/
• For
example,
in
mice
and
many
other
mammals,
coat
color
depends
on
two
genes
– One
gene
determines
the
pigment
color
(with
alleles
B
for
black
and
b
for
brown)
– The
other
gene
(with
alleles
C
for
color
and
c
for
no
color)
determines
whether
the
pigment
will
be
deposited
in
the
hair
Pedigree
Analysis
4 BC
×
4 BC
1/
4 bC
1/
4 Bc
1/
4 bc
BBCC
BbCC
BBCc
BbCc
BbCC
bbCC
BbCc
bbCc
BBCc
BbCc
BBcc
Bbcc
BbCc
bbCc
Bbcc
bbcc
9
: 3
: 4
Fig. 14-15a
• Pedigree
– family
tree
that
describes
the
interrelaDonships
of
parents
and
children
across
generaDons
– Inheritance
pa\erns
of
parDcular
traits
can
be
traced
and
described
using
pedigrees
Key
Male
Female
Affected
male
Affected
female
Mating
Offspring, in
birth order
(first-born on left)
6
Fig. 14-15b
1st generation
(grandparents)
2nd generation
(parents, aunts,
and uncles)
Fig. 14-15c
1st generation
(grandparents)
Ww
ww
ww
Ff
2nd generation
(parents, aunts,
and uncles)
FF or Ff ff
Ww ww ww Ww
Ww
Ff
ff
Ff
Ww
ff
Ff
Ff
ff
FF
or
Ff
ff
ww
3rd generation
(two sisters)
3rd generation
(two sisters)
WW
or
Ww
Widow’s peak
ww
No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?
Attached earlobe
Free earlobe
(b) Is an attached earlobe a dominant or recessive trait?
You
should
now
be
able
to:
1. Define
the
following
terms:
true
breeding,
hybridizaDon,
monohybrid
cross,
P
generaDon,
F1
generaDon,
F2
generaDon
2. DisDnguish
between
the
following
pairs
of
terms:
dominant
and
recessive;
heterozygous
and
homozygous;
genotype
and
phenotype
3. Use
a
Punne\
square
to
predict
the
results
of
a
cross
and
to
state
the
phenotypic
and
genotypic
raDos
of
the
F2
generaDon
4. Explain
how
phenotypic
expression
in
the
heterozygote
differs
with
complete
dominance,
incomplete
dominance,
and
codominance
5. Define
and
give
examples
of
pleiotropy
and
epistasis
6. Explain
why
lethal
dominant
genes
are
much
rarer
than
lethal
recessive
genes
7