2. Typical
Dihybrid
cross
Two
traits
are
considered-‐
• seed
color
(yellow
vs.
green)
• seed
texture
(wrinkled
vs.
round)
9:3:3:1
phenotypic
raDo
when
2
dihybrid
heterozygous
parents
are
crossed.
4. Independent assortment
An Interpretation from the Dihybrid cross
• During gamete formation, segregating pairs of unit
factors assort independently."
• In other words, segregation of 2 alleles at one genetic
locus has no effect on the segregation of 2 alleles at
another locus."
• For example, the assortment of yellow and green
alleles has no effect on the assortment of round and
wrinkled alleles, and vice versa."
• Results have expected, typical ratios"
5. Discovery
of
Linkage
• William
Bateson
and
R.C.
PunneP
were
working
with
several
traits
in
sweet
peas,
• a
gene
for
purple
(P)
vs.
red
(p)
flowers
• a
gene
for
long
pollen
grains
(L)
vs.
round
pollen
grains
(l).
6. Bateson and Punnett pea study:
Flower Color:
P = purple
p = red
Pollen seed shape:
L = long
l = round
True Breeding lines:
PPLL
x
ppll
PpLl
F1
Phenotype
Observed
Expected
Expected
Ra5o
Number
Purple
long
284
9
215
Purple
round
21
3
71
Red
long
21
3
71
Red
round
55
1
24
Crosses
produced
a
deviaDon
from
the
predicted
Mendelian
independent
assortment
raDos.
What is going on????
7. Results:
There
are
4
expected
genotypes
There
are
4
recombinant
genotypes
but
cannot
be
observed
from
phenotypes
There
are
2
recombinant
phenotypes
which
would
be
actually
observed
during
the
experiment
Results
did
not
show
a
typical
9:3:3:1
therefore
not
dihybrid
unlinked.
Results
did
not
show
a
totally
linked
raDo
either
(3:1)
Therefore,
there
must
be
something
else
happening
with
the
genes.
8. • Because the parental phenotypes reappeared
more frequently than expected, the
researchers hypothesized that there was a
coupling, or connection, between the parental
alleles for flower color and pollen grain shape
• This coupling resulted in the observed
deviation from independent assortment.
10. Thomas
Morgan’s
work
with
Drosophila
lead
to
the
discovery
of
linked
genes
and
recombina5on
due
to
crossing
over
• Morgan
proposed
that
the
chiasmata
visible
on
chromosomes
were
regions
of
crossing
over.
• Occurs
between
non-‐sister
chromaDds.
11. EvoluDon,
Natural
SelecDon
and
Crossing
Over
• From an evolutionary point of view, the purpose of sex is
to re-shuffle the combinations of alleles so the offspring
receive a different set of alleles than their parents had.
• Natural selection then causes offspring with good
combinations to survive and reproduce, while offspring
with bad combinations don’t pass them on.
• Genes are on chromosomes. Meiosis is a mechanism
for re-shuffling the chromosomes: each gamete gets a
mixture of paternal and maternal chromosomes.
• However, chromosomes are long and contain many
genes. To get individual genes re-shuffled, there needs
to be a mechanism of recombining genes that are on the
same chromosome. This mechanism is called “crossing
over.
13. Recombinants
RecombinaDon=
the
reassortment
of
alleles
into
combinaDons
different
from
those
of
the
parents,
as
a
result
of:
independent
assortment,
crossing
over
and/or
ferDlizaDon.
Example-‐
recombinants
due
to
crossing
over
Linked
genes:
A
with
B
AB
high
frequency
a
with
b
ab
high
frequency
Recombinant
a
with
B
aB
low
frequency
Recombinant
A
with
b
Ab
low
frequency
Recombinants
due
to
crossing
over
will
always
have
a
lower
frequency
15. Frequency
of
crossover
exchange...
of
chroma5ds
of
a
homologous
pair
at
synapsis
forming
a
chiasmata...
• Frequency
is
GREATER
the
FARTHER
apart
2
genes
are
is
propor5onal
to
rela5ve
distance
between
2
linked
genes
Rela5ve
distance
is
established
as...
• 1%
crossover
frequency
=
1
map
unit
of
map
distance
• 1%
CrossOver
Freq
=
1
cen5Morgan
(cM)
18. • Polygenic
inheritance-‐
more
than
one
gene
contributes
to
a
phenotype.
A
characterisDc
that
is
controlled
by
more
than
one
gene.
• Allows
for
conDnuous
variaDon
• Examples
include:
human
height,
human
skin
color,
wheat
seed
color
19. Polygenic
Traits
–
Skin
Color
About
5
genes
are
involved
in
human
skin
color
which
creates
a
conDnuous
range
of
skin
color
in
the
populaDon
from
very
light
to
very
dark.
Each
gene
has
2
melanin-‐producing
alleles
that
are
codominant.
Therefore,
many
genes
contribute
to
the
color
of
skin
EvoluDon
has
selected
for
greater
melanin-‐
producDon
in
areas
where
there
is
more
intense
sunlight
to
protect
against
UV
radiaDon
and
prevent
skin
cancer.
Lower
light
intensity
regions
selected
over
Dme
for
less
melanin
to
allow
for
more
Vitamin
D
producDon
SelecDve
advantage
of
skin
color
is
now
compensated/overcome
by
the
use
of
sunblock
and
vitamin
D
supplements.
Using
whitener
on
your
skin
is
a
terrible
idea!
