Transcript 11.2B Dihybrid Crosses File

Ch.11-2b
Independent Assortment
POINT > Define a dihybrid cross
POINT > Describe Mendel’s dihybrid experiments
POINT > Show how meiosis could lead to different
allele combinations
POINT > Use a Punnett square to predict genotypes
and phenotypes from a dihybrid cross
POINT > Define the Law of Independent Assortment
POINT > Define a dihybrid cross
So far you have examined monohybrid crosses:
crosses involving only one trait
Mendel wondered if inheritance of one trait affected
the inheritance of other traits, or if they all acted
independently
A dihybrid cross is a genetic cross that involves two
traits
Punnett squares are again useful in explaining what
Mendel found
POINT > Describe Mendel’s dihybrid experiments
Ex. Mendel crossed plants purebred for yellow seeds
and smooth seed coats with plants purebred for green
seeds and wrinkled seed coats
All the F1 had yellow seeds and smooth seeds coats
P
F1
(YYRR)
X
?
(yyrr)
(YyRr)
POINT > Describe Mendel’s dihybrid experiments
First generation (F1) plants self-pollinated:
All trait combinations show up in the F2
F1
F2
(YyRr)
X
?
(YyRr)
POINT > Describe Mendel’s dihybrid experiments
Every trait combination Mendel tried generated an F2
with the same proportions:
F2
9
:
3
:
3
:
1
WB CHECK:
Which of the following is a monohybrid cross?
a) Plant with black leaves X plant with pink flowers
b) Plant with red berries and wrinkled seeds X plant
with white berries and smooth seeds
c) Fruit fly with 2 sets of wings X fruit fly with
normal wings
d) Fruit fly with white eyes and 8 legs X fruit fly with
red eyes and 2 sets of wings
WB CHECK:
Which of the following is a dihybrid cross?
a) Plant with black leaves X plant with pink flowers
b) Plant with red berries and wrinkled seeds X plant
with white berries and smooth seeds
c) Fruit fly with 2 sets of wings X fruit fly with
normal wings
d) Fruit fly with white eyes and 8 legs X fruit fly with
red eyes and 2 sets of wings
POINT > Show
how meiosis
could lead to
different allele
combinations
YY
yy
RR
rr
F1 plant meiosis
(YyRr)
YY
yy
RR
R
Y
rr
R
Y
r
y
r
y
POINT > Show
how meiosis
could lead to
different allele
combinations
YY
yy
rr
RR
F1 plant meiosis
(YyRr)
YY
yy
rr
r
RR
Y
r
Y
R
y
R
y
25%
YR
25%
Yr
25%
Y
R
y
r
Y
r
y
R
yr
25%
yR
Expected gametes if traits move independently of each other
POINT > Use a Punnett square to predict
genotypes and phenotypes from a dihybrid cross
A Punnett square to
examine the inheritance
of two different traits
requires a 4x4 grid:
POINT > Use a Punnett square to predict
genotypes and phenotypes from a dihybrid cross
To generate gametes use FOIL:
Each gamete must have only one letter from each trait!!
Y
R
Y
YR
y
R
r
Y
r
y
R
Yr
yR
r
y
yr
WB CHECK:
Show the 4 possible gametes produced by a raccoon
heterozygous for tail length (Tt) and heterozygous
for ear tufts (Ee):
TE
Te
tE
te
POINT > Use a Punnett square to predict genotypes and phenotypes from a dihybrid
cross
YR
Yr
yR
yr
YR YYRR YYRr YyRR YyRr
Yr YYRr
YYrr
YyRr
Yyrr
yR YyRR YyRr yyRR yyRr
yr YyRr
Yyrr
yyRr
yyrr
POINT > Use a Punnett square to predict genotypes and phenotypes from a dihybrid
cross
YR
Yr
yR
yr
YR YYRR YYRr YyRR YyRr
Yr YYRr
YYrr
YyRr
Yyrr
yR YyRR YyRr yyRR yyRr
yr YyRr
Yyrr
yyRr
yyrr
Genotypes
YYRR
YYRr
YYrr
YyRR
YyRr
Yyrr
yyRR
yyRr
yyrr
1
2
1
2
4
2
1
2
1
POINT > Use a Punnett square to predict genotypes and phenotypes from a dihybrid
cross
YR
Yr
yR
yr
YR YYRR YYRr YyRR YyRr
Yr YYRr
YYrr
YyRr
Yyrr
yR YyRR YyRr yyRR yyRr
yr YyRr
Yyrr
yyRr
yyrr
POINT > Use a Punnett square to predict genotypes and phenotypes from a dihybrid
cross
YR
YR
Yr
yR
yr
Phenotypes
Yellow, Round
9
Yr
Yellow, Wrinkled
3
yR
Green, Round
3
yr
Green, Wrinkled
1
POINT > Define the Law of Independent Assortment
Law of Independent Assortment:
Genes for different traits segregate
independently during gamete formation (meiosis)
Helps explain great amount of variation in many
species
Read pages 313-318
Finish Assess p. 318 and any other missing
homework