1. Which is the correct set up for a
dihybrid cross of two heterozygous
parents RrYy x RrYy?
See following slides:
A
RR
YY
Yy
yY
yy
rR
Rr
rr
B.
R
R
r
Y
y
r
Y
y
RY
C.
RY
Ry
rY
ry
Ry
rY
ry
The correct set up for a heterozygous
dihybrid cross is
33%
C
33%
B
33%
A
A. A
B. B
C. C
• Each side of a Punnett Square represents all the possible
alleles in a gamete from each parent.
• Parent gametes always contain one allele for each gene.
(one R or r and one Y or y in this case).
• Four possible combinations of the alleles for the two
genes are possible- RY, Ry, rY and ry.
2. For the Punnett square of RrYy x
RrYy, what phenotype ratio would be
expected in the offspring?
25%
25%
1:
02
:0
1
25%
3:
01
25%
9:
3:
3:
1
1:1:1:1
9:3:3:1
3:1
1:2:1
1:
1:
1:
1
A.
B.
C.
D.
Dominant for both : Dom 1st/Rec. 2nd : Rec for 1st/Dom. for 2nd : Recessive for both
9
:
3
:
3
:
1
3. The 9:3:3:1 ratio for a heterozygous
dihybrid cross will apply if
A. the traits are on separate
chromosomes
B. the traits are closely linked on the
same chromosome
C. Either A or B
D. Neither A nor B
its
tra
th
e
th
e
tra
its
no
rB
or
B
A
er
Ne
ith
Ei
th
e
rA
lin
y
se
l
ar
e
ar
e
clo
on
se
pa
ra
...
..
25% 25% 25% 25%
A dihybrid Punnett square assumes that all 4
gamete combinations are equally likely.
This will occur if the two genes are on different
chromosomes. Independent assortment will make
all 4 combinations equally likely.
When two genes are on the
same chromosome, genetic
recombination can occur due
to crossing over.
However, if the two genes are
close together on the
chromosome, the
recombinants will not be
equally like as the original
combination.
4. Determine the phenotype ratio for a
cross between GgHh and gghh.
20%
20%
3:
01
20%
1:
02
:0
1
20%
3:
1:
3:
1
20%
1:
1:
1:
1
9:3:3:1
1:1:1:1
3:1:3:1
1:2:1
3:1
9:
3:
3:
1
A.
B.
C.
D.
E.
GH
Gh
gH
gh
gh GgHh GGhh ggHh gghh
Don’t need to fill out
additional rows.
All gametes of gghh will be
gh.
Ratio of 1:1: 1:1
25% chance dominant phenotype for both traits
25% chance dominant for G, recessive for h
25% chance recessive for g, dominant for H
25% chance recessive for both
1. C
2. B
3. A
4. B