Genetic Problems
1. Cross a homozygous tall (TT) pea plant with a (homozygous) short (tt) pea plant. How many
(expressed as a percentage) short pea plants will appear in the offspring of the cross?
TT x tt
T
T
Genotypes (in table)
t
Tt
Tt
Phenotypes: Tall
t
Tt
Tt
Genotypic ratio: 1:0
Phenotypic Ratio 1:0
Short Pea plants in the offspring = 0%
2. Cross two pink-flowered (RW) pea plants. How many (expressed as a percentage) red-flowered pea
plants will appear in the offspring of this cross?
RW x RW
R
W
Genotypes (in table)
R
RR
RW
Phenotypes: Red, Pink, White
W
RW
WW
Genotypic ratio: 1:2:1
Phenotypic Ratio 1:2:1
Red-flowered pea plants in the offspring = 25%
3. Cross a heterozygous Group A individual with a heterozygous Group B individual. What are the
chances of generating a Group O child from this cross?
IAi x IBi
IA
i
Genotypes (in table)
IB
IAIB
IBi
Phenotypes: AB, A, B, O
i
IAi
ii
Genotypic ratio: 1:1:1:1
Phenotypic Ratio 1:1:1:1
Group O child in the offspring = 25%
4.
Mildred, mother of 11 – Group AB
Charlie, husband of Mildred – Group O
Edward, groundskeeper at the Memorial Park – Group B
James, Mildred’s 11th Child – Group A
Charlie is convince that Mildred has been playing around in the park because he thinks that James,
Mildred’s most recent creation, looks more like Ed than himself. Based on their blood groupings, can
Charlie prove his case?
Charlie’s cross with Mildred
IAIB x ii
IA
IB
Genotypes (in table)
i
IAi
IBi
Phenotypes: A, B
i
IAi
IBi
Genotypic ratio: 1:1
Phenotypic Ratio 1:1
James could be Charlie’s Son
Edward’s cross with Mildred
IAIB x IBi
IA
IB
Genotypes (in table)
IB
IAIB
IBIB
Phenotypes: AB, A, B
i
IAi
IBi
Genotypic ratio: 1:1:1:1
Phenotypic Ratio 1:1:2
James could also be Edward’s son if Edward is heterozygous for the trait
but not if he is homozygous for the trait (I only show the first cross)
5. Sex-linked: White eyes versus Wild type Drosophilia. The parents (P generation) are male red-eyed
and female white-eyed. Show the punnet square for the original cross. The offspring in this square will
be the F1 generation.
Parent generation: X+y x XwXw
Genotypes (in table)
X+
Y
Xw
X +X w
XwY
Xw
X +X w
XwY
Phenotypes of the F1 generation: Normal females (carriers) and white-eyed males
Genotypic ratio: 1:1
Phenotypic Ratio 1:1
6. F2 generation – cross the F1 generation from above
F1 generation from above: X+Xw x XwY
XW
Genotypes (in table)
Y
X+
X+Xw
X +Y
Xw
XwXw
XwY
Phenotypes of the F2 generation: Normal females (carriers), white-eyed females, Normal males and
white-eyed males
Genotypic ratio: 1:1:1:1
Phenotypic Ratio 1:1:1:1
B. Dihybrid Crosses
1. In pea plants, tall (T) is dominant over short (t) and round seeds (R) is dominant over wrinkled (r).
Cross two heterozygous tall, heterozygous round-seeded plants.
Parents: TtRr X TtRr
Genotypes (in table)
TR
Tr
tR
tr
TR
TTRR
TTRr
TtRR
TtRr
Tr
TTRr
TTrr
TtRr
Ttrr
tR
TtRR
TtRr
ttRr
ttRr
tr
TtRr
Ttrr
ttRr
ttrr
Phenotypes: Tall round, Tall wrinkled, short round, short wrinkled
Genotypic ratio: 1:2:1:2:4:2:1:2:1
Phenotypic Ratio 9:3:3:1
2. Varies depends on your choices
3. Dihybrid cross involving complete dominance and incomplete dominance. Cross TtRW X TtRW where
height follows complete dominance and flower color follows incomplete dominance.
Parents TtRW X TtRW
TR
Genotypes (in table)
TW
tR
tW
TR
TTRR
TTRW
TtRR
TtRW
TW
TTRW
TTWW
TtRW
TtWW
tR
TtRR
TtRW
ttRR
ttRW
tW
TtRW
TtWW
ttRW
ttWW
Phenotypes: Tall Red, Tall Pink, Tall White, Short Red, Short Pink, Short White
Genotypic ratio: 1:2:1:2:4:2:1:2:1
Phenotypic Ratio: 3:6:3:1:2:1