Question 4
A homozygous tall, red flower plant was crossed with a homozygous dwarf, white flower plant. The F1 were selfed and 160 F2 plants were obtained.
(a) (i) Name the type of genetic cross.
[1 marks]
(ii) State one reason for the answer in 4(a)(i).
[1 marks]
(b) (i) What is the phenotype of F1 generation?
[2 marks]
(ii) With the aid of a genetic diagram, state the phenotypic ratio of F2 generation.
[13 marks]
(iii) Calculate the number of tall and white flowers that would be obtained in the F2 generation.
[3 marks]
Observation
Some candidates performed poorly in this question because they could not properly identify that the type of genetic cross is dihybrid.
The expected answers are:
-
- Type of genetic cross
- Reason for answer
Dihybrid
Two characters are involved in the cross/involves two characters separable in inheritance.
- Phenotype of F1
- Genetic Diagram
Tall and Red
Let T represent the gene for Tallness;
t represent the gene for Dwarfism
R represent the gene for Red
r represent the gene for White.
|
Tr |
Tr |
tR |
tr |
TR |
TTRR Tall Red |
TTRr Tall Red |
TtRR Tall Red |
TtRr Tall Red |
Tr |
TTRr Tall Red |
TTrr Tall Whit |
TtRr Tall Red |
Ttrr Tall White |
tR |
TtRR Tall Red |
TtRr Tall Red |
ttRR Dwarf Red |
ttRr Dwarf Red |
tr |
TtRr Tall Redt |
Ttrr Tall White |
ttRr Dwarf Redt |
ttrr Dwarf White |
Crosses of parental genotypes
Tall and Red - 9
Tall and White - 3
Dwarf and Red - 3
Dwarf and White - 1
Parental genotype
Parental gametes (TR, Tr, tR, tr) shown
Correct Crosses of parental gamete
Phenotypic ratio 9:3:3:1
(iii) Calculation of Tall and White flowers in F2
3 x 160 = 30
16 1
1 mark for using ‘3’ i.e. Tall and White
1 mark for working i.e. 3 x 160
1 mark for answer i.e. 30