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 evolution? [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:
(a) (i) Type of genetic cross
Dihybrid
(ii) Reason for answer
Two characters are involved in the cross/involves two characters separable in inheritance.
(b) (i) Phenotype of F1
Tall and Red.
(ii) Genetic Diagram
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.
OR
Parental genotype TtRr X TtRr
| X | TR | Tr | tR | tr |
|---|---|---|---|---|
| TR | TTRR Tall Red | TTRr Tall Red | TtRR Tall Red | TtRr Tall Red |
| Tr | TTRr Tall Red | TTrr Tall White | TtRr Tall Red | Ttrr Tall White |
| tR | TtRR Tall Red | TtRr Tall Red | ttRR Dwarf Red | ttRr Dwarf Red |
| tr | TtRr Tall Red | Ttrr Tall White | ttRR Dwarf Red | 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
3/16 x 160/1 = 30
1 mark for using ‘3’ i.e. Tall and White
1 mark for working i.e. 3/16 x 160/1
1 mark for answer i.e. 30