Biology Paper 2, WASSCE (PC 1ST)2019

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:

    1. Type of genetic cross

    2. Dihybrid


    3. Reason for answer

    4. Two characters are involved in the cross/involves two characters separable in inheritance.


    1. Phenotype of F1

    2. Tall and Red


    3. Genetic Diagram

    4. 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