A homozygous tall, red flower plant was crossed... | WAEC SSCE - Biology - 2019

Question 1 Report

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]

Answer

(a)(i) The cross is a dihybrid cross.

(a)(ii) It involves two pairs of contrasting characters, namely plant height (tall/dwarf) and flower colour (red/white).

(b)(i) Evolution is the gradual change in the inherited characteristics of a population over many generations, which may result in new forms or species.

(b)(ii) Genetic diagram

Let T represent tallness and t dwarfness; R represent red flowers and r white flowers. Tallness and red flower colour are dominant.

P generation: Tall, red plant (TTRR) × Dwarf, white plant (ttrr)

Gametes: TR × tr

F₁ generation: all TtRr, tall and red.

Selfing of F₁: TtRr × TtRr

Each F₁ plant produces the four types of gametes: TR, Tr, tR and tr.

Genetic diagram showing the F₂ offspring from selfing two heterozygous tall, red F₁ plants.

From the genetic diagram, the F₂ phenotypic ratio is:

9 tall red : 3 tall white : 3 dwarf red : 1 dwarf white.

(b)(iii) Tall, white plants are represented by 3 out of 16 plants.

\[\text{Number of tall, white plants}=\frac{3}{16}\times160=30\]

Therefore, 30 tall, white-flowered plants would be obtained in the F₂ generation.

Read lesson note on Chromosomes: The Basis Of Heredity (WAEC)

Answer Details