The question was not popular among candidates.

Many candidates could not give the correct Parental gamete horizontal (PGH) and Parental game vertical (PGV) thereby getting wrong answers in Offspring (F1) in the punnett square.

In question 4(b), many wrote 9:3:3:1 or 1:3:3:9 as the number of offspring that would have the required phenotypic characters.

Most candidates could not differentiate somatic from reproductive cells and so gave wrong number of chromosomes in question 4(c).

Many candidates loss marks in this question due to poor knowledge of genetics and poor interpretation of the question.

The expected answer is as follows:

 (a) Punnett squares of a dihybrid cross

X	Bt	Bt	Bt	Bt
bT	BbTt	BbTt	BbTt	BbTt
bT	BbTt	BbTt	BbTt	BbTt
bT	BbTt	BbTt	BbTt	BbTt
bT	BbTt	BbTt	BbTt	BbTt

         
Parental gametes horizontal (PGH): Bt      4x½   [2 marks]
Parental gametes vertical (PGV): bT         4x½   [2 marks]
Offspring (F1)                                             16x½ [8 marks]                                                                                                                                                                                                                      
(b)   Offspring with:
(i)    black fur and short tail – 0
(ii)   brown fur and long tail – 0
(iii)  black fur and long tail – 16
(iv)  brown fur and short tail – 0

(c)   Number of Chromosomes in
(i)  Pollen grain:      10
(ii) Guard cell:        20
(iii) Ovule:              10
(iv) Root cell:          20

QUESTION 5

• (i) What are sense organs?                                                                             [3 marks]

(ii) Name three sense organs that respond to the stimulus of chemicals.      [3 marks]

• List three animals each that exhibit the following courtship behaviours.
• territoriality
• pairing                                                                                                   [6 marks]
• (i) State one similarity between the eggs of toads and the eggs of birds.        [1 mark]

(ii) State three differences between the eggs of toads and the eggs of birds.  [3 marks]

• Complete the table below by listing four organisms involved in the nitrogen cycle and state one role each of the organisms.                                                                       

Four organisms involved in the nitrogen cycle	One role of each organism

                                                                                                                          [8 marks]

• Complete the table below by naming two types of heterotrophic modes of nutrition in animals and give two examples each of the organisms that carry out the modes of nutrition.

Two modes of heterotrophic nutrition	Two examples of organisms that carry out mode of nutrition

                                                                                                                                     [6 marks]
This question was attempted by many candidates.

Most candidates were able to define sense organs with examples.

Poor spellings made some candidates to lose marks for example spelling tongue as tounge, yoke instead of yolk etc.

In question 5(b)(i), many candidates could not give correct examples of animals that exhibit territoriality some instead were mentioning animals like Zebra, Elephant, Antelope etc.

Many candidates gave wrong responses in question 5(c)(iii). They wrote it is hydrated/it is not hydrated; absence of amniotic fluid/presence of amniotic fluid; cannot survive on land/can survive on land etc.

The organisms involved in nitrogen fixation and their roles were wrongly answered by many candidates.

Many candidates gave correct answers in question 5(e) Few however could not give correct examples for the mode of nutrition mentioned.

 

 

The expected answers are as follows: 

 

(a)      (i)    Sense organs
Organs with sensory receptors; that detect changes from the environment; and relay
the impulses to the nervous system; for interpretation and appropriate response sent
to the effector organ.                                                                          
(ii)   Sense organs that respond to the stimulus of chemicals

• Tongue;
• Nose;
• Skin;
• Eye.                                                                                             

          (b)   (i)    Animals that exhibit territoriality

• Agama lizard/lizard;
• Leopard/tiger;
• Jaguar;
• Lion;
• Monkey;
• Chimpanzee/baboon;
• Dog;
• Cat;
• Male Tilapia.                                                                               

                 (ii)   Animals that exhibit pairing

• Termite;
• Human;
• Gibbon;
• Domestic fowl/duck/pigeon;
• Fish;
• Spider;
• Monkey;
• Toad.                                                                                           

                       
(c)   (i)    Similarities between eggs of toads and eggs of birds
Both have yolk; embryo; vitelline membrane.                                  

 

                            
(ii)   Differences between eggs of toads and eggs of birds

Eggs of toads	Eggs of birds
Surrounded by jelly coat	surrounded by calcareous shell;
Relatively smaller	relatively larger;
Shell membrane absent	shell membrane present;
Presence of black part	no black part;
Eggs are laid in chains	eggs are laid in single piece;
Many/numerous eggs	one egg at a time;
Transparent	opaque;
Airspace absent	airspace present;
Yolk not suspended by chalaza	Yolk suspended by chalaza.

                

(d)  Organisms involved in Nitrogen Cycle

Organisms involved in the nitrogen cycle	One role of organism
Animals	Excrete nitrogenous compounds/ Eat proteins in plant/ Die/decay to form ammonium compound;
Plants	Absorbs nitrate from soil/ Die/decay to form ammonium compound;
Putrefying bacteria	Break down dead plants/animals;
Azotobacter	Converts atmospheric nitrogen to nitrate;
Rhizobium	Converts atmospheric nitrogen to nitrates;
Nitrobacter	Converts nitrite to nitrates;
Nitrosomonas	Converts ammonium compound to nitrite;
Pseudomonas	Converts amino compound to ammonium compounds.

                

(e)   Types of heterotrophic nutrition

Two modes of heterotrophic Nutrition	Two examples of organisms that carry out mode of Nutrition
Holozoic	Human; domestic fowl; dog; cat; goat; sheep; cow; snake; lion; rabbit; Amoeba; Any 2x1 [2 marks]
Parasitic	Lice; tick; tapeworm; liver fluke; Guinea worm; hookworm; Any 2x1[2 marks]
Saprophytic	Fungi; mushroom; soil bacteria; yeast; Rhizopus; Mucor. Any 2x1 [2 marks]