Form 4:
Genetics kcse questions

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1. Differentiate between phenotype and genotype as used in genetics (1 mark)

Phenotype is outward appearance of an organism while genotype is genetic constitution of an organism.

2. State two structural differences between ribonucleic acid ( RNA) and deoxyribonucleic acid ( DNA) ( 2 marks)

3. Name a disorder of human blood that is caused by mutation ( 1 mark)

Haemophilia (sickle cell anaemia)

4. State the function of deoxyribonucleic acid (DNA) molecule ( 1 mark)

Controls / regulates enzyme / synthesis for the material for inheritance.

5. Give a reason why it is only mutation in genes of gametes that influence evolution ( 2 marks)

Gametes form new offspring

6. In an experiment, red flower were crossed with plants with white flower. All the plants in the F1 generation had pink flowers.

(a) Give a reason for the appearance of pink flower in the F1 generation ( 1 mark)

Both alleles expressed themselves equally resulting in blending of characteristics.

(b) If the plants from F1 generation were selfed, state the phenotype ratio of the F2 generation ( 2 marks)

1 red flower:2 pink flowers:1 white flower

7. State two characteristics that researchers select in breeding programmes.

( 2 marks)

- High yielding Hybrid vigour

- Resistance to disease

-early maturity

-Resistance to drought

8. Give an example of sex- linked trait in humans on Y chromosome( 2 marks)

Hairy pinna, tuft and hair sprouting from the pinna, baldness.

X chromosome

Colour blindness; Haemophilia

9. In an experiment, a variety of garden peas having a smooth seed coat was crossed with a variety with a wrinkled seed coat. All the seeds obtained in the F1 had a smooth seed coat. The F1 generation was selfed. The total number of F2 generation was 7324.

(a) Using appropriate letter symbols, work out the genotype of the F1 generation. ( 4 marks)

Smooth seed coat is dominant to wrinkled seed coat. Let R represent gene for smooth and parental genotype RR x rr

(b) From the information above, work out the following for the F2 generation

(i) Genotype ratio ( 2 marks)

1RR : 2Rr: 1rr

(ii) Phenotype ratio ( 1 mark)

3 smooth: 1 wrinkled

(iii) Wrinkled number ( 1 mark)

¼ x 7324 = 1831

10. In a certain plant species, some individual plant may have white, red or pink flower.  All the plants from F1 generation were pink. Using letter R to represent the gene for red colour and letter W for white colour;

(a) Work out the genotype of F1 generation ( 3 marks)

(b) If the plants from F1 generation were selfed, what would be the phenotypic ratio of the F2 generation? ( 3 marks)

1:2:1 for ratio, 1 white: 2 pink: 1 red

(c) What is the genetic explanation for the absence of plants with red and white in the flower F1 generation? ( 2 marks)

Co-dominance / incomplete dominance / partial dominance / equal dominance.

11. In a breeding experiment plants with red flower were crossed. They produced 123 plants with red flowers and 41 with white flowers.

(a) Identify the recessive character. Give a reason


- Fewer number / lower rates / absence of white in parent and its presence in offspring.

(b) What were the genotypes of the parent plants that give rise to the plants with red and white flowers?

Heterozygous/ Rr

(c) If the white flowers were selfed, what would be the genotypes of their offspring?

Homozygous / rr / double recessive.

12. (a) Name two disorders in humans caused by gene mutation

( 2 marks)

i) Haemophilia

ii) Sickle cell anaemia

iii) Colour blindness

iv) Leukemia

v) Albinism

(b) Describe the following chromosomal mutations

(i) Inversion ( 2 marks)

A result of a chromosomal break up and rejoining with the middle piece turned by 1800

(ii) Translocation

A section of chromosome breaking and joining a homologous chromosome.

(c) In mice the allele for black fur is dominant to the allele for brown fur. What percentage offspring would have brown fur from a cross between heterozygous black mice and brown mice? Show your working. Use letter B to represent the allele for black colour. ( 4 marks)

13. (a) What is meant by the term allele? ( 1 mark)

It is alternative form of a chromosome been similar in structure but may have different composition.

(b) Explain how the following occur during gene mutation

(i) Deletion ( 1 mark)

Occurs when the nucleotides of a gene break off and disappear

(ii) Inversion ( 1 mark)

Occurs when the nucleotides of a part of a gene become inverted by taking a 1800 turn.

(c) What is a test- cross? ( 1 mark)

Testing the genotype of an individual by crossing with the recessive trait.

14. In maize the gene for purple colour is dominant to the gene for white colour. A pure breeding maize plant with purple grains was crossed with a heterozygous plant.

(a) (i) Using letter G to represent the gene for purple colour, work out the genotypic ratio of the offspring ( 5 marks)

1st filial generation

(Off springs)

- The genotype ratio:

- 2 homozygous purple coloured grains

- 2 heterozygous purple coloured grains

(ii) State the phenotype of the offspring ( 1 mark)

All purple coloured grained maize plants maize plants.

(b) What is genetic engineering? ( 1 mark)

-Deliberate modification of characteristics of an organism by manipulation of genes and DNA by transferring genes from one organism to another.

-It is when best characteristics are developed from both parents and offspring better than either parent.

15. Define the following terms as used in genetics.

(i) Alleles -Alleles are alternative forms of the same gene which control the inheritance of contrasting features of the same position in homologous chromosomes.

(ii) Genotype -Genotype is the genetic makeup or composition of an organism.

(iii) Phenotype-Phenotype is the outward appearance of an organism with reference to a particular trait.

16. A farmer mated his dark red cow with a white bull. The cow gave birth to a light red calf

(a) State why the calf is light red and not dark red or white

The genes for dark red colour and white colour are co-dominant. Since the calf  is heterozygous it gets a coat colour that is intermediate between dark red and white

(b) If a light red bull is mated with a dark red cow, work out using appropriate letter symbols the probability of getting a light offspring

Let R represent the gene for dark red coat colour and W the gene for white coat colour.

The light red bull must be heterozygous (RW) and the dark red cow must be homozygous (RR)

Parental phenotypes Light red x dark red

Offspring phenotypes: Dark Red Light red

17. (a) What is meant by linked genes?

Linked genes are those genes that are found in the same chromosomes. They are usually inherited together.

(b) (i) In fruit flies (Drosophila) the gene for red eyes ® is dominant over  the one for white – eye (r). If a true breeding white – eyed male, all the offspring will be red eyed. However, if a true – breeding white- eyed female is mated with a true- breeding red- eyed male, all the female offspring will be red – eyed. Explain this apparent contradiction.

These observation show that eye colour in fruit flies is a sex-linked trait. Since it is well known that the Y chromosomes carries very few genes, we can assume that eye colour in fruit flies is an x-linked trait.

- When a true-breeding, red-eyed female is mated with a white-eyed male, all the  offspring received an X-chromosome carrying the dominant gene from the mother. Because of this, they all develop red eyed. This is illustrated below.

When a true breeding white eyed female is mated with a red-eyed male, the female offspring receive an X-chromosome carrying the dominant gene from the father and develop red eyed. The male offspring received an X-chromosome carrying the recessive gene from the mother. They also receive a Y chromosome contains no genes for eye colour the male offspring develop white eyes.

(ii) Work out the ratio of the expected phenotypes if a red- eyed female offspring from the cross- described in (i) above is mated with red- eyed males.


18. (a) Explain the term variation with reference to the study of genetics.

Variation refers to the difference in specific characteristics that exist between  members of a species e.g. in humans, characteristics that exist between members of a species e.g. in human, characteristics such as height, blood group.

(b) Using relevant examples distinguish between discontinuous variation and continuous variation

-Discontinuous variation refers to the existence of two or more distinct forms between them e.g. pink or white flowers in pea plants

- Continuous variation refers to the existence of a characteristic in a continuous gradation between two extremes e.g. height, weight and fruit size in trees.

(c) What is the importance of genetic variation?

Genetic variation provides the raw materials for evolution by natural selection. It increases the chances of survival in an ever-changing environment. If a particular species is highly adapted to a specific habitat, it might find it difficult to survive in case there is a sudden change in the environmental conditions. In absence of variation, such a change may lead to death of all members.With a variation that adapts them to the new conditions they will survive. They will reproduce and multiply rapidly in the absence of competition from other forms. Since the variation is genetic, the adaptation is passed on to subsequent generations.

(d) Describe one example where genetic variations has helped a species to survive

When DDT was first used in the 1950’s it was very effective in killing mosquitoes. In areas where mosquito populations were previously large, their numbers were greatly reduced. However, in the 1960’s the number of mosquitoes in these areas began to rise again despite the continued application of DDT.

- It then became evident that a few mutant forms had a variation that made them resistant to DDT. In the absence of competition from other forms, the resistant forms reproduced and multiplied very fast. The result was that the number of mosquitoes started to arise again despite the continued application of DDT.

Thus the presence of a variation in the few mutant forms saved mosquitoes from extinction.

19. The diagram below shows the base sequence of part of a nucleic acid stand. Observe it and answer the questions that follow


(a) What do the letters G, T , C and A represent?

They represent the bases guamine, thymine, cytosine, adenine

(b) Show the complementary DNA strand


(c) Show the complimentary RNA strand


20. In human couples the sex of a baby is determined by the man. Explain this statement.

Human females produce one type of egg containing an X chromosome. Males produce two types of sperms; half contains a X-chromosome and the other half contains a Y chromosome.

Fertilization of the egg by a sperm carrying an X chromosome gives rise to a baby girl.

Fertilization of the egg by a sperm carrying a Y-chromosome gives rise to a baby boy.