Form Four Biology Exam - Genetics (Version 2) With Marking Scheme

FORM FOUR BIOLOGY EXAM – TOPIC: GENETICS

Time: 3 Hours

Instructions:

• Answer all questions in Sections A and B.
• Answer two (2) questions only from Section C.
• All diagrams must be neat and well-labelled.

SECTION A (16 Marks)

1. Multiple Choice Questions (10 Marks)

Choose the correct answer and write its letter:

1. What is the main function of DNA?
   a) Transport oxygen
   b) Provide energy
   c) Store genetic information
   d) Break down proteins
2. Mendel’s Law of Independent Assortment refers to:
   a) Genes for different traits segregate independently
   b) Dominant traits always mask recessive
   c) Only one gene affects a trait
   d) Gametes are produced randomly
3. In Mendelian inheritance, the recessive trait is expressed only when:
   a) One dominant allele is present
   b) Two dominant alleles are present
   c) Two recessive alleles are present
   d) The trait is codominant
4. Which term refers to different forms of a gene?
   a) Traits
   b) Chromatids
   c) Alleles
   d) Codons
5. A cross that studies two traits at the same time is called:
   a) Monohybrid
   b) Test cross
   c) Back cross
   d) Dihybrid
6. If black fur (B) is dominant over white fur (b), what will be the phenotype of Bb?
   a) White
   b) Grey
   c) Black
   d) Both black and white
7. A change in the DNA sequence is called:
   a) Evolution
   b) Mutation
   c) Translation
   d) Inheritance
8. What type of inheritance is shown when a red and white flower produce a flower with both red and white patches?
   a) Incomplete dominance
   b) Codominance
   c) Dominance
   d) Mutation
9. The sex of a human baby is determined by:
   a) Ova
   b) X chromosome only
   c) Sperm
   d) Environment
10. What is the genetic material found in the nucleus?
    a) mRNA
    b) Ribosome
    c) DNA
    d) ATP

2. Matching Items (6 Marks)

Match the items in List A with responses in List B (from Mendelian Laws):

List A	List B
I. Law of Segregation	___
II. Law of Dominance	___
III. Hybrid	___
IV. Homozygous	___
V. Recessive gene	___
VI. Mendel	___

List B:

1. Masked by dominant allele
2. First scientist to study inheritance
3. Two similar alleles
4. Two alleles separate during gamete formation
5. Expressed over recessive gene
6. Product of two different alleles

SECTION B (54 Marks)

Answer all questions.

3.

(a) Define the terms heredity and variation. (4 marks)

(b) Explain how identical twins show similarity and differences. (5 marks)

4.

(a) Given that brown eyes (B) are dominant over blue eyes (b), use a Punnett square to show the offspring from a cross between Bb and bb. (5 marks)

(b) Distinguish between genotype and phenotype with one example each. (4 marks)

5.

(a) List three components of a nucleotide and explain their functions. (5 marks)

(b) State two roles of RNA in protein synthesis. (4 marks)

6.

Explain the contribution of genetic engineering in industry and agriculture. (9 marks)

7.

(a) Explain how mutation can lead to genetic disorders. (4 marks)

(b) State three examples of genetic disorders and their causes. (5 marks)

8.

(a) Explain what is meant by sex-linked inheritance and give two examples. (5 marks)

(b) Suggest two reasons why genetic counselling is important. (4 marks)

SECTION C (30 Marks)

Answer two (2) questions only.

9.

Discuss the importance of understanding Mendelian inheritance in agriculture and medicine. (15 marks)

10.

Describe the structure of a DNA molecule and explain how it is involved in protein synthesis. (15 marks)

11.

Explain the types of variation, their causes, and their significance in the process of evolution. (15 marks)

FORM FOUR BIOLOGY EXAM – TOPIC: GENETICS (Version 2) Answers

SECTION A (16 Marks)

1. Multiple Choice Questions (10 Marks)

1. What is the main function of DNA?
   c) Store genetic information
   Explanation: DNA stores and transmits genetic information for traits and functions, unlike oxygen transport (hemoglobin), energy provision (ATP), or protein breakdown (enzymes).
2. Mendel’s Law of Independent Assortment refers to:
   a) Genes for different traits segregate independently
   Explanation: This law states that genes for different traits assort independently during gamete formation, provided they are on different chromosomes, not related to dominance or single-gene effects.
3. In Mendelian inheritance, the recessive trait is expressed only when:
   c) Two recessive alleles are present
   Explanation: A recessive trait requires two recessive alleles (e.g., bb) to be expressed, as a dominant allele (B) masks it in heterozygous (Bb) or homozygous dominant (BB) conditions.
4. Which term refers to different forms of a gene?
   c) Alleles
   Explanation: Alleles are alternative forms of a gene (e.g., B or b for fur color), unlike traits (observable characteristics), chromatids (chromosome halves), or codons (mRNA sequences).
5. A cross that studies two traits at the same time is called:
   d) Dihybrid
   Explanation: A dihybrid cross examines two traits simultaneously (e.g., seed color and shape), unlike a monohybrid (one trait), test cross (genotype test), or back cross (with a parent).
6. If black fur (B) is dominant over white fur (b), what will be the phenotype of Bb?
   c) Black
   Explanation: Bb is heterozygous, and since B (black) is dominant, the phenotype is black, not white, grey, or a mix.
7. A change in the DNA sequence is called:
   b) Mutation
   Explanation: A mutation is an alteration in DNA sequence, unlike evolution (population change), translation (protein synthesis), or inheritance (trait transmission).
8. What type of inheritance is shown when a red and white flower produce a flower with both red and white patches?
   b) Codominance
   Explanation: Codominance occurs when both alleles are expressed equally (e.g., red and white patches), unlike incomplete dominance (blended phenotype) or simple dominance.
9. The sex of a human baby is determined by:
   c) Sperm
   Explanation: Sperm carries either an X or Y chromosome, determining sex (XX female, XY male), while ova always carry X, and environment has no role.
10. What is the genetic material found in the nucleus?
    c) DNA
    Explanation: DNA is the primary genetic material in the nucleus, unlike mRNA (messenger), ribosomes (protein synthesis sites), or ATP (energy molecule).

2. Matching Items (6 Marks)

List A	List B
I. Law of Segregation	d) Two alleles separate during gamete formation
II. Law of Dominance	e) Expressed over recessive gene
III. Hybrid	f) Product of two different alleles
IV. Homozygous	c) Two similar alleles
V. Recessive gene	a) Masked by dominant allele
VI. Mendel	b) First scientist to study inheritance

SECTION B (54 Marks)

3.

(a) Define the terms heredity and variation. (4 marks)

• Heredity: The process by which traits are passed from parents to offspring through genes, ensuring continuity of characteristics.
• Variation: Differences in traits among individuals of the same species, resulting from genetic and environmental factors.

(b) Explain how identical twins show similarity and differences. (5 marks)

Identical twins arise from a single fertilized egg splitting into two embryos, sharing nearly identical DNA, leading to similarities in appearance, blood type, and genetic traits. Differences arise due to:

• Environmental factors: Diet, lifestyle, or exposure to different conditions can affect traits like weight or health.
• Epigenetic changes: Modifications in gene expression (e.g., methylation) can cause variations in traits like behavior.
• Minor mutations: Rare DNA changes during cell division may lead to slight differences.

4.

(a) Given that brown eyes (B) are dominant over blue eyes (b), use a Punnett square to show the offspring from a cross between Bb and bb. (5 marks)

Parent genotypes: Bb (heterozygous brown) × bb (homozygous blue) Gametes: B, b b, b Punnett Square: | b | b ------|---------|--------- B | Bb | Bb b | bb | bb Results: 50% Bb (brown eyes), 50% bb (blue eyes) Phenotypic ratio: 1 brown : 1 blue

(b) Distinguish between genotype and phenotype with one example each. (4 marks)

• Genotype: The genetic makeup of an organism for a trait; e.g., Bb (heterozygous for eye color).
• Phenotype: The observable expression of a trait; e.g., brown eyes (resulting from Bb or BB).

5.

(a) List three components of a nucleotide and explain their functions. (5 marks)

• Sugar (deoxyribose in DNA, ribose in RNA): Forms the backbone, providing structural support.
• Phosphate group: Links nucleotides via phosphodiester bonds, stabilizing the nucleic acid chain.
• Nitrogenous base (A, T, C, G in DNA; A, U, C, G in RNA): Encodes genetic information through base pairing.

(b) State two roles of RNA in protein synthesis. (4 marks)

• mRNA (messenger RNA): Carries genetic code from DNA to ribosomes for translation.
• tRNA (transfer RNA): Transfers specific amino acids to the ribosome to build the protein chain.

6.

Explain the contribution of genetic engineering in industry and agriculture. (9 marks)

Industry:

• Pharmaceuticals: Genetically modified bacteria (e.g., E. coli) produce insulin for diabetes treatment, improving yield and affordability.
• Enzymes: Engineered microbes produce enzymes for detergents, food processing, and biofuels, enhancing efficiency.
• Bioremediation: Modified organisms degrade pollutants (e.g., oil spills), aiding environmental cleanup.

Agriculture:

• Crop improvement: Genetically modified crops (e.g., Bt maize) resist pests, reducing pesticide use and increasing yield.
• Disease resistance: Engineered plants (e.g., virus-resistant papaya) withstand diseases, ensuring food security.
• Nutritional enhancement: Golden rice, modified to produce vitamin A, combats malnutrition in developing regions.

7.

(a) Explain how mutation can lead to genetic disorders. (4 marks)

Mutations are changes in DNA sequences that can disrupt gene function. If a mutation occurs in a critical gene, it may produce defective proteins, leading to disorders. For example, a point mutation in the hemoglobin gene causes sickle cell anemia, altering red blood cell shape and function.

(b) State three examples of genetic disorders and their causes. (5 marks)

• Sickle cell anemia: Caused by a point mutation in the HBB gene, leading to abnormal hemoglobin.
• Down syndrome: Caused by trisomy 21 (extra chromosome 21), affecting physical and mental development.
• Haemophilia: Caused by mutations in X-linked genes (e.g., F8), impairing blood clotting.

8.

(a) Explain what is meant by sex-linked inheritance and give two examples. (5 marks)

Sex-linked inheritance involves genes on sex chromosomes (usually X), affecting males and females differently due to their chromosome composition (XX vs. XY). Males are more affected as they have one X chromosome, lacking a second to mask recessive alleles.

Examples:

• Haemophilia: X-linked recessive disorder causing impaired blood clotting.
• Color blindness: X-linked recessive disorder affecting color vision.

(b) Suggest two reasons why genetic counselling is important. (4 marks)

• Risk assessment: Helps couples understand the likelihood of passing genetic disorders (e.g., sickle cell anemia) to offspring.
• Informed decisions: Guides family planning, including options like prenatal testing or adoption, to prevent severe disorders.

SECTION C (30 Marks)

Answer two (2) questions only.

9. Discuss the importance of understanding Mendelian inheritance in agriculture and medicine. (15 marks)

Agriculture:

• Selective breeding: Mendelian principles guide breeding for desirable traits (e.g., high-yield wheat, Tt × Tt for tall plants), predicting offspring ratios.
• Hybrid vigor: Crossing heterozygous plants (hybrids) enhances traits like drought resistance, using Mendel’s segregation law.
• Pest resistance: Understanding dominance helps develop crops with dominant resistance alleles, reducing pesticide use.

Medicine:

• Genetic disorders: Mendel’s laws predict inheritance patterns of disorders (e.g., cystic fibrosis, recessive), aiding diagnosis.
• Carrier identification: Test crosses identify heterozygous carriers of recessive disorders, informing genetic counseling.
• Personalized medicine: Understanding inheritance patterns supports tailored treatments based on genetic profiles.

10. Describe the structure of a DNA molecule and explain how it is involved in protein synthesis. (15 marks)

Structure: DNA is a double-stranded helix composed of nucleotides, each with:

• Deoxyribose sugar, forming the backbone.
• Phosphate group, linking sugars via phosphodiester bonds.
• Nitrogenous base (adenine, thymine, cytosine, guanine), pairing A-T and C-G via hydrogen bonds.

Protein Synthesis:

• Transcription: In the nucleus, DNA unwinds, and RNA polymerase synthesizes mRNA using one DNA strand as a template, replacing thymine with uracil.
• Translation: In the cytoplasm, mRNA binds to ribosomes. tRNA brings amino acids matching mRNA codons, forming a polypeptide chain based on the DNA code.

DNA’s sequence determines the amino acid order, thus protein structure and function.