Form Four Biology Exam - Genetics (Version 3) 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 drawings must be neat and properly labelled.

SECTION A (16 Marks)

1. Multiple Choice Questions (10 Marks)

Choose the most correct answer:

1. The basic unit of heredity is:
   a) Chromosome
   b) Nucleotide
   c) Gene
   d) RNA
2. In test cross, an individual with unknown genotype is crossed with:
   a) A homozygous dominant
   b) A heterozygous
   c) A homozygous recessive
   d) A hybrid
3. The type of RNA that carries amino acids to ribosomes is:
   a) mRNA
   b) rRNA
   c) tRNA
   d) sRNA
4. A condition where both alleles are equally expressed in the phenotype is:
   a) Mutation
   b) Codominance
   c) Incomplete dominance
   d) Dominance
5. Which of the following is a sex-linked disorder?
   a) Diabetes
   b) Colour blindness
   c) Tuberculosis
   d) Malaria
6. Gregor Mendel used which organism in his genetics experiments?
   a) Maize
   b) Rats
   c) Pea plant
   d) Fruit flies
7. A condition where an organism has two identical alleles is:
   a) Heterozygous
   b) Homozygous
   c) Genotype
   d) Phenotype
8. What is the name of the molecule responsible for storing genetic information?
   a) ATP
   b) Protein
   c) RNA
   d) DNA
9. A test cross is useful in determining:
   a) Dominant traits
   b) Recessive traits
   c) Genotype of a parent
   d) Sex of offspring
10. In incomplete dominance, the resulting offspring:
    a) Show blending of traits
    b) Exhibit dominant traits only
    c) Show codominant expression
    d) Are always recessive

2. Matching Items (6 Marks)

Match the following items on Types of Inheritance:

List A	List B
(i) Codominance	___
(ii) Incomplete dominance	___
(iii) Sex-linked trait	___
(iv) Recessive allele	___
(v) Back cross	___
(vi) Mutation	___

List B:

1. Blending of traits
2. Involves sudden gene change
3. When both alleles show up equally
4. Gene appears only in homozygous state
5. Determines parental genotype
6. Trait found on X or Y chromosome

SECTION B (54 Marks)

Answer all questions.

3.

(a) Define the following genetic terms: allele, phenotype, and genotype. (6 marks)

(b) Explain how phenotype may be affected by the environment. (3 marks)

4.

(a) A man with genotype Tt is crossed with a woman who is also Tt. Using a Punnett square, show the possible genotypes and phenotypes of their offspring. (5 marks)

(b) What is the probability of getting a child with the recessive trait? (4 marks)

5.

(a) Describe the structure of DNA. (5 marks)

(b) State two major differences between DNA and RNA. (4 marks)

6.

(a) Explain the purpose and procedure of a test cross. (5 marks)

(b) Differentiate between a back cross and a test cross. (4 marks)

7.

(a) What is a mutation? (2 marks)

(b) Give three types of gene mutations and briefly describe each. (6 marks)

(c) State one possible effect of a harmful mutation. (1 mark)

8.

(a) List two sex-linked traits and explain how they are inherited. (4 marks)

(b) Why are sex-linked conditions more common in males than females? (5 marks)

SECTION C (30 Marks)

Answer two (2) questions only.

9.

Explain the importance of genetics in crop and animal breeding, disease control, and understanding heredity. (15 marks)

10.

Describe the structure and role of DNA in genetic inheritance. (15 marks)

11.

Discuss the types and causes of mutations and explain their impact on living organisms. (15 marks)

FORM FOUR BIOLOGY EXAM – TOPIC: GENETICS (Version 3) Answers

SECTION A (16 Marks)

1. Multiple Choice Questions (10 Marks)

1. The basic unit of heredity is:
   c) Gene
   Explanation: A gene is a segment of DNA that codes for a specific trait, making it the basic unit of heredity. Chromosomes carry genes, nucleotides are DNA building blocks, and RNA is involved in gene expression.
2. In test cross, an individual with unknown genotype is crossed with:
   c) A homozygous recessive
   Explanation: A test cross uses a homozygous recessive (e.g., tt) to determine if the unknown is homozygous dominant (TT) or heterozygous (Tt).
3. The type of RNA that carries amino acids to ribosomes is:
   c) tRNA
   Explanation: tRNA (transfer RNA) transports amino acids to ribosomes during protein synthesis, unlike mRNA (carries genetic code), rRNA (ribosome component), or sRNA (not a standard RNA type).
4. A condition where both alleles are equally expressed in the phenotype is:
   b) Codominance
   Explanation: In codominance, both alleles are fully expressed (e.g., AB blood type), unlike incomplete dominance (blended traits) or simple dominance.
5. Which of the following is a sex-linked disorder?
   b) Colour blindness
   Explanation: Colour blindness is an X-linked recessive disorder, unlike diabetes (metabolic), tuberculosis (infectious), or malaria (parasitic).
6. Gregor Mendel used which organism in his genetics experiments?
   c) Pea plant
   Explanation: Mendel used pea plants for their clear traits and ease of crossbreeding, not maize, rats, or fruit flies.
7. A condition where an organism has two identical alleles is:
   b) Homozygous
   Explanation: Homozygous means two identical alleles (e.g., TT or tt), unlike heterozygous (different alleles), genotype (genetic makeup), or phenotype (observable traits).
8. What is the name of the molecule responsible for storing genetic information?
   d) DNA
   Explanation: DNA stores genetic information in the nucleus, unlike ATP (energy), proteins (functional molecules), or RNA (gene expression).
9. A test cross is useful in determining:
   c) Genotype of a parent
   Explanation: A test cross reveals whether a parent with a dominant phenotype is homozygous or heterozygous, not just traits or sex.
10. In incomplete dominance, the resulting offspring:
    a) Show blending of traits
    Explanation: Incomplete dominance results in an intermediate phenotype (e.g., pink flowers from red and white), unlike codominance or simple dominance.

2. Matching Items (6 Marks)

List A	List B
(i) Codominance	c) When both alleles show up equally
(ii) Incomplete dominance	a) Blending of traits
(iii) Sex-linked trait	f) Trait found on X or Y chromosome
(iv) Recessive allele	d) Gene appears only in homozygous state
(v) Back cross	e) Determines parental genotype
(vi) Mutation	b) Involves sudden gene change

SECTION B (54 Marks)

3.

(a) Define the following genetic terms: allele, phenotype, and genotype. (6 marks)

• Allele: Different forms of a gene controlling a trait (e.g., T or t for plant height).
• Phenotype: The observable expression of a genotype (e.g., tall plant for TT or Tt).
• Genotype: The genetic makeup of an organism for a trait (e.g., TT, Tt, or tt).

(b) Explain how phenotype may be affected by the environment. (3 marks)

Phenotype is influenced by both genotype and environment. For example, plant height (genotype) can be stunted by poor soil nutrients. Human skin color darkens with sun exposure, and malnutrition can affect growth despite genetic potential.

4.

(a) A man with genotype Tt is crossed with a woman who is also Tt. Using a Punnett square, show the possible genotypes and phenotypes of their offspring. (5 marks)

Parent genotypes: Tt (heterozygous) × Tt (heterozygous) Gametes: T, t T, t Punnett Square: | T | t ------|---------|--------- T | TT | Tt t | Tt | tt Genotypes: 25% TT, 50% Tt, 25% tt Phenotypes: 75% tall (TT, Tt), 25% dwarf (tt) Phenotypic ratio: 3 tall : 1 dwarf

(b) What is the probability of getting a child with the recessive trait? (4 marks)

The recessive trait (dwarf, tt) appears in 25% of offspring, as shown in the Punnett square. Probability = 1/4 or 25%.

5.

(a) Describe the structure of DNA. (5 marks)

DNA (deoxyribonucleic acid) is a double-stranded helix composed of nucleotides. Each nucleotide contains:

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

The strands run antiparallel (5’ to 3’ and 3’ to 5’).

(b) State two major differences between DNA and RNA. (4 marks)

• DNA is double-stranded; RNA is single-stranded.
• DNA contains thymine; RNA contains uracil.

6.

(a) Explain the purpose and procedure of a test cross. (5 marks)

Purpose: A test cross determines the genotype of an individual with a dominant phenotype (e.g., TT or Tt) by crossing it with a homozygous recessive (tt).

Procedure:

• Cross the individual with unknown genotype (e.g., tall plant, T?) with a homozygous recessive (tt).
• Observe offspring phenotypes: if all are dominant (e.g., tall), the parent is likely TT; if mixed (tall and dwarf), the parent is Tt.

(b) Differentiate between a back cross and a test cross. (4 marks)

• Back cross: Crossing an offspring with one of its parents (e.g., Tt × TT) to enhance desired traits.
• Test cross: Crossing an individual with unknown genotype with a homozygous recessive (e.g., T? × tt) to determine genotype.

7.

(a) What is a mutation? (2 marks)

A mutation is a change in the DNA sequence, altering gene function or expression.

(b) Give three types of gene mutations and briefly describe each. (6 marks)

• Point mutation: A single nucleotide change (e.g., A to G), altering one codon (e.g., sickle cell anemia).
• Insertion: Addition of nucleotides, shifting the reading frame (e.g., causing nonfunctional proteins).
• Deletion: Removal of nucleotides, disrupting the gene sequence (e.g., cystic fibrosis).

(c) State one possible effect of a harmful mutation. (1 mark)

A harmful mutation can cause a genetic disorder, such as haemophilia, impairing blood clotting.

8.

(a) List two sex-linked traits and explain how they are inherited. (4 marks)

• Colour blindness: X-linked recessive; males (XY) inherit the defective allele (X^c) from their mother, expressing the trait if X^cY.
• Haemophilia: X-linked recessive; males inherit the mutant allele (X^h) from their mother, leading to impaired clotting if X^hY.

(b) Why are sex-linked conditions more common in males than females? (5 marks)

Males (XY) have one X chromosome, so a single recessive allele on it (e.g., X^h) expresses the condition. Females (XX) need two recessive alleles (X^hX^h) to express the trait; a single normal allele (X^H) makes them carriers but unaffected.

SECTION C (30 Marks)

Answer two (2) questions only.

9. Explain the importance of genetics in crop and animal breeding, disease control, and understanding heredity. (15 marks)

Crop Breeding:

• Genetics enables selective breeding for desired traits (e.g., high-yield rice via Tt crosses).
• Hybrid vigor from heterozygous crosses improves crop resilience.
• Genetic modification (e.g., Bt corn) enhances pest resistance.

Animal Breeding:

• Predicts inheritance of traits like milk production in cows using Mendelian ratios.
• Reduces undesirable traits via selective mating (e.g., avoiding recessive disorders).

Disease Control:

• Identifies genetic markers for disorders (e.g., BRCA1 for breast cancer).
• Facilitates gene therapy to correct mutations (e.g., cystic fibrosis).

Understanding Heredity:

• Explains trait transmission via Mendel’s laws.
• Aids genetic counseling to predict disorder risks.

10. Describe the structure and role of DNA in genetic inheritance. (15 marks)

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

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

Role in Genetic Inheritance:

• Storage: DNA stores genetic information in its base sequence.
• Replication: DNA copies itself before cell division, ensuring offspring inherit parental genes.
• Expression: Through transcription (mRNA synthesis) and translation (protein synthesis), DNA determines traits like eye color.