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.
- Write your answers clearly.
SECTION A (16 Marks)
1. Multiple Choice Questions (10 Marks)
Choose the correct answer and write its letter:
- The physical appearance of an organism is known as:
a) Genotype
b) Phenotype
c) Trait
d) Chromosome - Which of the following is a sex-linked disorder?
a) Albinism
b) Haemophilia
c) Sickle cell anemia
d) Down syndrome - The unit of heredity is called:
a) Chromosome
b) Allele
c) Gene
d) DNA - A cross between a red-flowered and a white-flowered snapdragon giving pink flowers is an example of:
a) Dominant inheritance
b) Recessive inheritance
c) Codominance
d) Incomplete dominance - A test cross is used to determine:
a) DNA sequence
b) Genotype of an individual
c) Number of chromosomes
d) Mode of inheritance - If a heterozygous tall plant is crossed with a homozygous dwarf plant, the expected ratio is:
a) 1 tall: 3 dwarf
b) 2 tall: 2 dwarf
c) 1 tall: 1 dwarf
d) All tall - In DNA, adenine pairs with:
a) Thymine
b) Guanine
c) Cytosine
d) Uracil - Which of the following is not a Mendelian law?
a) Law of segregation
b) Law of independent assortment
c) Law of dominance
d) Law of gene linkage - RNA differs from DNA in that:
a) RNA is double stranded
b) RNA contains thymine
c) RNA contains uracil instead of thymine
d) RNA is found in chromosomes - Which of the following is an example of a monohybrid cross?
a) Tt × Tt
b) AaBb × AaBb
c) TtAa × TtAa
d) TTAA × TTAa
2. Matching Items (6 Marks)
Match the items in List A with the correct responses from List B:
| List A | List B |
|---|---|
| I. Homozygous | ___ |
| II. Test cross | ___ |
| III. Codominance | ___ |
| IV. Alleles | ___ |
| V. Phenotype | ___ |
| VI. Hybrid | ___ |
List B:
- Physical expression of genes
- Two identical alleles
- Crossing with homozygous recessive
- Offspring from genetically different parents
- Pair of genes controlling a trait
- Both traits are equally expressed
SECTION B (54 Marks)
Answer all questions.
3.
(a) Define genetics and explain the role of Gregor Mendel in the study of genetics. (5 marks)
(b) State four reasons why Mendel used pea plants in his experiments. (4 marks)
4.
(a) Given that tall (T) is dominant over dwarf (t), use a genetic diagram to show the result of a cross between Tt and tt plants. (5 marks)
(b) Explain what is meant by heterozygous and homozygous with one example each. (4 marks)
5.
(a) Explain the structure and function of DNA. (5 marks)
(b) State four differences between DNA and RNA. (4 marks)
6.
(a) Explain three differences between Mendelian and non-Mendelian inheritance. (6 marks)
(b) Name any three non-Mendelian inheritance patterns. (3 marks)
7.
(a) A couple has a son with haemophilia. Explain how the disorder was inherited. (5 marks)
(b) Suggest two genetic disorders caused by abnormal number of chromosomes. (2 marks)
(c) Why are males more affected by sex-linked disorders than females? (2 marks)
8.
(a) Describe how variation arises during sexual reproduction. (5 marks)
(b) State two types of variation and give two examples for each. (4 marks)
SECTION C (30 Marks)
Answer two (2) questions only.
9.
Describe the principles of Mendel’s Laws of inheritance and their importance in modern genetics. (15 marks)
10.
With the aid of diagrams, describe the stages and results of a monohybrid and a dihybrid cross. (15 marks)
11.
Discuss the causes and types of genetic disorders, and explain how genetic counseling can help in preventing them. (15 marks)
FORM FOUR BIOLOGY EXAM – TOPIC: GENETICS Answers
SECTION A (16 Marks)
1. Multiple Choice Questions (10 Marks)
- The physical appearance of an organism is known as:
b) Phenotype
Explanation: Phenotype refers to observable traits (e.g., eye color), while genotype is the genetic makeup, trait is a characteristic, and chromosome is a DNA structure. - Which of the following is a sex-linked disorder?
b) Haemophilia
Explanation: Haemophilia is linked to the X chromosome, unlike albinism and sickle cell anemia (autosomal recessive) or Down syndrome (chromosomal abnormality). - The unit of heredity is called:
c) Gene
Explanation: Genes are segments of DNA carrying hereditary information, unlike chromosomes (gene carriers), alleles (gene variants), or DNA (the molecule). - A cross between a red-flowered and a white-flowered snapdragon giving pink flowers is an example of:
d) Incomplete dominance
Explanation: Incomplete dominance results in an intermediate phenotype (pink flowers), unlike codominance (both traits expressed) or dominant/recessive inheritance. - A test cross is used to determine:
b) Genotype of an individual
Explanation: A test cross with a homozygous recessive individual reveals whether an organism is homozygous dominant or heterozygous. - If a heterozygous tall plant is crossed with a homozygous dwarf plant, the expected ratio is:
c) 1 tall: 1 dwarf
Explanation: A Tt × tt cross yields 50% Tt (tall) and 50% tt (dwarf), resulting in a 1:1 phenotypic ratio. - In DNA, adenine pairs with:
a) Thymine
Explanation: In DNA, adenine pairs with thymine, and guanine pairs with cytosine. Uracil is found in RNA, not DNA. - Which of the following is not a Mendelian law?
d) Law of gene linkage
Explanation: Mendel’s laws include segregation, independent assortment, and dominance. Gene linkage is a non-Mendelian concept discovered later. - RNA differs from DNA in that:
c) RNA contains uracil instead of thymine
Explanation: RNA is single-stranded, contains uracil (not thymine), and is found outside chromosomes, unlike DNA. - Which of the following is an example of a monohybrid cross?
a) Tt × Tt
Explanation: A monohybrid cross involves one trait (e.g., Tt × Tt), while others involve multiple traits (dihybrid crosses).
2. Matching Items (6 Marks)
| List A | List B |
|---|---|
| I. Homozygous | b) Two identical alleles |
| II. Test cross | c) Crossing with homozygous recessive |
| III. Codominance | f) Both traits are equally expressed |
| IV. Alleles | e) Pair of genes controlling a trait |
| V. Phenotype | a) Physical expression of genes |
| VI. Hybrid | d) Offspring from genetically different parents |
SECTION B (54 Marks)
3.
(a) Define genetics and explain the role of Gregor Mendel in the study of genetics. (5 marks)
Definition: Genetics is the branch of biology that studies heredity and variation in organisms, focusing on how traits are passed from parents to offspring.
Mendel’s Role: Gregor Mendel, known as the father of genetics, conducted experiments on pea plants in the 19th century. He established the principles of inheritance (segregation, independent assortment, and dominance) through controlled crosses, laying the foundation for modern genetics.
(b) State four reasons why Mendel used pea plants in his experiments. (4 marks)
- Short generation time, allowing quick observation of results.
- Distinct, contrasting traits (e.g., tall vs. dwarf).
- Self- and cross-pollination possible, enabling controlled breeding.
- Produces many offspring, providing large sample sizes for analysis.
4.
(a) Given that tall (T) is dominant over dwarf (t), use a genetic diagram to show the result of a cross between Tt and tt plants. (5 marks)
(b) Explain what is meant by heterozygous and homozygous with one example each. (4 marks)
- Heterozygous: Having two different alleles for a trait; e.g., Tt (tall plant with one dominant and one recessive allele).
- Homozygous: Having two identical alleles for a trait; e.g., TT (tall plant with two dominant alleles).
5.
(a) Explain the structure and function of DNA. (5 marks)
Structure: DNA (deoxyribonucleic acid) is a double-stranded helix composed of nucleotides, each with a sugar (deoxyribose), phosphate, and one of four bases (adenine, thymine, cytosine, guanine). The strands are held by hydrogen bonds between complementary bases (A-T, C-G).
Function: DNA stores genetic information, directs protein synthesis via transcription and translation, and replicates to pass genetic material to offspring.
(b) State four differences between DNA and RNA. (4 marks)
- DNA is double-stranded; RNA is single-stranded.
- DNA contains thymine; RNA contains uracil.
- DNA has deoxyribose sugar; RNA has ribose sugar.
- DNA is primarily nuclear; RNA functions in nucleus and cytoplasm.
6.
(a) Explain three differences between Mendelian and non-Mendelian inheritance. (6 marks)
- Gene interaction: Mendelian inheritance follows simple dominant-recessive patterns; non-Mendelian involves complex interactions like codominance or polygenic traits.
- Predictability: Mendelian traits predict fixed ratios (e.g., 3:1); non-Mendelian shows variable or continuous outcomes (e.g., height).
- Gene location: Mendelian traits are typically autosomal; non-Mendelian includes sex-linked or mitochondrial inheritance.
(b) Name any three non-Mendelian inheritance patterns. (3 marks)
- Incomplete dominance
- Codominance
- Polygenic inheritance
7.
(a) A couple has a son with haemophilia. Explain how the disorder was inherited. (5 marks)
Haemophilia is an X-linked recessive disorder. The son (XY) inherited the recessive haemophilia allele (Xh) from his mother (XHXh or XhXh), as males inherit their X chromosome from their mother. The father (XHY) contributes the Y chromosome, so he does not pass the disorder but may be unaffected.
(b) Suggest two genetic disorders caused by abnormal number of chromosomes. (2 marks)
- Down syndrome (trisomy 21)
- Turner syndrome (monosomy X)
(c) Why are males more affected by sex-linked disorders than females? (2 marks)
Males (XY) have one X chromosome, so a single recessive allele on it expresses the disorder. Females (XX) need two recessive alleles, and a dominant normal allele can mask the recessive one.
8.
(a) Describe how variation arises during sexual reproduction. (5 marks)
Variation arises through:
- Crossing over: Homologous chromosomes exchange segments during meiosis, creating new gene combinations.
- Independent assortment: Random alignment of chromosomes during meiosis leads to diverse gametes.
- Random fertilization: Random fusion of gametes increases genetic diversity.
(b) State two types of variation and give two examples for each. (4 marks)
- Continuous variation: Traits with a range of phenotypes; e.g., height, skin color.
- Discontinuous variation: Traits with distinct phenotypes; e.g., blood groups, tongue rolling.
SECTION C (30 Marks)
Answer two (2) questions only.
9. Describe the principles of Mendel’s Laws of inheritance and their importance in modern genetics. (15 marks)
Mendel’s Laws:
- Law of Segregation: Each individual has two alleles for a trait, which separate during gamete formation, so each gamete carries one allele (e.g., Tt → T or t).
- Law of Independent Assortment: Genes for different traits assort independently during gamete formation, provided they are on different chromosomes (e.g., TtYy → TY, Ty, tY, ty).
- Law of Dominance: In a heterozygous individual, the dominant allele masks the recessive allele’s expression (e.g., Tt appears tall).
Importance: Mendel’s laws provide the basis for predicting inheritance patterns, understanding genetic disorders, breeding crops and animals, and developing genetic technologies like CRISPR.
10. With the aid of diagrams, describe the stages and results of a monohybrid and a dihybrid cross. (15 marks)
Monohybrid Cross:
Stages: Cross two individuals differing in one trait (e.g., Tt × Tt for height).
Dihybrid Cross:
Stages: Cross two individuals differing in two traits (e.g., TtYy × TtYy for height and seed color).
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