Chapter 2 — Classical Genetics Exercise Solutions
Genetics problems require setting up Punnett squares systematically. Always write the parental genotypes, gametes, then the cross.
How to Use This Page
1. Begin with the Key Concepts table for rapid review of all important terms and definitions. 2. Solve MCQs and Fill in the Blanks to check factual recall and terminology. 3. Work through True or False to identify and correct common misconceptions. 4. Practise Match the Following to reinforce term-definition associations. 5. Write Brief Answers for 2-mark questions — keep them concise (3-5 sentences). 6. Study Detailed Answers for 5-mark questions — use sub-points and diagrams where applicable. 7. Review Common Mistakes before exams to avoid repeat errors. 8. Use the Exam Checklist to ensure you have covered every topic.
Key Concepts (Quick Recap)
| Term | Definition |
|---|---|
| Gene | Unit of heredity; a segment of DNA that codes for a protein |
| Allele | Alternative form of a gene occupying the same locus on homologous chromosomes |
| Dominant allele | Allele that masks the expression of the recessive allele in heterozygote |
| Recessive allele | Allele whose expression is masked in the presence of a dominant allele |
| Homozygous | Two identical alleles for a trait (e.g., TT or tt) |
| Heterozygous | Two different alleles for a trait (e.g., Tt) |
| Genotype | Genetic constitution of an organism (e.g., TT, Tt, tt) |
| Phenotype | Observable expression of genotype (e.g., tall, dwarf) |
| Monohybrid cross | Cross between individuals differing in one trait |
| Dihybrid cross | Cross between individuals differing in two traits |
| Incomplete dominance | Heterozygote shows intermediate phenotype (e.g., pink flowers) |
| Codominance | Both alleles expressed equally in heterozygote (e.g., AB blood group) |
| Multiple alleles | More than two allele forms exist in a population (e.g., ABO blood group) |
| Pleiotropy | Single gene affects multiple phenotypic characters (e.g., PKU) |
| Polygenic inheritance | Multiple genes control one phenotypic character (e.g., skin colour) |
I. Choose the Best Answer (MCQs)
1. Mendel's Law of Segregation is also known as: (a) Law of Dominance (b) Law of Purity of Gametes (c) Law of Independent Assortment (d) Law of Inheritance
2. In a monohybrid cross between TT and tt, the F1 generation will be: (a) All TT (b) All tt (c) All Tt (tall) (d) 1 TT : 2 Tt : 1 tt
3. The phenotypic ratio in an incomplete dominance cross is: (a) 3:1 (b) 9:3:3:1 (c) 1:2:1 (d) 2:1
4. In ABO blood grouping, a person with blood group AB has the genotype: (a) IᴬIᴬ (b) IᴮIᴮ (c) ii (d) IᴬIᴮ
5. A test cross is used to determine: (a) Whether a trait is dominant (b) Whether an individual showing dominant phenotype is homozygous or heterozygous (c) The sex of the offspring (d) Whether a trait is sex-linked
6. The dihybrid phenotypic ratio is: (a) 1:2:1 (b) 3:1 (c) 9:3:3:1 (d) 1:1:1:1
7. Pleiotropy is illustrated by: (a) Skin colour (b) Phenylketonuria (PKU) (c) ABO blood group (d) Mirabilis jalapa flower colour
8. In codominance, the heterozygous phenotype is: (a) Intermediate between two parents (b) Both parental phenotypes expressed simultaneously (c) Same as the dominant parent (d) Same as the recessive parent
9. Polygenic inheritance is seen in: (a) Pea flower colour (b) ABO blood group (c) Sickle cell anaemia (d) Human skin colour
10. When genes are located on the same chromosome, they show: (a) Independent assortment (b) Linkage (c) Incomplete dominance (d) Codominance
11. Recombination frequency between two linked genes is 20%. The map distance between them is: (a) 10 centimorgans (b) 20 centimorgans (c) 40 centimorgans (d) 5 centimorgans
12. In Mirabilis jalapa, when red-flowered plants are crossed with white-flowered plants, the F1 plants produce: (a) Red flowers (b) White flowers (c) Pink flowers (d) Red and white flowers
II. Fill in the Blanks
1. Mendel selected pea plant (Pisum sativum) for his experiments. 2. The phenotypic ratio of a monohybrid cross in F2 generation is 3:1. 3. In incomplete dominance, the F2 phenotypic ratio is 1:2:1. 4. The genotype of a person with blood group O is ii. 5. A test cross involves crossing an individual with a homozygous recessive individual. 6. When two alleles are expressed equally, it is called codominance. 7. The recombination frequency is directly proportional to the distance between genes on a chromosome. 8. Polygenic inheritance shows a bell curve (continuous variation) in the population. 9. In a dihybrid cross, the number of phenotypes in F2 is 4. 10. The law that explains 9:3:3:1 ratio is the Law of Independent Assortment.
III. True or False (with corrections for false ones)
IV. Match the Following
| Column A | Column B |
|---|---|
| 1. Law of Segregation | e. Purity of gametes |
| 2. Incomplete dominance | a. Mirabilis jalapa (1:2:1) |
| 3. Codominance | d. ABO blood group (IᴬIᴮ) |
| 4. Pleiotropy | f. One gene, multiple characters |
| 5. Polygenic inheritance | b. Skin colour, height |
| 6. Linkage | c. Genes on same chromosome |
| 7. Test cross | g. Cross with homozygous recessive |
| 8. Dihybrid cross ratio | h. 9:3:3:1 |
V. Answer Briefly (2-mark)
1. State Mendel's Law of Dominance. The Law of Dominance states that when two homozygous individuals with contrasting traits are crossed, only the dominant trait appears in the F1 generation. The recessive trait is masked but not lost. For example, when tall (TT) pea plants are crossed with dwarf (tt) plants, all F1 plants are tall (Tt) because the tall allele is dominant over the dwarf allele.
2. What is a test cross? Why is it important? A test cross is a cross between an individual showing a dominant phenotype and a homozygous recessive individual. It is used to determine whether the dominant individual is homozygous (TT) or heterozygous (Tt). If all offspring show the dominant trait, the parent is homozygous. If half the offspring show the dominant trait and half show the recessive trait (1:1 ratio), the parent is heterozygous.
3. Differentiate between incomplete dominance and codominance. In incomplete dominance, the heterozygote shows an intermediate phenotype between the two parents — e.g., red × white flowers produce pink flowers in Mirabilis jalapa. In codominance, both alleles are expressed equally and simultaneously in the heterozygote — e.g., IᴬIᴮ genotype produces both A and B antigens, resulting in AB blood group. The key difference is that incomplete dominance produces a blended trait while codominance shows both parental traits together.
4. What are multiple alleles? Give an example. Multiple alleles refer to the existence of more than two allele forms for a gene in a population. Although many alleles may exist in the population, each individual carries only two alleles (one on each chromosome). The ABO blood group system is a classic example: the gene has three alleles — Iᴬ, Iᴮ, and i. Iᴬ and Iᴮ are codominant, while i is recessive to both. This produces four blood groups: A, B, AB, and O.
5. Define linkage. How does it affect independent assortment? Linkage is the tendency of genes located on the same chromosome to be inherited together during meiosis. Linked genes do not follow Mendel's Law of Independent Assortment because they do not segregate independently. However, crossing over during prophase I of meiosis can separate linked genes, producing recombinant offspring. The closer two genes are on a chromosome, the stronger the linkage and the lower the recombination frequency.
6. What is pleiotropy? Give an example. Pleiotropy occurs when a single gene influences multiple phenotypic traits. The gene has a wide-ranging effect because the protein it produces is involved in multiple pathways. A classic example is phenylketonuria (PKU) — a mutation in the PAH gene leads to multiple effects including intellectual disability, skin pigmentation changes, and musty body odour. Another example is Marfan syndrome, where a fibrillin gene mutation affects the skeleton, eyes, and cardiovascular system.
VI. Answer in Detail (5-mark)
1. Explain Mendel's monohybrid cross with a Punnett square. State the laws it demonstrates. A monohybrid cross involves individuals differing in one trait:
- Parental generation: TT (tall) × tt (dwarf)
- F1 generation: All Tt (tall) — demonstrates the Law of Dominance
- F1 × F1: Tt × Tt
- Gametes: T, t from each parent
- Punnett square:
- F2 genotype ratio: 1 TT : 2 Tt : 1 tt
- F2 phenotype ratio: 3 Tall : 1 Dwarf
- The reappearance of the dwarf trait in F2 demonstrates the Law of Segregation (Law of Purity of Gametes) — the two alleles separate during gamete formation, so each gamete carries only one allele.
2. Explain a dihybrid cross and the Law of Independent Assortment. A dihybrid cross involves two traits simultaneously:
- Parents: YYRR (yellow, round) × yyrr (green, wrinkled)
- F1: All YyRr (yellow, round) — yellow and round are dominant
- F1 × F1: YyRr × YyRr
- Gametes: Each parent produces 4 types: YR, Yr, yR, yr
- F2 phenotype ratio: 9 Yellow Round : 3 Yellow Wrinkled : 3 Green Round : 1 Green Wrinkled
- Law of Independent Assortment: Alleles of different genes assort independently during gamete formation. The inheritance of seed colour does not affect the inheritance of seed shape.
- Condition: This law applies only when genes are on different chromosomes or are far apart on the same chromosome (not linked).
3. Explain incomplete dominance with a suitable example. Draw a Punnett square. Incomplete dominance is a non-Mendelian inheritance pattern where neither allele is completely dominant:
- Example: Flower colour in Mirabilis jalapa (Four O'Clock plant)
- Cross: RR (red) × rr (white)
- F1: All Rr (pink) — intermediate phenotype, NOT red
- F1 × F1: Rr × Rr
- Gametes: R, r from each parent
- Punnett square:
- F2 phenotype ratio: 1 Red : 2 Pink : 1 White (1:2:1)
- F2 genotype ratio: 1 RR : 2 Rr : 1 rr (also 1:2:1)
- Note: In incomplete dominance, the phenotype ratio equals the genotype ratio because the heterozygote has a distinct phenotype.
- This violates Mendel's Law of Dominance because the dominant allele does not completely mask the recessive allele.
4. Explain linkage and crossing over. How is recombination frequency used to construct linkage maps? Linkage and crossing over are fundamental to understanding inheritance beyond Mendel's laws:
- Linkage: Genes located on the same chromosome tend to be inherited together. They do not assort independently. Morgan demonstrated this with Drosophila — body colour (b⁺/b) and wing size (vg⁺/vg) were inherited together more often than expected.
- Crossing over: During prophase I of meiosis, non-sister chromatids exchange segments, producing recombinant gametes. This is the mechanism that breaks linkage.
- Recombination frequency (RF): RF = (Number of recombinant offspring / Total offspring) × 100%. RF is directly proportional to the distance between genes on a chromosome.
- Linkage maps: Genes are mapped based on recombination frequency. 1% recombination = 1 map unit = 1 centimorgan (cM). For example, if RF between genes A and B is 10%, they are 10 cM apart.
- Types of linkage: Complete linkage (RF = 0%, no crossing over) and incomplete linkage (RF between 0-50%, some crossing over occurs).
- Significance: Linkage maps help determine gene order and relative distances on chromosomes, which is essential for genome mapping.
Mendel's Laws — Summary
| Law | Statement | Ratio |
|---|---|---|
| Law of Dominance | In a heterozygote, one allele (dominant) masks the other (recessive) | 3:1 (F2 monohybrid) |
| Law of Segregation | Alleles separate during gamete formation | 3:1 |
| Law of Independent Assortment | Genes on different chromosomes assort independently | 9:3:3:1 (dihybrid) |
Monohybrid Cross — Steps
1. Parents: TT (tall) × tt (dwarf) 2. F1: All Tt (tall) 3. F1 × F1: Tt × Tt 4. F2 gametes: T, t × T, t 5. F2 genotype ratio: 1 TT : 2 Tt : 1 tt 6. F2 phenotype ratio: 3 Tall : 1 Dwarf
Dihybrid Cross — F2 Ratio
YYRR × yyrr → F1 YyRr
F2 phenotype: 9 Yellow Round : 3 Yellow Wrinkled : 3 Green Round : 1 Green Wrinkled
Non-Mendelian Inheritance Patterns
| Pattern | Key Feature | Example | F2 Ratio |
|---|---|---|---|
| Incomplete dominance | Intermediate phenotype | Mirabilis jalapa | 1:2:1 |
| Codominance | Both alleles expressed equally | ABO blood group (IA IB) | — |
| Multiple alleles | More than two alleles in population | ABO blood group | — |
| Pleiotropy | One gene → multiple characters | PKU, Marfan syndrome | — |
| Polygenic inheritance | Multiple genes → one character | Skin colour, height | bell curve |
Linkage — Key Points
- Genes on the same chromosome tend to be inherited together — called linked genes.
- Morgan's experiments: Drosophila — body colour (b) and wing size (vg) linked.
- Crossing over breaks linkage and creates recombinants.
- Recombination frequency = (recombinants / total offspring) × 100%
Problem-Solving Pattern
1. Identify dominance type (complete / incomplete / codominance). 2. Write parental genotypes. 3. Determine gametes. 4. Complete Punnett square. 5. State phenotype and genotype ratios.
Worked Examples
Example 1 — Monohybrid cross (Tt × Tt)
Example 2 — Test cross
A tall pea plant is crossed with a short plant (tt). The offspring are 50% tall, 50% short. What was the tall plant's genotype?
Example 3 — Dihybrid ratio
Common Mistakes to Avoid
1. ❌ Writing gametes incorrectly for a dihybrid cross (only 2 types instead of 4) ✅ For RrYy, all 4 gametes must be written: RY, Ry, rY, ry
2. ❌ Confusing genotype ratio with phenotype ratio ✅ Monohybrid F2 genotype ratio is 1:2:1; phenotype ratio is 3:1
3. ❌ Stating incomplete dominance and codominance are the same ✅ Incomplete dominance = intermediate phenotype (pink from red + white); Codominance = both expressed (AB blood group shows both A and B antigens)
4. ❌ Forgetting that test cross uses a homozygous recessive parent ✅ Test cross = unknown genotype × homozygous recessive (e.g., T_ × tt)
5. ❌ Saying 9:3:3:1 is the genotype ratio ✅ 9:3:3:1 is the phenotype ratio of a dihybrid cross; the genotype ratio has 9 classes
6. ❌ Confusing multiple alleles with polygenic inheritance ✅ Multiple alleles = many alleles of ONE gene (ABO blood group); Polygenic = many genes for ONE trait (skin colour)
7. ❌ Stating linked genes assort independently ✅ Linked genes do NOT assort independently; only unlinked genes on different chromosomes do
8. ❌ Writing that pleiotropy and polygenic inheritance are the same ✅ Pleiotropy = one gene affects many traits; Polygenic = many genes affect one trait
Exam Checklist
- [ ] Can you state and explain Mendel's three laws with examples?
- [ ] Can you set up and solve a monohybrid cross with a Punnett square?
- [ ] Can you set up and solve a dihybrid cross and derive the 9:3:3:1 ratio?
- [ ] Can you differentiate between genotype and phenotype?
- [ ] Can you explain incomplete dominance with Mirabilis jalapa and draw the cross?
- [ ] Can you explain codominance with ABO blood group inheritance?
- [ ] Can you define multiple alleles and give the ABO blood group genotypes for all four blood groups?
- [ ] Can you explain pleiotropy and polygenic inheritance with examples?
- [ ] Can you define linkage and crossing over and calculate recombination frequency?
- [ ] Can you explain what a test cross is and interpret its results?
- [ ] Can you distinguish between Mendelian and non-Mendelian inheritance patterns?
- [ ] Can you explain how recombination frequency is used to construct linkage maps?