📗 Case Study 1: Mendel’s Dihybrid Cross
Mendel crossed a pure-breeding pea plant with round yellow seeds (RRYY)
with a pure-breeding plant having wrinkled green seeds (rryy). All plants
in the F₁ generation showed round yellow seeds. On self-pollination of F₁
plants, four different phenotypes appeared in the F₂ generation.
1. What is the genotype of the F₁ generation?
A. RRYY
B. rryy
C. RrYy
D. RRYy
✅ Answer: C
F₁ receives one allele from each parent → RrYy.
2. Which phenotypic ratio is observed in F₂?
A. 3 : 1
B. 1 : 2 : 1
C. 9 : 3 : 3 : 1
D. 1 : 1 : 1 : 1
✅ Answer: C
Independent assortment produces 9:3:3:1 ratio.
3. Which phenotype appears least frequently?
A. Round yellow
B. Round green
C. Wrinkled yellow
D. Wrinkled green
✅ Answer: D
Wrinkled green (rryy) occurs once out of 16.
4. Which Mendelian law is proved?
A. Dominance
B. Segregation
C. Independent assortment
D. Linkage
✅ Answer: C
Genes assort independently.
📗 Case Study 2: Formation of Gametes
A student studies gamete formation in a pea plant with genotype RrYy.
5. How many types of gametes are formed?
A. 1
B. 2
C. 3
D. 4
✅ Answer: D
Gametes: RY, Ry, rY, ry.
6. Which process causes independent assortment?
A. Fertilization
B. Mitosis
C. Meiosis
D. Cytokinesis
✅ Answer: C
Occurs during meiosis I.
7. Stage of meiosis for assortment?
A. Prophase I
B. Metaphase I
C. Anaphase II
D. Telophase I
✅ Answer: B
Random alignment at Metaphase I.
📗 Case Study 3: Dihybrid Test Cross
A heterozygous plant (RrYy) is crossed with rryy.
8. What type of cross?
A. Self
B. Back
C. Test cross
D. Reciprocal
✅ Answer: C
Cross with homozygous recessive.
9. Expected phenotypic ratio?
A. 3 : 1
B. 9 : 3 : 3 : 1
C. 1 : 2 : 1
D. 1 : 1 : 1 : 1
✅ Answer: D
Equal gamete contribution.
10. Why is test cross important?
A. Increase variation
B. Identify dominant genes
C. Determine genotype
D. Create hybrids
✅ Answer: C
Reveals homozygous/heterozygous nature.
📗 Case Study 4: Linked Genes vs Independent Assortment
In an experiment, two traits were observed to be inherited together
without forming new combinations.
11. What does this observation indicate?
A. Independent assortment
B. Mutation
C. Gene linkage
D. Polygenic inheritance
✅ Answer: C
Linked genes do not assort independently.
12. Why does Mendel’s law of independent assortment not apply here?
A. Genes are dominant
B. Genes are recessive
C. Genes are linked
D. Genes mutate
✅ Answer: C
Linked genes remain together during inheritance.
📗 Case Study 5: Genetic Variation
Dihybrid crosses are important in understanding natural variation in organisms.
13. Which factor increases variation in dihybrid crosses?
A. Mitosis
B. Independent assortment
C. Binary fission
D. Cloning
✅ Answer: B
Independent assortment creates new combinations.
14. Recombinant phenotypes are formed due to
A. Mutation
B. Independent assortment
C. Dominance
D. Purity of gametes
✅ Answer: B
Recombinants result from new allele combinations.
15. Which of the following is a recombinant phenotype?
A. Round yellow
B. Wrinkled green
C. Round green
D. None of these
✅ Answer: C
Round green was absent in parental generation.