Case Study 1: Sex Determination in Humans
In humans, sex is determined by the combination of sex chromosomes present
in the zygote. Females possess XX chromosomes, whereas males possess XY.
The Y chromosome carries a gene essential for male development.
1. Which chromosomal combination results in a male child?
A. XX
B. XO
C. XY
D. YY
✅ Answer: C
Presence of Y chromosome leads to male development.
2. Who determines the sex of a child in humans?
A. Mother
B. Father
C. Both equally
D. Environment
✅ Answer: B
Father produces X and Y sperms.
3. Which gene on the Y chromosome is responsible?
A. Hox gene
B. SRY gene
C. lac gene
D. Pax gene
✅ Answer: B
SRY gene initiates testis formation.
4. If an X-bearing sperm fertilizes the ovum?
A. Male
B. Female
C. Intersex
D. Sterile
✅ Answer: B
X ovum + X sperm = XX (female).
Case Study 2: Gamete Formation and Sex Ratio
During spermatogenesis, males produce X and Y sperms. Females produce only X ova.
5. Which statement is correct?
A. Females produce X and Y ova
B. Males produce only X sperms
C. Females produce only X ova
D. Males produce only Y sperms
✅ Answer: C
Females are homogametic.
6. Why is sex ratio ~1:1?
A. Equal ova produced
B. Equal X & Y sperms
C. Environment decides
D. Selective fertilization
✅ Answer: B
Equal chance of X or Y sperm.
Case Study 3: Sex Determination in Birds
7. Which sex is heterogametic?
A. Male
B. Female
C. Both
D. None
✅ Answer: B
Females are ZW.
8. Chromosomes of male birds?
A. XX
B. XY
C. ZZ
D. ZW
✅ Answer: C
9. System present?
A. XX–XY
B. XX–XO
C. ZZ–ZW
D. Haplodiploid
✅ Answer: C
Case Study 4: Haplodiploid Sex Determination
10. Haploid individuals?
A. Queen
B. Worker
C. Male
D. Female
✅ Answer: C
11. Female honey bees produced from?
A. Unfertilized eggs
B. Fertilized eggs
C. Mutation
D. Environment
✅ Answer: B
12. Which system operates?
A. XX–XY
B. ZZ–ZW
C. Haplodiploid
D. XX–XO
✅ Answer: C
Case Study 5: Environmental Sex Determination
13. Which factor determines sex?
A. Chromosomes
B. Genes
C. Temperature
D. Hormones
✅ Answer: C
14. Which organism shows this?
A. Human
B. Bird
C. Crocodile
D. Honey bee
✅ Answer: C
15. How is it different from genetic determination?
A. Occurs after fertilization
B. Depends on chromosomes
C. No genes involved
D. Occurs before fertilization
✅ Answer: A
Environmental factors act during development.