Chromosome Number (Basic Idea) MCQs Quiz | Class 9
This quiz covers Class IX Science (Code 086), Unit II: Organization in the Living World. It focuses on the topic of Chromosome Numbers, specifically the concept that chromosome number is species-specific and constant. Test your understanding, view the correct answers, and download the PDF result sheet for your records.
Overview of Chromosome Numbers
Chromosomes are thread-like structures located inside the nucleus of animal and plant cells. Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA). The most critical concept for Class 9 students is that the number of chromosomes is fixed and constant for a particular species. This specific number ensures that offspring inherit the correct amount of genetic material from their parents.
Key Concepts
- Species-Specific: Every biological species has a specific number of chromosomes in its somatic (body) cells. For example, all normal human body cells contain 46 chromosomes.
- Diploid (2n): Somatic cells (body cells like skin, muscle, nerve) contain two complete sets of chromosomes, one from each parent. This is called the diploid number. In humans, 2n = 46.
- Haploid (n): Gametes (reproductive cells like sperm and eggs) contain only one set of chromosomes. This is called the haploid number. In humans, n = 23.
- Fertilization: When a sperm (n) and egg (n) fuse, they form a zygote (2n), restoring the species-specific chromosome number.
Examples of Chromosome Numbers
Different species have vastly different numbers of chromosomes. The number does not indicate the complexity of the organism.
| Organism | Diploid Number (Body Cell) | Haploid Number (Gamete) |
|---|---|---|
| Human | 46 | 23 |
| Chimpanzee | 48 | 24 |
| Dog | 78 | 39 |
| Cat | 38 | 19 |
| Fruit Fly | 8 | 4 |
Quick Revision List
- Chromosomes are visible clearly only during cell division.
- The constancy of chromosome number is maintained by meiosis (which halves the number for gametes) and fertilization (which restores it).
- A change in chromosome number typically leads to genetic disorders or developmental issues.
- Bacteria typically have a single circular chromosome (haploid state).
Extra Practice Questions
1. Why is meiosis important for chromosome number?
It reduces the chromosome number by half in gametes so that fertilization restores the original number.
2. If a horse has 64 chromosomes in body cells, how many are in its pollen/sperm?
32 chromosomes.
3. Are chromosomes visible throughout the cell cycle?
No, they are most distinct and visible during cell division (metaphase).
4. What is a Karyotype?
It is an organized profile or image of a person’s chromosomes, arranged by size and number.
5. Do plants have chromosomes?
Yes, all eukaryotic organisms, including plants, have chromosomes in their nuclei.
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