2 marksShort AnswerSection A

“The chromosomes number of the sexually reproducing parents and their offspring is the same.” Justify this statement.

How do Organisms Reproduce?

Sexual Reproduction - Chromosome Number Maintenance

Official Answer

The chromosome number of sexually reproducing parents and their offspring remains the same due to the following mechanisms:

Meiosis (Reduction Division): Organisms have specialized lineages of cells in reproductive organs that undergo a special type of cell division called meiosis. This process halves the number of chromosomes and the amount of DNA in the germ-cells (gametes) compared to the non-reproductive body cells (somatic cells). Thus, if the parent body cells have $2n$ chromosomes, the gametes will have only $n$ chromosomes.

Fertilisation: During sexual reproduction, the male gamete ( $n$ ) and the female gamete ( $n$ ) fuse to form a zygote. This fusion re-establishes the original diploid number of chromosomes ( $2n$ ) and the original DNA content in the offspring. Therefore, the chromosome number remains constant from generation to generation.

meiosisreduction divisiongerm-cellsgameteshalvesfertilisationzygotere-establisheschromosome numberDNA content

Marking Scheme

1State that specialized germ-cells undergo meiosis (reduction division) which halves the chromosome number and DNA content: 1 Mark
2State that the fusion of male and female gametes (fertilisation) restores the original chromosome number and DNA content in the zygote: 1 Mark

Hint

Think about the special type of cell division that halves the chromosome number in germ-cells, and what happens when these germ-cells fuse during fertilisation.

Quick Oral Answer

The chromosome number is kept constant because gametes are formed by meiosis, which halves the chromosome number, and when they fuse during fertilisation, the original chromosome number is restored.

Analysis & Explanation

If gametes were formed by normal cell division (mitosis), they would contain the full amount of DNA ( $2n$ ). Upon fusion, the zygote would have double the DNA ( $4n$ ), which would disrupt the cellular control mechanisms. Meiosis solves this by reducing the chromosome number to half ( $n$ ) in gametes, and fertilisation restores it to $2n$ .

Common Mistakes

1Confusing meiosis with mitosis.
2Forgetting to mention that fertilisation restores the original diploid number of chromosomes.

Interesting Facts

Meiosis comes from the Greek word 'meioun', which means 'to make smaller', referring to the halving of the chromosome number.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2013?

This question carries 2 marks in the CBSE Class 10 Science 2013 examination.

Which chapter does this question come from in Science?

This question is from the chapter "How do Organisms Reproduce?" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Sexual Reproduction - Chromosome Number Maintenance" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2013 paper?

This is a Short Answer question from Section A in the CBSE Class 10 Science 2013 paper.