CBSE Class 12 Biology Chapter 1: Reproduction in Organisms NCERT Solutions

Let's analyze each statement:

• I. Gametic fusion takes place: This is characteristic of sexual reproduction only, where male and female gametes fuse to form a zygote. It does not occur in asexual reproduction.
• II. Transfer of genetic material takes place: This is a fundamental aspect of all reproduction, both asexual and sexual. Genetic material (DNA) is passed from the parent(s) to the offspring.
• III. Reduction division takes place: Reduction division (meiosis) occurs during the formation of gametes in sexual reproduction. It does not occur in asexual reproduction, which typically involves mitosis.
• IV. Progeny have some resemblance with parents: Offspring produced through both asexual and sexual reproduction inherit traits from their parent(s), leading to resemblance. In asexual reproduction, the resemblance is exact, while in sexual reproduction, it is partial due to genetic recombination.

Therefore, statements II and IV are true for both asexual and sexual reproduction.

Explanation:

• Clones are individuals that are genetically identical to one another and to their parent. This occurs in asexual reproduction where a single parent produces offspring through processes like budding, fission, or vegetative propagation.
• In sexual reproduction, offspring are produced from the fusion of gametes from two parents. While genetic material is transferred, the process involves recombination and independent assortment of genes during meiosis, and the combination of genetic material from both parents. This results in offspring that are genetically similar but not exact copies of either parent.
• Option (b) is incorrect because DNA from both parents contributes to the offspring in sexual reproduction.
• Option (c) is irrelevant to the definition of a clone.
• Option (d) is incorrect; offspring from sexual reproduction share significant genetic material with their parents, they are not completely different.

Therefore, the term 'clone' is not applicable to offspring of sexual reproduction because they do not possess exact copies of parental DNA.

Explanation:

• Unicellular organisms possess a relatively simple body organization. Asexual reproduction is a common and efficient mode of reproduction for them because it allows for rapid multiplication. For example, Amoeba reproduces by binary fission, and yeast reproduces by budding.
• While Amoeba and yeast are also microscopic (a) and heterotrophic (b), their unicellular nature (c) is the primary reason why asexual reproduction is their predominant mode.
• Although they are uninucleate (d), this is a characteristic of many unicellular organisms and not the sole reason for their mode of reproduction; for instance, some multicellular algae are also uninucleate and can reproduce asexually.

Thus, being unicellular allows for simple and rapid asexual reproduction.

Let's evaluate each statement:

• I. Sexual reproduction does not always require two individuals: This statement is true. Hermaphroditic organisms (like earthworms or tapeworms) possess both male and female reproductive organs and can reproduce sexually, sometimes even with themselves (self-fertilization), although cross-fertilization is more common.
• II. Sexual reproduction generally involves gametic fusion: This statement is true. The defining characteristic of sexual reproduction is the fusion of male and female gametes (syngamy) to form a zygote.
• III. Meiosis never occurs during sexual reproduction: This statement is false. Meiosis is essential during sexual reproduction for the production of haploid gametes from diploid cells.
• IV. External fertilisation is a rule during sexual reproduction: This statement is false. Fertilisation can be either external (e.g., in many aquatic organisms) or internal (e.g., in reptiles, birds, mammals).

Therefore, statements I and II are correct.

Explanation:

The life cycle described is one where meiotic division occurs after the formation of the zygote. This means the zygote is diploid ($$2n$$), and it undergoes meiosis to produce haploid ($n$) cells.

• Since the adult filament is formed from these haploid cells resulting from meiosis, the vegetative cells of the adult filament will be haploid.
• In a haploid organism, gametes are produced by mitosis. The structures that produce gametes are called gametangia. If the vegetative cells are haploid, then the gametangia, which develop from these cells, will also be haploid.

This type of life cycle, where the haploid phase is dominant and the diploid phase is represented only by the zygote which immediately undergoes meiosis, is known as a haploid life cycle. Examples include many algae and fungi.