Q.Why are no variations seen in clones ? State two reasons.
Clones are genetically identical copies, so no genetic variation arises from sexual reproduction; the two key reasons are the absence of meiosis (no crossing over or independent assortment) and the absence of fertilisation (no combining of different parental genomes).
The Concept: Why Clones Are Carbon Copies
When we talk about clones, we mean organisms produced by asexual reproduction — not by the fusion of male and female gametes. In nature, this happens in bacteria (binary fission), yeast (budding), plants (runners, cuttings), and even some animals (starfish regeneration). In the lab, it’s done via techniques like somatic cell nuclear transfer (think Dolly the sheep).
The core idea is simple: no mixing of DNA from two parents means no new combinations of genes. But let’s dig deeper into the two specific biological processes that normally create variation — and why they are completely absent in clones.
Step-by-Step Reasoning
1. Absence of Meiosis (No Crossing Over or Independent Assortment)
In sexual reproduction, gametes (sperm and eggs) are produced by meiosis. During meiosis, two major events shuffle the genetic deck:
- Crossing over in prophase I: Homologous chromosomes exchange segments, creating new combinations of alleles on a single chromosome.
- Independent assortment in metaphase I: The way homologous pairs line up at the equator is random, so each gamete gets a unique mix of maternal and paternal chromosomes.
In a clone, the new individual is formed by mitosis — a simple, exact duplication of the parent cell’s DNA. Mitosis produces two daughter cells with identical genetic material. No crossing over, no independent assortment. The clone’s genome is a perfect copy of the parent’s.
A common mistake is to think that mutations during mitosis could create variation. While mutations can occur, they are rare and random — not a systematic source of variation like meiosis. The question asks why no variations are seen in clones, meaning the normal, predictable process produces zero variation. Mutations are exceptions, not the rule.
2. Absence of Fertilisation (No Combining of Two Genomes)
Even after meiosis, variation in sexual reproduction isn’t complete until fertilisation — the fusion of two genetically different gametes. This brings together chromosomes from two distinct parents, creating a new, unique diploid genome.
Clones skip this entirely. There is no second parent, no sperm, no egg fusion. The clone inherits all its DNA from a single parent. For example, when a plant grows from a cutting, every cell in the new plant has the same DNA as the original plant. No new alleles are introduced from outside.
Think of it like photocopying a document. A photocopy (clone) is identical to the original. To get a new document with different content, you need to combine text from two different sources (like fertilisation) or shuffle paragraphs within the original (like meiosis). A photocopy does neither.
The Two Reasons — Final Answer
- Clones are produced by mitosis, not meiosis. Mitosis does not involve crossing over or independent assortment, so no genetic recombination occurs.
- Clones involve only one parent. There is no fertilisation, so no mixing of two different parental genomes takes place.
The two reasons are: (1) clones are formed by mitosis (no crossing over or independent assortment), and (2) there is no fertilisation (no fusion of two different gametes).
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