Step‑by‑step solution

Given:

• Clones of a plant are grown in a field → all plants are genetically identical (same genotype).

• Plants show different heights → phenotypic variation in height.

• A frequency distribution of height (frequency vs. height) gives a bell‑shaped (normal) curve.

We are to conclude what causes this variation in height.

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Option (1): It being a polygenic trait

• A polygenic trait is controlled by many genes (quantitative trait) and usually shows continuous variation with a bell‑shaped curve.

• However, here all plants are clones, so they have the same genotype.

• If height were purely polygenic, all clones should have the same height (or very similar) in the same environment, but here they differ.

• So, the bell‑shaped curve here is not due to segregation of many genes (polygenic inheritance), because there is no genetic variation among clones.

• Hence, option (1) is not correct.

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Option (2): Environmental effect

• Since all plants are clones (same genotype), any variation in height must be due to differences in the environment (light, water, nutrients, microclimate, etc.).

• Environmental factors like soil fertility, water availability, shading, and temperature can cause genetically identical plants to grow to different heights.

• When many individuals of the same genotype are exposed to slightly different microenvironments, their phenotypes (heights) vary continuously and often form a normal (bell‑shaped) distribution.

• So, the observed bell‑shaped curve of height in clones is best explained by environmental variation.

• Thus, option (2) is correct.

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Option (3): Variation in genotype

• The question clearly states that clones of a plant are used → all plants are genetically identical.

• There is no variation in genotype among the plants.

• Therefore, the observed variation in height cannot be due to genotypic differences.

• So, option (3) is incorrect.

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Option (4): Influence of environment on different genotypes

• This option implies that there are different genotypes and the environment affects them differently (genotype × environment interaction).

• But in this experiment, all plants are clones → same genotype.

• Since there is only one genotype, we cannot talk about “different genotypes” being influenced by environment.

• So, option (4) is not applicable and therefore incorrect.

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Final answer

The observed bell‑shaped curve in height of cloned plants is due to environmental effect.

Correct option: (2).

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Introduction

In plant biology experiments, clones (genetically identical individuals) of a plant are often grown together in a field to study how environment affects growth. Surprisingly, even though all plants have the same genotype, they often show a range of heights. When height is plotted against frequency, a bell‑shaped (normal) curve is obtained. This article explains why this variation occurs and why it is due to environmental effects, not genetic differences.

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What the Experiment Shows

In the described experiment:

• A single plant is cloned, so all individuals have identical DNA (same genotype).

• These clones are grown in a field, where each plant experiences slightly different microenvironments (soil, water, light, nutrients, competition, etc.).

• As a result, the plants grow to different heights, even though they are genetically the same.

• When the frequency of plants is plotted against height, a bell‑shaped (normal) curve is obtained, indicating continuous variation in the phenotype.

This pattern is a classic example of how environment can cause phenotypic variation in a genetically uniform population.

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Why the Bell‑Shaped Curve?

A bell‑shaped curve in a frequency distribution of a trait (like height) is typical of quantitative traits. Such traits are influenced by:

• Many genes (polygenic inheritance), or

• Environmental factors acting on a single genotype.

In this case:

• Since all plants are clones, there is no genetic variation among them.

• Therefore, the continuous variation and bell‑shaped curve cannot be due to polygenic inheritance (option 1) or variation in genotype (option 3).

• Instead, the variation arises because each plant experiences a slightly different microenvironment in the field.

Small differences in light, water, nutrients, temperature, or crowding cause small differences in growth, leading to a normal distribution of heights.

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Why Not Polygenic Trait (Option 1)?

• A polygenic trait is controlled by multiple genes, and in a genetically diverse population, this leads to continuous variation and a bell‑shaped curve.

• However, in a population of clones, all individuals have the same set of genes.

• If height were purely polygenic, all clones should have very similar heights under uniform conditions.

• The fact that clones show a wide range of heights in a field shows that the variation is not due to segregation of many genes, but due to environmental differences.

So, option (1) is incorrect in this context.

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Why Not Variation in Genotype (Option 3)?

• The question explicitly states that clones of a plant are used.

• Clones are produced by vegetative propagation (cuttings, tissue culture, etc.) and are genetically identical to the parent.

• Therefore, there is no variation in genotype among the plants in this experiment.

• Since genotype is the same, the observed differences in height cannot be attributed to genotypic variation.

Thus, option (3) is clearly wrong.

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Why Not Influence of Environment on Different Genotypes (Option 4)?

• Option (4) refers to a genotype × environment interaction, where different genotypes respond differently to environmental changes.

• For example, one genotype may grow tall in good conditions but short in poor conditions, while another genotype may be less sensitive to environment.

• However, in this experiment, there is only one genotype (all plants are clones).

• So, we cannot talk about “different genotypes” being influenced by environment; the variation is simply due to environment acting on a single genotype.

Therefore, option (4) is not applicable here and is incorrect.

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Why Environmental Effect (Option 2) is Correct

• Since all plants are genetically identical, any phenotypic variation (in height) must be due to non‑genetic factors, i.e., the environment.

• Environmental factors that affect plant height include:

  • Light intensity and duration (photoperiod),

  • Water availability and soil moisture,

  • Nutrient supply (N, P, K, etc.),

  • Temperature,

  • Competition from neighboring plants.

• In a field, these factors vary slightly from spot to spot, leading to small differences in growth among clones.

• When many such individuals are measured, the small environmental effects add up to produce a normal distribution of heights.

Hence, the observed bell‑shaped curve in height of cloned plants is due to environmental effect on a single genotype.

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Conclusion

In an experiment where clones of a plant are grown in a field and show a bell‑shaped distribution of height, the variation is due to environmental effects, not genetic differences. Since clones are genetically identical, differences in height arise from microenvironmental variation (light, water, nutrients, etc.) acting on the same genotype. This is a classic example of how environment shapes quantitative traits in plants, and the correct choice is option (2).