What is a Chimeric Organism?

A chimeric organism refers to an organism that contains cells with different genetic compositions. This occurs when cells from different individuals, or from different species, are combined in a single organism, resulting in a genetically diverse entity.

How are Chimeric Organisms Generated?

The generation of chimeric organisms can happen in several ways. Let’s break down each potential mechanism:

1. Cells of Different Genetic Constitution Appear in the Same Organism

   • Definition: This is the most common description of a chimeric organism. It happens when different types of cells, each with distinct genetic material, coexist within one organism.

   • Example: In some cases, the chimerism is the result of fusion between two embryos at early developmental stages, where the cells from the two embryos mix and contribute to the tissue of the resulting organism.

   • Answer: This statement correctly describes the generation of chimeric organisms.

2. Union of Two Different Genomes Occur Within a Cell

   • Definition: This describes a process known as somatic cell fusion. While it might seem like a chimera, it is more accurately referred to as a hybrid cell or a somatic hybrid. In this case, the genomes of two cells are fused in the laboratory to produce a hybrid, but it doesn’t describe the traditional chimeric organism, where distinct cell populations coexist in an organism.

   • Explanation: This option describes something different from natural or artificial chimeric organisms and is not the best fit for the description.

3. A Genome of Another Individual is Injected into an Adult Organism

   • Definition: This process is often referred to as genetic modification or transgenesis rather than chimerism. When one genome is inserted into another individual, it is typically done for research or therapeutic purposes, such as gene therapy, rather than creating a chimeric organism where two genetic populations co-exist.

   • Explanation: While this option refers to genetic modification, it doesn’t describe the creation of a chimeric organism.

4. A Part of the Genome is Inactivated by Chromosomal Rearrangement

   • Definition: This description refers to genetic inactivation or mutation within a single genome, which can lead to diseases or other conditions. It doesn’t create a chimeric organism, as only one genetic composition remains in the organism.

   • Explanation: This statement doesn’t describe chimerism, as there is no genetic diversity created in the organism.

Conclusion

The most accurate definition of a chimeric organism is when cells of different genetic constitutions appear in the same organism. Therefore, the correct answer is:

(1) Cells of different genetic constitution appear in the same organism.

This phenomenon can occur naturally or be induced in laboratories for research purposes, leading to organisms that have genetically distinct cells within their tissues.