Chromosomal Theory of Inheritance: Observations and Inheritance

The Chromosomal Theory of Inheritance explains how traits are passed from one generation to the next through chromosomes. It highlights how genes, located on chromosomes, determine characteristics such as eye colour, height, and even inherited diseases. The chromosomal theory of inheritance was proposed by Walter Sutton and Theodor Boveri independently in 1902–1903. This topic is part of the Class 12 chapter Principles of Inheritance and Variation in Biology.

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This Story also Contains

1. What are Chromosomes?
2. Chromosomal Theory of Inheritance
3. Chromosome Theory Linkage and Genetic Recombination
4. Observations of Chromosomal Theory of Inheritance
5. Chromosomal Abnormalities

What are Chromosomes?

Chromosomes are thread-like structures found in the nucleus of a cell that carry genetic information in the form of DNA. They carry genes of traits determining an organism's growth and also how it works. Such chromosomes occur in pairs, one from each parent and function as a main channel for the passing of traits from generation to generation.

Chromosomal Theory of Inheritance

The Chromosomal Theory of Inheritance carries genes on chromosomes and passes information from generation to generation:

Chromosome Pairs: Chromosomes occur in pairs, one from each parent. During meiosis, each pair splits, so each gamete gets one chromosome from each pair.

Genetic Diversity: Meiosis helps in variation by making sure that gametes are different. This is critical in producing genetic diversity.

Meiosis and Haploid Cells: Meiosis reduces the chromosome number to half, resulting in haploid cells. This is necessary so that when fertilization takes place, the zygote will have the correct diploid number of chromosomes.

Separation of alleles: When gametes are being produced, alleles for each gene separate. That is to say, any one allele for a gene can be found only in one gamete and only one allele for the gene can be present in the other gamete, just as happens when homologous chromosomes are separated at anaphase I of meiosis.

Independent Assortment: Different genes for different traits assort independently of each other, provided they are located on different chromosomes or different parts of the same chromosome. This is because chromosomes line up at random during metaphase I of meiosis.

Also Read:

• MCQ Practice on Genetics
• Principles of Inheritance and Variation
• Mendelian Genetics
• Molecular Basis of Inheritance

Diagram Showing Chromosomal Theory of Inheritance

Given below is the diagrammatic representation of the chromosomal theory of Inheritance

Chromosome Theory Linkage and Genetic Recombination

The Chromosomal Theory of Inheritance explains linkage and genetic recombination:

• Genes that are near each other on the same chromosome tend to be inherited together. Such inheritance is said to be in linkage.
• During meiosis, there might be an exchange of parts between a pair of homologous chromosomes. This phenomenon is known as crossing over.
• During recombination, it breaks the linkage between closely associated genes and produces new combinations of genes. Thus, an increase in genetic variation is ensured through recombination.

Genetic diversity allows species to adapt and evolve. It makes sure that the offspring's traits differ from that of the parents.

Observations of Chromosomal Theory of Inheritance

• In diploid cells, chromosomes occur in pairs, one chromosome of each pair is inherited from each parent.
• While making gamete (meiosis), the chromosome pairs segregate. Thus, this corresponds to the law of segregation by Mendel.
• Non-homologous chromosomes are oriented by meiosis independently of one another and correspond to Mendel's law of independent assortment.
• Genes occur on chromosomes, specific loci on specific chromosomes.
• Meiosis leads to a reduction in the number of chromosomes in gametes so that at the time of fertilization, the diploid state is regained.
• The behaviour of chromosomes during meiosis directly relates to the patterns identified by Mendel concerning the inheritance of traits.
• Specialized chromosomes such as the human X and Y determine sex too, thus establishing that chromosomes carry genetic information.
• Experiments carried out on Drosophila melanogaster, known as the fruit fly, showed that some characteristics were linked to specific chromosomes, such as eye colour being linked with the X chromosome. Inheritance was therefore directly linked to the action of chromosomes.

Chromosomal Abnormalities

• Numerical abnormalities: These are generally the abnormalities in the number of chromosomes. Some of the examples include Down syndrome, also known as trisomy 21, which is caused due to an extra chromosome number 21. The other one is Turner syndrome, wherein females have only one X chromosome.

• Structural Abnormalities: Such rearrangements in the chromosome structure include translocation, where segments of chromosomes are shuffled, deletions, during loss of chromosome segments, duplications, extra copies of chromosome segments, or inversions, where the chromosome segments are reversed.

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