WHAT IS THE CHROMOSOME THEORY OF INHERITANCE? (SUTTON AND MORGAN) - BIOLOGY - 2025

The chromosomal theory or chromosomal theory of inheritance is one of those that was developed in the way of biologists to try to explain the transmission of phenotype and genotype from parents to their offspring.

This theory aims to integrate the observations of cell biology with the hereditary principles proposed by Gregor Mendel, concluding that genes are found in chromosomes and that they are distributed independently in meiosis.

Chromosome theory can be summarized as follows: the physical location of genes resides on the chromosomes and these are arranged in a linear fashion. In addition, there is a phenomenon of exchange of genetic material between pairs of chromosomes, known as recombination, which depends on the proximity of the genes.

History

At the time that Mendel enunciated his laws, there was no evidence on the mechanism of distribution of chromosomes in the processes of meiosis and mitosis.

However, Mendel suspected the existence of certain "factors" or "particles" that were distributed in the sexual cycles of organisms, but had no knowledge of the true identity of these entities (now it is known that they are genes).

Due to these theoretical gaps, Mendel's work was not appreciated by the scientific community of the time.

Contributed by Walter Sutton

In 1903 the American biologist Walter Sutton emphasized the importance of a pair of chromosomes of similar morphology. During meiosis, this homologous pair separates and each gamete receives a single chromosome.

In fact, Sutton was the first person to note that chromosomes obey Mendel's laws, and this claim is considered the first valid argument to support the chromosome theory of inheritance.

Sutton's experimental design consisted of studying the chromosomes in the spermatogenesis of the grasshopper Brachystola magna, demonstrating how these structures segregate in meiosis. In addition, he was able to determine that the chromosomes were grouped in pairs.

With this principle in mind, Sutton proposed that Mendel's results can be integrated with the existence of chromosomes, thus assuming that genes are part of them.

Contribution of Thomas Hunt Morgan

In 1909 Morgan managed to establish a clear relationship between a gene and a chromosome. This he achieved thanks to his experiments with Drosophila, showing that the gene responsible for white eyes was located on the X chromosome of this species.

In her research, Morgan found that the fruit fly possessed four pairs of chromosomes, of which three were homologous or autosomal chromosomes and the remaining pair was sexual. This discovery was awarded the Nobel Prize in Physiology or Medicine.

As in mammals, females have two identical chromosomes, denoted XX, while males are XY.

Morgan also made another crucial observation: In a significant number of cases, certain genes were inherited together; I call this phenomenon linked genes. However, on some occasions it was possible to "break" this link, thanks to genetic recombination.

Finally, Morgan noted that genes were arranged in a linear fashion along the chromosome, and each was located in a physical region: the locus (the plural is loci).

Morgan's conclusions achieved full acceptance of the chromosomal theory of inheritance, completing and corroborating the observations of his colleagues.

Principles of the chromosome theory of inheritance

The evidence presented by these researchers allowed to enunciate the principles of the chromosomal theory of inheritance:

Genes located on chromosomes

Genes are found on chromosomes and are arranged in a linear fashion. To corroborate this principle there is direct evidence and indirect evidence.

As indirect evidence we have to consider chromosomes as the vehicles of genes. Chromosomes are capable of transmitting information through a semi-conservative replication process that certifies the molecular identity of sister chromatids.

In addition, chromosomes have the peculiarity of transmitting genetic information in the same way that Mendel's laws predict.

Sutton postulated that genes associated with seed color - green and yellow - were carried on a particular pair of chromosomes, while genes related to texture - smooth and rough - were carried on a different pair.

Chromosomes have specific positions called loci, where genes are located. Likewise, it is the chromosomes that are distributed independently.

Following this idea, the 9: 3: 3: 1 ratios found by Mendel are easy to explain, since now the physical particles of inheritance were known.

Chromosomes exchange information

In diploid species, the meiosis process allows the number of chromosomes that the gametes will have in half. In this way, when fertilization occurs, the diploid condition of the new individual is restored.

If there were no meiosis processes, the number of chromosomes would double as generations progress.

Chromosomes are capable of exchanging regions with each other; This phenomenon is known as genetic recombination and occurs in the meiosis processes. The frequency at which recombination occurs depends on how far apart the genes are located on the chromosomes.

There are linked genes

The closer the genes are, the more likely they are to be inherited together. When this happens, the genes are "linked" and passed on to the next generation as a single building block.

There is a way to quantify the proximity in genes in units of centimorgan, abbreviated cM. This unit is used in the construction of genetic linkage maps and is equivalent to 1% recombination frequency; it corresponds to approximately one million base pairs in DNA.

The maximum frequency of recombination - that is, in separate chromosomes - occurs at more than 50%, and this scenario is one of "no ligation".

Note that when two genes are linked, they do not comply with the law of transmission of characters proposed by Mendel, since these laws were based on characters that were located on separate chromosomes.

References

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4. Lacadena, JR (1996). Cytogenetics. Editorial Complutense.
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