Chromosome Theory and the Discovery of the Gene

FROM THE LECTURE SERIES: Redefining Reality—The Intellectual Implications of Modern Science

By Steven Gimbel, Ph.D., Gettysburg College

Evolutionary theory was one of the defining moments of the scientific world, forcing humans to reconceptualize their understanding of the world. While Charles Darwin gave a sense of the dynamic nature of the biological world, he himself did not know the mechanism through which these changes happen—the processes that have since been understood as genetics. 

Image of an X-chromosome against a backdrop of DNA.
The chromosome theory gave way to the understanding that the gene is the seat of heredity in the human body and the foundation of modern genetics. (Image: ustas7777777/Shutterstock)

The first great name in the study of genetics and the chromosome theory was Gregor Mendel, the Czech monk who first disclosed the regularities in inheritance through his work with pea plants. He examined several characteristics of the plants in his Church’s garden—the shape and color of pods, flowers, and seeds, and the height of the plant.

Mendel’s Genetic Tests on Pea Plants

Mendel expected a mixture of properties due to hybridization—crossing a tall and short plant to get a medium one, for instance. But to draw a mathematical inference, he tried to keep his investigation in binary properties, starting with purebred strains that exhibited the requisite properties cleanly. He took the pollen from purebred plants exhibiting one property and inserted it into a plant that exhibited the other property to see what would happen. The result was not blending at all, but the prevalence of one property over the other. 

Flow chart to depict how genetic properties get transferred to the next generation.
The 3:1 ratio could only be explained if there were two property giving elements passed from each parent and if one of the properties was dominant over the other. (Image: Benutzer:Magnus Manske/Public domain)

Looking at thousands of hybrids and tracking the properties from purebred parent plants, he found that of the pairs of binary properties, one had the tendency to show up more than the other. With a sizeable sample, he was able to show that the dominant trait appeared three times more frequently than the recessive trait. 

Mendel found the 3:1 ratio for all inherited properties in the second generation, but absolutely no correlation for all other properties, just a random distribution. He realized that the factor determining inherent properties was independent of the factors causing the other properties. There was something in the reproductive process that allowed the inheritance to occur such that the traits were not linked. 

Relevance for Darwinian Concepts

Mendel’s findings would have been very important to Darwin, and he published these in the journal The Proceedings of the Natural History Society of Brünn in 1866. A copy of that very issue of the journal was found on Darwin’s desk after he died, unopened. 

The simplest way to account for the 3:1 ratio was to conclude that there were two of ‘something’ in each plant that determined the property. Consider fair coins. If on flipping them, the result is two heads, trait A is created, and if the result is two tails, trait B is created. If the result is a head and a tail, whichever is the dominant trait, say Trait A, is created. This is a helpful understanding of the 3:1 ratio discovered by Mendel. The question then is, what exactly is the mechanism here, and what is being passed on? 

This is where the discovery of the cell enters the equation. 

Embryology in the Study of Genetics

Around the same time that Darwin and Mendel were working on their research in genetics, embryology was born. Around this time, the microscope allowed the human eye to see that living tissue was made up of even smaller elements. Just as chemists and physicists had discovered atoms, life scientists discovered the basic unit of living organisms—the cell. Originally, it was thought that the protoplasm, the bulk of the cell, was responsible for the transmission of heritage traits. But in 1844, there were suggestions that it was the nucleus that was key. 

In 1866, the German biologist Ernst Haeckel declared that the inner nucleus was responsible for the transmission of inheritable characteristics, while the outer plasma was responsible for the accommodation or adaptation to the conditions of the environment. It was now that the focus of biology turned to the importance of the nucleus in cell division. 

The study of the nucleus became an increasingly prevalent subfield of cell biology. 

Learn more about atoms, the bedrock of matter.

The Nucleus and the Chromosome 

For studying nuclei, dye was injected into the cell, and it was found to stick very well to a particular structure in the nucleus, which was christened ‘the colorful thing’ in Greek, or ‘chromosome’. 

Chromosomes seemed to be good candidates for the purpose of transferring inherited information for three main reasons. First, the number of chromosomes in a cell nucleus was the same in all the cells of every organism of a given species but was different for different species. Second, the chromosomes were two seemingly identical strands that were tangled together during cell division in most cells; when they untangled themselves each new nucleus got one of them. Third, for sperm and egg cells, the cells contained only one strand, so that when they combined to form a zygote, the new potential offspring received one from each of the parents. It was, therefore, a reasonable hypothesis that it was the chromosome that was responsible for carrying the units of inheritance. 

The chromosomal theory of inheritance had certain critics, though, one of the most vocal ones being the American biologist Thomas Hunt Morgan.
The objection was based on the idea that the correctness of the chromosome theory implied that the environment had little to do with the determination of traits, which seemed counter-intuitive. 

This is a transcript from the video series Redefining Reality: The Intellectual Implications of Modern Science. Watch it now, on The Great Courses Plus.

Attempts to Disprove the Chromosome Theory

Morgan set to disprove the chromosome theory, focusing his research on fruit flies, which offered several advantages. To start with, they had several easily identifiable traits: the sex of the fruit fly, the color of the fruit fly’s body, the color of the fruit fly’s eyes, and whether the wings were short or curly. Further, the lifespan of the fruit fly was so short that a new generation could be studied every two weeks. Also, fruit flies possessed a small number of chromosomes: five pairs in each nucleus, and finally, they were low maintenance, easy to keep subjects. 

Single fruit fly on a white background.
The fruit fly possessed certain attributes that made it a great focus subject for Morgan’s research. (Image: Roblan/Shutterstock)

Morgan tinkered with environmental factors and noted the consequent traits of the parents and the offspring, factor by factor. He even went to the extent of whirling the eggs in a centrifuge to see if there would be any mutations. He did not see any changes. 

It was after years of working on these experiments that Morgan realized the fact: no matter what environmental factors were altered, the only things that seemed statistically relevant were the properties of the parents, and thereby the chromosomes. Reluctantly, but surely, one of the biggest opponents of the theory became one of its most important supporters. 

The next challenge was to figure out what it was about the chromosome that made it the seat of genetics. 

From Chromosomes to Genes

Morgan and his students scrutinized chromosomes in great detail and discovered that they had a structure. The chromosomes had tangible parts that were named ‘genes’. 

The term was initially ‘pangene’, as coined by the Dutch botanist, Hugo de Vries. It was later shortened to ‘gene’ by the Danish researcher Wilhelm Johannsen, who then used the idea to create a distinction between two other terms that he coined: genotype, which was the difference in chromosomal makeup, and phenotype, which was the difference in observable properties of an organism. 

The gene became the seat of genetics thereon, and the scientific community dedicated itself to understanding how the gene worked. 

Learn more about how genes work.

Common Questions About Chromosome Theory and the Discovery of the Gene

Q: How did Gregor Mendel contribute to the study of genetics?

Gregor Mendel was one of the first to disclose the nature of inheritance through his work with pea plants. Through his work, Mendel was able to understand the working of dominant traits and recessive traits, and thus discover the cell, paving the way for the discovery of the chromosome and the gene.

Q: Why were chromosomes thought to be the carriers for inheritance?

Within the nucleus, chromosomes were thought to be the seat for genetic material, as the number of chromosomes in a cell nucleus was the same in all the cells of every organism of a given species, but was different for different species. Further, the chromosomes were two seemingly identical strands that were tangled together during cell division in most cells; when they untangled themselves each new nucleus got one of them. Finally, for sperm and egg cells, the cells contained only one strand, so that when they combined to form a zygote, the new potential offspring received one from each of the parents.

Q: How did the term ‘gene’ come to be?

When the chromosome was closely studied, it was discovered to have a structure, elements of which are now known as ‘genes’. The term, coined by the Dutch Botanist Hugo De Vries, was originally ‘pangene’ but was later shortened to ‘gene’ by Danish scientist Willhelm Johannsen.

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