Unlocking Genetic Secrets: A Summary of the Chromosomal Theory of Inheritance

Have you ever wondered why you look the way you look or why you share similar characteristics as your parents (for better or for worse)? Why is there a resemblance between your facial features or your hair color or skin tone? These are just some interesting questions that haunted other people’s minds too. And thankfully, some of them had the scientific rigor to figure out the answers to these questions.

1. Elementary Knowledge

So in case, Biology was never your cup of tea in school, I’ve got you covered with this sort of recap of all the terms and words you’ll often come across while reading any genetics-related material.

1. First, the most basic terminology of it all is DNA. So what is DNA? To just put the full form out there, it stands for Deoxyribonucleic Acid. This is the flour to the cake of all the genetic material. DNA consists of two strands of molecules known as nucleotides that are wound around each other in an anti-parallel manner.
2. Next up we have, a gene- The gene is the part of our genetic material that produces proteins in our body. Each person has two genes for one characteristic, one that our mothers contribute and the other that out fatherds do. The number of genes is also organism specific, for example, humans have about 25k genes whereas an ant has about 17k.
3. Chromosomes are the last on this list. Chromosomes are present in the nuclei. The nucleus is the main character in the cell. It is the center of all major activities in the cell. Chromosomes are DNA molecules wrapped around proteins (which are known as histones). DNA is negatively charged so this protein (histone) is positively charged and hence DNA can bind to it. This compact structure leads to the possibility of humans which is about 2m of fitting in cell nuclei which is so much smaller than that.

2. The Beginning

So when did we start thinking about inheritance and genes and why are there any similarities between family members? It started with the Father of Genetics- Gregor Mendel. This man did not work with humans. He worked instead on pea plants. And he began his work in the 1800s when the concept of chromosomes was unimaginable. The patterns of inheritance observed by Mendel were the same as others observed later. In all honesty, his work formed all the foundations of the this theory.

Let’s go through what Mendel worked out through his research. The following were the conclusions of Mendel’s Laws that he postulated-

There is a pair of heritable factors that determines various characteristics of the plant such as height or flower color

In this pair, one of the factors is the dominant one while the other one is recessive. What does this mean? It means that one of the factors stops the other one from showing itself in the physical characteristics of the plant.

And third, Mendel stated that the factors(what are now known to us as genes) were inherited with no correlation to each other.

To read all about his work, give this article a read.

Anyways, during his time, Gregor Mendel’s work was left largely unrecognized because of his unorthodox way of incorporating mathematics in biology along with no actual microscopic evidence of everything he was suggesting (chromosomes had not been visualized under the microscope yet).

3. The Three Musketeers

Our story’s Athos, Porthos, and Aramis are Walter Sutton, Theodor Boveri, and Thomas Hunt Morgan.

These were some of the scientists who decided to revisit Mendel’s work after it had been long ignored. So while Mendel’s organism of choice was the pea plant, Walter Sutton worked majorly on grasshoppers, Boveri on sea urchins, and Morgans chose fruit flies (scientific name- Drosophila melanogaster).

3.1. The Meaning of This Theory

In one sentence, this theory states that chromosomes are the genetic material that is responsible for the Mendelian form of inheritance (mentioned above). This theory helped solidify the observations that Mendel had seen. It very quickly became a pillar of genetic studies.

Morgan even went on to win the Nobel Prize in Medicine or Physiology in 1933. No prizes for Sutton and Boveri, I would’ve been a little salty, not going to lie. But Morgan was gracious enough to acknowledge their work in his speech (here’s the link).

3.2. Walter Sutton and Theodor Boveri’s Work

In 1902, in the USA, Sutton’s observed how the chromosomes in a cell were separating and dividing into daughter cells during meiosis. He was  25 when he made this important discovery. That means I have about 5 years to make a revolutionary discovery.

Meiosis is a type of cellular division in which the number of chromosomes in the gamete (i.e, the egg or sperm) reduces to half of the original number. So basically what Sutton noted in grasshopper cells undergoing meiosis, was that chromosomes were dividing from x to x/2.

While all this is exciting, his life had an unexpected ending when he died at the age of 39 due to an appendicitis surgery gone wrong.

The title of his paper was- “The Chromosomes in Heredity” and it further elaborated on his results and how they directly correlated to Mendel’s observation

Also in 1902, but in a different country, Theodor Boveri was working on similar concepts in Germany. Boveri discovered that unless chromosomes were present in the cells, sea urchins were unable to develop properly (proper embryonic development was hindered)

Theodor Boveri was initially more of a humanities kid until changing his mind (a still popular phenomenon).

He lived a longer life than his companion in this game.

4. Facets of This Theory

Each chromosome pair exists in its world. Its migration is independent of the other pairs of homologous chromosomes.

The parents’ gametes contain only half of their initial chromosome number.

Each parent (i.e., male and female gametes) contributes the same amount of chromosomes to the offspring, no gender inequality here.

These gametic chromosomes combine during fertilization which leads to the original number in the offspring.

So, while Sutton and Boveri did some remarkable work in this field, they could not prove that traits or characteristics (like hair color, eye color, etc) were present in chromosomes.

To provide evidence for the same, Thomas Hunt Morgan came to the rescue.

5. Thomas Hunt Morgan- The Fruit Fly Guy

He started his work in 1910, in the USA. Now, it’s a very common question to wonder why he only chose fruit flies (Drosophila melanogaster) and not any other insect/animal. There are numerous reasons for the same-

• Short reproduction period
• Generated in large numbers
• Can be grown and handled in labs easily
• Only 4 pairs of chromosomes (3 autosomes or the non sex chromosomes and 1 pair of sex chromosome). So out of the four pairs of chromosomes in fruit flies, three are autosomes while one is the pair of sex chromosomes (sex determining chromosomes).

For a better understanding, check this article.

His experiments consisted of breeding normal flies with red eyes and flies with a mutation in their eye color because of which they had white eyes.

The wild type of eye color in flies, by default, is red (XW), while the white eye color (Xw) is the recessive trait.

5.1. The X Chromosome and The White-Eyed Females

So let’s dive into the actual experiment a little bit to understand it better.

Morgan took a red-eyed female (this is the normal wild type) and crossed it with mutated flies, i.e, a white-eyed male. What was the result of this crossing? We observed red-eyed female and red-eyed male offspring in the F1 generation, with no white-eyed male in the scene. What was inferred from this? That the white-eyed trait was the recessive one among the two. This was predicted and an expected result.

Now, the twist came when the red-eyed female and the red-eyed male were crossed.

In the F2 generation, Morgan found that the female flies were all red-eyed while half of the male population was red-eyed, the other half being white-eyed. There were no white-eyed females in the collection of progeny. This was food for thought. It led to the conclusion that males and females were inheriting eye color in different ways.

So where we are right now in the story is that we know the X chromosome is playing some role here. Just a reminder, in Drosophila melanogaster (like in humans), females have the characteristic two X chromosomes and males have one X chromosome and another Y chromosome.

Okay so let me also introduce a diagram known as Punnett Square. It’s a square (shocker) with four smaller squares in each square assigned one chromosome. So, in total four chromosomes are seen on it.

So when this analysis was done with the F2 cross of red-eyed females (Xw+Xw) and red-eyed males (Xw+Y), it gave us the result we observed before. If the gene for eye color was indeed on the X chromosome, then half the males will have white eyes (white-eyed male) while all the females have red eyes. There cannot be any white-eyed females.

So, the story concludes, that the gene for eye color was present solely on the X chromosome. And since males get the X chromosome only from their mother, they get a fifty-fifty split of phenotypes (the visual result of genes, here, eye color). Thus the eye color trait was the first trait to be X linked or the first trait that was linked to the X chromosome.

The male will show the feature when he has the recessive allele according to this inheritance pattern but a female would require two X chromosomes to be affected to be expressing the recessive allele. Now, we have a variety of X chromosome-linked diseases that are passed on from mothers to offspring.

Science is nothing if we don’t share our knowledge and skills with others and Morgan understood that. One of his students, Alfred Sturtevant who worked under him also went on to construct the first genetic map.

Conclusion

Thomas Hunt Morgan’s experiments and contribution to genetic linkage and the field of genetics were invaluable and he continued to nurture talented scientists who worked under him. His enthusiasm to contribute to this field was unmatched. From the “fly room” at Columbia University to winning the Nobel Prize, his legacy will be defined by his work.

Without his work, we would not be at the precipice of exponential biological discoveries. Understanding heredity led us to this day, with the progress of genetic biology and biotechnology moving at a rapid pace.

I hope this article was a reminder that everything stems from one seed, one thought, or one idea. No idea is small enough. We just need to work on it and have the patience to follow through with it. Science will always have unanswered questions and sure the money isn’t always great in this field but maybe outcomes be more rewarding than monetary comforts.

Frequently Asked Questions (FAQs)

1. Who gave the this theory?

We will do the right thing and give credit to all three men, Theodor Boveri, Walter Sutton, and Thomas Hunt Morgan.

2. What are the three postulates of this theory?

The main three postulates (Mendel’s Laws that this theory is based on, are-

• Principle of Fertilization- Gametes from each parent fuse and this leads to the complete same chromosome number in the offspring.
• Principle of Segregation- Only one of the two alleles is passed to the gamete. This passing on is a random event. During meiosis, only one member of the homologous pair is transferred to the gamete, the concept of independently segregating traits comes into play here.
• Law of Independent Assortment- Different genes tend to separate irrespective of each other. Homologous chromosome pairs migrate without caring for the other chromosome pairs.

3. Who is the father of inheritance biology?

Gregor Mendel, undoubtedly is known as the Father of Inheritance Biology.

4. What is Wilson’s chromosomal theory?

His observation is also part of this theory. He determined that during gamete formation, the sperm or egg only receives half of the chromosomes.

5. What is this theory mean by Thomas Morgan?

To put it in a few words, this theory states that genes lie on chromosomes. Chromosomes carry genetic information. Maybe give the article one more read if this question is still unclear. You can also view this article for a different version.

Last Updated on January 12, 2024 by kalidaspandian