As a biology student, you have often heard the term ‘genes’, ‘chromosomes’, or ‘inheritance.’ You may also be quite aware of Mendel’s famous biologist. He was the first to propose the fundamental law of inheritance. But, do you know what it was exactly? Or who detected that genes lie in the chromosome and precisely where genes lie in the cells? And what is the theory that proposed such an idea?
This article will help you with the answers to all these questions. Read till the end to get more information.
In 1843, the father of modern genetics, Gregor Mendel, initiated studying parentage. Due to the absence of advanced microscopes during that period, chromosomes had not been practised, and his work got disregarded by scientists for a long time. There are more reasons for that, like lack of communication mediums.
Later, after the invention of advanced microscopes, some scientists started to study his work and re-evaluate his models in terms of chromosomal practices.
Interested in Medical Courses? Click Here to Register for NEET Crash Course 2022 by Aakash Byju’s
Chromosome
The DNA molecule is bundled into a thread-like structure in each cell nucleus. These structures are called chromosomes. Each chromosome is made up of histones, i.e., tightly coiled DNA around protein. That holds up the structure of chromosomes.
Chromosomes help ensure that DNA is reproduced and distributed properly during cell division. In humans, there are 46 chromosomes present in each individual’s body. These 46 chromosomes are arranged in 23 pairs. And out of 23 pairs, 22 pairs of chromosomes are called autosomes. Other than autosomes, human has one pair of sex chromosomes called an allosome.
Chromosomes are the location of genes, and genes are the stretches of DNA that specify proteins. Therefore, they are interlinked with each other. Or precisely, genes are part of chromosomes.
Also see: Important Concepts at Aakash Byjus’s
The Chromosomal Theory of Inheritance
The hypothesis that chromosomes might be the way to understand extraction led several scientists to investigate Mendel’s publications. They reanalyse his model regarding the conduct of chromosomes during meiosis and mitosis. In 1900, scientists could carefully observe cell division due to the advancement in microscopes. It led to the discovery of some double and divided structures in the nucleus just before each cell division. They called them chromosomes.
Later, in 1902, Theodore Boveri and Walter Sutton studied the chromosomes’ actions and found similarities in-between the bearing of genes and chromosomes. They use this movement of chromosomes to explain Mendel’s theory. Their observations taught them that chromosomes are found in pairs. Similarly, genes also occur in pairs. The two allelomorphs of a gene pair are located on equivalent sites on homologous chromosomes.
Boveri and Sutton argued that the pairing and separation of chromosomes would separate a pair of genes they carried. Sutton consolidated the knowledge of chromosomal partitioning with Mendelian principles and called it the chromosomal theory of inheritance.
Later, Thomas Hunt Morgan and his colleagues followed this idea of the chromosomal theory of inheritance and provided the required experimental verification. Hence, it has become the foundational genetic theory that acknowledges chromosomes as the messenger of genetic material.
The main points of this theory are as follows:
- Gametes (sperm and egg) transfer hereditary characters from one generation to another.
- The two allelomorphs of a gene pair are placed on homologous sites on the comparable chromosomes.
- Chromosomes and genes are both found in pairs.
- The nucleus is the area of hereditary characters.
- Homologous chromosomes get nervous during meiosis and get separated to pass into different cells, which becomes the basis of separation and independent diversity during meiosis.
- The gametes having haploid sets of chromosomes fuse to amalgamate the diploid state.
Check our visual story on Principles of Inheritance & variations
Principles of Inheritance & Variation: Class 12 CBSE Revision Notes
Chromosomal Theory – Linkage and Genetic Recombination
Thomas Hunt Morgan and his colleagues perform experiments to verify Sutton and Boveri’s theory. Morgan selected a fruit fly, Drosophila melanogaster, for his experiments. He wanted to study sex-linked genes.
It was suggested by Mendel’s work that characteristics are inherited independently of each other. As per Mendel’s law, Morgan found out that the isolation result of the two genes did not show the expected result. It resulted in the interference of Mendel’s predicted outcomes. If two genes were present on the same chromosome, the expectation of obtaining a parental combination in the next generation was much higher than the non-parental type. It suggested that the random segregation of chromosomes was the physical basis of Mendel’s model.
Morgan designated this due to the physical cooperation or the linkage of the two genes and cast the term ‘linkage’ to describe the physical cooperation of genes on the same chromosome. While the generation of non-parental gene combination through dihybrid cross is known as ‘recombination.’ When genes are brought to light on the same chromosome, they are firmly linked and show very low recombination.
Observations of Chromosomal Theory of Inheritance
Given below are some observations that support this theory:
- The chromosomal inheritance theory keeps up with Mendel’s laws.
- Gametes are produced by each parent and contain half of their chromosomal companion.
- Each pair of homologous chromosomes are erratically distributed into pre-gametes.
- During the cell division – meiosis process, some pairs of chromosomes move as individual and independent structures. These pairs are of homologous chromosomes.
- The gametic chromosomes fuse with the same number of chromosomes as their parents to produce offspring during fertilisation.
- Although the size of male and female gametes is different, the number of chromosomes remains the same in both. It indicates that each parent contributes equal inheritance.
Also see: NCERT Solutions for Class 12 Biology Chapter 5 – Principles of Inheritance and Variation
FAQs
1. Why are Chromosomes important?
Chromosomes are a critical part of all living beings. Their functions are vital for all living organisms’ survival and proper functioning. They carry the basic genetic material called genes. Genes provide genetic information to many cells.
In addition, the chemical composition of the chromosomes is covered by histones (a type of protein found in chromosomes) and proteins that play a great role in metabolism. They also protect chemical enzymes. Hence, DNA functions without any obstacles during cell division. Furthermore, non-histone proteins regulate gene action and are also present in chromosomes.
Watch our video on Chromosomal Theory of Inheritance – Principles of Inheritance and Variation Class 12
2. What are the differences between chromosomes and genes?
The main differences between chromosomes and genes are given below:
| Chromosome | Gene |
| It is DNA’s packed structure with proteins. | It is located on chromosomes. |
| It is composed of DNA, RNA, and histones ( a type of protein). | It is composed of either DNA or RNA. |
| The chromosomal mutation leads to chromosomal abnormalities. | The gene mutation leads to a frameshift mutation and point mutation. |
| The chromosomal mutation is large. | The gene mutation is small. |
| It is visible under the microscope. | It is not visible under the microscope. |
| A single chromosome is made up of many genes. | A single gene is an emplacement on the chromosome. |
3. What are the key points of the chromosomal theory of inheritance?
The key points of the chromosomal theory of inheritance are given below:
- The chromosomal theory of inheritance (proposed by Boveri and Sutton) states that genes are present at specific locations on chromosomes. The action of chromosomes throughout meiosis can explain Mendel’s laws of inheritance.
- Thomas Hunt Morgan provided the first strong confirmation of the chromosome theory by studying fruit flies. He found a mutation that affected the colour of the fly’s eye.
- Morgan observed that the alteration was inherited differently by sexually different flies. And because of this patrimony pattern, he finalised that the gene of eye color must be located on the X chromosome.
4. What are Mendel’s three laws of inheritance?
During his study of pea plants, Mendel stated three inheritance laws. These are as follows:
- Law of dominance:
It states that genes control characters. When two alternate forms of genes are present in an organism, only one factor expresses itself. A dominant gene will express itself over the introverted gene.
- Law of segregation:
It states that parental genes are erratically separated into the germ cells so that each germ cell can acquire only one gene from each pair. They do not show any merging.
- Law of independent assortment:
According to this law, when two pairs of characteristics are combined in a hybrid, the segregation of one pair of genes is independent of the other pair of genes, i.e., genes with different traits are classified separately from one another so that the inheritance of one trait is not dependent on the other trait.
Out of these three laws, the law of segregation is the most important. It has no exceptions and is universally accepted. Hence, it becomes the basis of the chromosomal theory of inheritance.
5. What are the differences between linkage and recombination?
The differences between linkage and recombination are as follows:
| Linkage | Recombination |
| It keeps the genes together. | It escorts to separate the linked genes. |
| It reduces irregularity. | It increases irregularity by forming new gene combinations. |
| The number of linked groups can never be more than haploid chromosome numbers. | The frequency of recombination can never be more than 50%. |
| It involves individual chromosomes. | It includes the exchange of segments between homologous chromosomes. |
6. What is the purpose of Morgan’s observations?
Morgan wanted to understand heredity and mutation, which are genetic changes. Like other scientists, he was reluctant to accept Boveri and Sutton’s chromosomal theory of inheritance. Morgan argued that scientists were biassed toward related phenomena like the inheritance of characters, etc. He argued that if one gene did not explain the phenomenon, they could argue with any other number of genes.
Later, he made an observation that ultimately provided evidence supporting the chromosomal theory. He started his experiment with white eye fruit flies and made some hypotheses to get more information. And to test his hypothesis, he mated other specific groups of flies and observed the offspring. His experiments showed that the inheritance of a trait could differ between sexes.
In future experiments, he established the chromosomal theory of inheritance, which describes the role of chromosomes in heredity.
Also see: Medical Courses at Aakash
Conclusion
Boveri and Sutton came up with The Inheritance Chromosomal Theory. It states that chromosomes are the mechanism of genetic ancestry. Neither Mendel’s genetics laws nor gene interaction is perfectly faultless. Instead, chromosome behaviour involves isolation, independent selection, and sometimes linkage.
Sturtevant, with Morgan, in 1911, concocted a method to assess recombination recurrence. He was part of Morgan’s team and completed his doctoral under him. While researching with Morgan, Sturtevant deduced the relative positions and distances of linked genes on a chromosome. He performed so based on the average number of crossovers in the interceding region between the genes.
Sturtevant correctly imagined that genes are sorted out in serial order on chromosomes. He said that reuniting between resemblances could occur, with equal possibility, anywhere on a chromosome. According to him, linkage causes genes on the same chromosome to be inherited together. And the homologous recombination prejudice genes toward the legacy pattern of the independent array.
