Chromosomal Theory of Inheritance: Sutton and Boveri Experiments, Practice Problems and FAQs

We have some characteristics similar to parents. Yes, because we carry genes from our parents. You all heard a lot about genes. So can you tell me, where are the genes found in a cell? Yes, they lie on chromosomes indeed. So before understanding more about the inheritance of genes, we must study about chromosomes.

Fig: Chromosome

Do you know who found out that genes are on chromosomes? It was Walter Sutton and Theodor Boveri who found this. Sutton was an American, who studied chromosomes and meiosis in grasshoppers. Boveri was a German, and studied the same things in sea urchins. In 1902 and 1903, Sutton and Boveri published independent papers related to the chromosome theory of inheritance. Let us understand more about chromosomes and their inheritance in detail in this article.

Table of contents

Introduction
Experiments of Sutton and Boveri
Conclusion of experiments of Sutton and Boveri
The hypothesis
Practice Problems
FAQs

Introduction

Scientists Walter Sutton and Theodor Boveri independently performed some experiments.

They were able to carefully observe cell division and discovered that chromosomes separate during cell division and are present in pairs, specifically in meiosis. Their conclusions reinforce Mendel's work which is now known as the chromosomal theory of inheritance.

Fig: Developers of chromosomal theory of inheritance

Experiments of Sutton and Boveri – 1

Walter Sutton conducted the experiment on lubber grasshopper. He observed the cells of the grasshopper under a microscope and observed that it had 22 chromosomes in each cell.

Initially, all 22 chromosomes appeared quite different from each other. But when he started arranging them based on their structure and size, he found a pattern. He saw that every chromosome had an identical pair which are now known as homologous pairs.

Fig: Lubber grasshopper

The conclusion of this experiment is that every chromosome has an almost identical partner.

So, chromosomes occur in pairs.

Fig: Chromosomal pairs

Experiment of Sutton and Boveri - 2

Boveri worked on Ascaris and found that this organism has 4 chromosomes. He observed that there are 2 types of cells present in Ascaris. In the present era, they are called somatic cells and the germ cells. The way these cells divide is also different.

Fig: Ascaris

Division in somatic cells

In somatic cells, during S-Phase the genetic material gets doubled, which is condensed to form chromosomes during prophase. The chromosomes now align at the centre during the metaphase.

During anaphase, the chromosomes separate and travel to the opposite poles. Now the cells start to divide during telophase, and at the time of cytokinesis each daughter cell has 4 chromosomes equal to the number of chromosomes present in the mother cell.

Fig: Sutton and Boveri Experiments - 2

Division in germ cells

In germ cell, the genetic material doubles during the S-phase, and during the prophase I, 4 chromosomes are formed and crossing-over occurs between homologous chromosomes. Homologous pairs of chromosomes align at the equatorial plate during metaphase I and during the anaphase I stage, the identical pairs separate and move to opposite poles. At the telophase I stage, karyokinesis is completed which will be followed by cytokinesis.

Fig: Sutton and Boveri Experiment - 2

During metaphase II, the chromosomes get arranged at the centre, and the sister chromatids separate and travel to opposite poles during anaphase II. By the end of the division, i:e during telophase II and cytokinesis, 4 cells were formed and 2 chromosomes were present in each cell.

Fig: Sutton and Boveri Experiment - 2

So the conclusions of experiment 2 are as follows:

The germs cells follow meiosis and the number of chromosomes is reduced to half.
During this process of gamete formation, chromosomes segregate.

Experiment of Sutton and Boveri – 3

Sutton conducted another experiment on the same lubber grasshopper. He had observed that grasshoppers had a total of 22 chromosomes in a cell. Let us consider only 4 chromosomes now.

With these 2 sets of homologous chromosomes, there are two possibilities for meiosis.

Case I

In case I, yellow and red are together,

Fig: Sutton and Boveri Experiment - 3

Case II

In case II, yellow and green are together.

Fig: Sutton and Boveri Experiment - 3

Comparison of both possibilities

On comparing both possibilities, we find the following:

Each chromosome therefore segregates independently of others. When they were separated into gametes, the set of chromosomes in each daughter cell ended up having a mixture of the parental traits, but not necessarily the same mixture as that of other daughter cells. Hence, it can be concluded that during gamete formation, chromosomes in a homologous pair separate independently of other homologous pairs of chromosomes.

Conclusion of Sutton and Boveri experiments

The results of experiments of Sutton and Boveri are summarised below:

Chromosomes come in pairs.
Chromosomes segregate during gamete formation.
Chromosomes segregate independently of each other.

The hypothesis

Sutton and Boveri also came up with a hypothesis, based on their experiments, that Mendel's factors are present on specific locations of the chromosomes.

Inheritance of chromosomes = Inheritance of factors

Fig: Similarities between factors and chromosomes

The Chromosomal Theory of Inheritance

According to the chromosomal theory of inheritance by Sutton and Boveri “genes are found on specific locations of the chromosomes. It is the chromosomes that segregate and assort independently during meiosis for gamete formation.”

Practice problems

What happens during the separation of the pair of chromosomes at the time of gamete formation?

result in zygote formation
further lead to the beginning of meiosis
result in segregation of a pair of factors
affect zygote formation

Solution: Separation of the pair of chromosomes at the time of gamete formation neither results in the formation of zygotes nor affects the process. It results in the segregation of a pair of factors. The male gamete fuses with the female gametes to form zygotes in the fertilisation process. Separation of pairs of the homologous pair of chromosomes takes place during meiosis to ensure reduction in the number of chromosomes in the gametes. It is not followed by meiosis again. Hence option ‘c’ is correct.

According to the chromosomal theory of inheritance, the behaviour of ________ was parallel to the behaviour of _______.

chromosomes ; genes
genes ; proteins
chromosomes ; proteins
genes ; cell organelles

Solution: The chromosomal theory of inheritance is a famous theory proposed by Sutton and Boveri. Sutton and Boveri identified that the behaviour of chromosomes is parallel to the behaviour of genes. It occurs in the following ways:

Both chromosomes and genes occur in pairs.
Both chromosomes and genes segregate at the time of gamete formation such that only one member of each pair is transmitted to a gamete.

Hence, they used chromosome movement during cell division to explain Mendel’s laws.

According to Mendel’s law of segregation, the pair of factors controlling a character separate during gamete formation. This was proved by Sutton and Boveri. They argued that the pairing and separation of a pair of chromosomes during cell division would lead to the segregation of a pair of factors (genes) they carried. Sutton united the knowledge related to the chromosomal segregation with the famous Mendelian principles and called it the chromosomal theory of inheritance. Hence option ‘a’ is correct.

3. Chromosomal theory of inheritance was put forth by___________.

(a) Sutton in 1902
(b) Boveri in 1902
(c) Correns in 1909
(d) Sutton and Boveri in 1902

Solution: The chromosomal theory of inheritance was given by Boveri and Sutton in the early 1900s. It is the fundamental theory of genetics. According to the theory of chromosomal inheritance, genes are considered as the units of heredity and are found in the chromosomes. The chromosomal theory of Inheritance came into existence after Mendelian genetics only.

4. The chromosomal theory of inheritance is a combination of

knowledge of chromosomal segregation with Mendelian principles
knowledge of the dihybrid cross with the motion of the chromosome
knowledge of the law of dominance with chromosomal appearance
knowledge of classical genetics with modern molecular biology

Answer: Chromosomal theory of inheritance was worked on by Sutton and Boveri. They noted that behaviour of chromosomes was parallel to behaviour of genes and used chromosomal movement to explain Mendelian principles. Like, both genes and chromosomes occur in pairs and segregate at the time of gamete formation such that each gamete receives one member of each pair and independent pairs segregate independently of each other. With so many similarities, they united the knowledge of chromosomal segregation with Mendelian principles and called the theory, the chromosomal theory of inheritance. This theory is not comparing dihybrid cross with chromosomal movement or law of dominance with chromosomal appearance or classical genetics with molecular biology.

5. Factors : Mendel :: Chromosomal theory of inheritance : __________ .

a) Correns
b) Sutton and Boveri
c) Hugo deVries
d) Gregor J. Mendel

Solution: Mendel performed hybridisation experiments with pea plants and suggested that factors are discrete units passed from parents to offspring. They contain the information that is required to express a particular trait in an organism. Sutton and Boveri noted that the behaviour of chromosomes during cell division was similar to behaviour of factors and put forward the chromosomal theory of inheritance. Correns and Hugo de Vries rediscovered Mendel’s work independently. Hence option ‘b’ is correct.

FAQs

If hereditary units are related to the term, genes which carry genetic information from one generation to the next. Then, which term will be related to the coloured bodies found inside the nucleus?

Answer: The chromosomes are coloured bodies when stained. They are found inside the nucleus and visible under the microscope. Factors are hereditary units. This term (factors) was used by Mendel which are called genes nowadays. Genes are located on the chromosomes.

How do Sutton and Boveri conclude that genes are located on chromosomes?

Answer: Walter Sutton and Boveri studied the behaviour of chromosomes and genes during meiotic division. They observed that the movement of the chromosome and the gene during meiosis was similar. Based on this observation, they proposed that genes are located on chromosomes only.

Why did Sutton use grasshoppers for his experiments?

Answer: The testes of the lubber grasshoppers were large, so Sutton was able to observe meiotic divisions of the chromosomes. It allowed him to describe the processes that occurred in the cells.