Chromosomal Theory of Inheritance

Untill 1800s, information related to the molecular behaviour of genes remained unclear but the use of microscope after 1840 made things clear to Biologists. By using microscope, cell biologists could stain and watch for subcellular structures and their actions during cell division and meiosis. Chromosomes are the coloured bodies found inside the nucleolus of cells. They are easily attracted to some stains making it easy for the biologists to observe. Prior to the Mendel`s time, a number of attempts were made to explain the phenomenon of inheritance, but Mendel remained the most influential scientist to do this. This is the reason he was named father of genetics. 

Mendel`s work 

Although Mendel published his work in 1865, it failed to gain the attention of many modern age scientists because of many reasons. Mendel clealry argued that the chromosomes act as a key to the genetic behaviour. This has led many scientists to re-examins examine Mendel’s publications and re-evaluate his model in by basing the behaviour of chromosomes during cell division. In 1902, Theodor Boveri identified that proper embryonic development of sea urchins id due to the chromosomes. In the same year,  separation of chromosomes into daughter cells was microscopically observed by Walter Sutton. These 2 observations led to the development of the Chromosomal Theory of Inheritance. According to this theory, chromosomes are the basis for genetic behaviour responsible for Mendelian inheritance. Prior to this, it is important to understand some  reasons why Mendel`s work was inrecognized. They are:

1. Firstly, Mendel worked alone in an isolated setup in which he couldn't communicate his findings to the world in a timely manner. This has led to the poor acknowledgment by the people. 

2. His concept of genes or factors, as a stable and discrete units facilitating the expression of traits through a pair of alleles, did not ‘blend’ with the principles of modern-day scientists. Many geneticists have argued in a contradicting manner stating that, genes are not stable and discrete as they are constantly influenced by variation.

3. The last reason is that, his mathematical approach to explain the biological phenomena was totally new and many disliked it because most of them were biologists who failed to accept the mathematical base of his work.

Basis for Mendel`s experiments

Genetic science has witnessed a lot of discoveries during the 19th and 20th centuries due to the advent of modern technology. By 1900, scientists like de Vries, Correns and von Tschermak have edited the Mendelian results. In the year 1902, they have clearly noted that, during mitosis,  chromosomal movement is apparent, which further added a strong foundation for Mendel`s postulates.  Scientists like  Walter Sutton and Theodore Boveri found that the chromosomes and genes behave in a  similar way resulting in the cell division. All these led to the development of a theory called Boveri–Sutton chromosome theory –also known as chromosome theory of inheritance.

Similarities between genes and chromosomes

Genes  

Chromosomes

Occur as a pair of alleles

Usually occur in pairs( 23 pairs ) but the numbers might  increase or decrease due to chromosomal defects.

Genes are segregated and independently assorted at the time of gamete formation to transmit only one pair from each parent.

Chromosomes also segregated and independently assorted at the time of gamete formation to transmit only one pair from each parent.

Genes has subunits called genomes

The subunits of chromosomes are genes/DNA.

Boveri–Sutton`s chromosome theory

The theory was proposed by Walter Sutton and Theodor Boveri. According to this, genes and chromosomes are distinguished by their structural and functional similarities , some of these are :

1. Both chromosomes and genes come in pairs. The 2 alleles of a gene pair are located on the same locus of a 2 chromosomes that come in pairs (homologous chromosomes).

2. Sutton and Boveri argued that the pairing and separation of a pair of chromosomes would also lead to segregation or pairing of factors (genes) so, it was concluded that both chromosomes and genes behave similarly. Sutton related his knowledge of chromosomal segregation with that of Mendelian principles and he named this as the chromosomal theory of inheritance.

3. Later on, Thomas Hunt Morgan worked on Drosophila melanogaste, a type of fruit fly to explain the influence of sexual reproduction to cause variations in their offsprings. He carried out a dihybrid crossing between white-eyed, yellow-bodied females and red-eyed, brown-bodied males. For which he conducted a self-crossing of the F1 generation which resulted in the offerings ratio of 9:3:3:1. The outcome exhibited a divergence from Mendel’s dihybrid cross in peas.

Linkage and Recombination

Morgan recognised that,  as per Mendel’s law,  2 genes failed to segregate normally while crossing. Furthermore, he estimated that the likelihood of receiving the parental combinations is higher than the non-parental combination if 2 genes were present on the same chromosome. Such physical association between genes was termed as linkage. Due to the high frequency of linkage and recombination, there cannot be a person exactly the copy of others. Also, such dissimilarity between 2 persons is possible because the genes in them recombine differently causing diffrent phenotypical expression from each other. Our cells contain 23 pairs of chromosomes transmitted by our parents during reproduction. Out of these, it is difficult to speculate on which chromosome has the genes/ alleles that control a particular trait. Adding to this, humans have around 8,324,608 possible genetic combinations due to the enormous number of genes (70,368,744,177,6640) which provides a clue about high degree of recombinations.

 

Types of Linkage

There are 2 types of linkages : complete and incomplete. Complete linkage occurs when 2 or more combination of characters appears together in more than two generations in a consistent manner while in an incomplete linkage, new gene combinations are freshly formed in offsprings due to chiasma or crossing over between the linked genes.

 

Observations of Chromosomal Theory of Inheritance

  1. During meiosis, 2 or more pairs of homologous chromosome move as discrete structures independently from other pairs of chromosomes.

  2. Though eggs and sperms vary in their size and morphology, the number of chromosomes on both of them are same which result in the equal genetic contribution from each parent.

  3. The gametic chromosomes fuse during fertilization to produce offspring with the same number of a chromosome as their parents.

  4. Chromosomes distribute in a random manner into the pre-gametes from each homologous pair.

  5. Each parent synthesizes only half of their chromosomal complement.

Read More

  1. Chromosomes-structure and functions

  2. Disorders And Defects Of Chromosomes

  3. Chromosomal theory of inheritance

  4. Gene mutations

Check your understanding

  1. Why Mendel`s work was not recognized in the beginning?

  2. Write any 4 similarities between the genes and chromosomes.

  3. Why the linkage and recombination are essential?

  4. How chromosomes help in the inheritance?

 



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