Chromosomal Theory of Inheritance

Introduction

Chromosomes are the coloured bodies(chromo means colour) found inside the nucleolus of cell. Although a number of attempts have been made to explain the phenomenon of inheritance, Mendel remained the most influential and well-known biologist with this regard. This is the reason he was named as the father of genetics. Although Mendel published his work in 1865 but it failed to gain the attention of many modern age scientists. Later on, scientists like, de Vries, Walter Sutton, Theodore Boveri Correns and von Tschermak, have revived the  Mendel`s work to prove that he was right.This article helps to explain the reasons behind Mendel`s success, chromosomes and genes,and experiments for Linkage and Recombination.

 

Reasons, why Mendel`s work had a hard time recognizing

1. Firstly, Mendel worked alone in an isolated setup with no scope for communicating the world about what he was doing .Therefore, his work was poorly and acknowledged hardly in a narrow range of populaces. 

2. The second reason was that 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 that genes are not stable and discrete, instead, they undergo continuous 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 couldn’t accept or understand the mathematical base of his work.

With the advent of modern technology, genetic science has witnessed a lot of discoveries during the 19th and 20th centuries. 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 ) nevertheless, in the case chromosomal disorders, the numbers might  increase or decrease.

Segregate and independently assort at the time of gamete formation to make sure that only one of each pair is transmitted to a gamete

Segregated and independently assort at the time of gamete formation to ensure only one of each pair is transmitted to a gamete

Independent  pairs  segregate independently of each other

One pair segregates independently of another pair

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 hence it was named after them . According to the theory, genes and chromosomes are characterized by a few structural and functional similarities. Some of them are:

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

2. Sutton and Boveri argued that the pairing and separation of a pair of chromosomes would lead to segregation of a pair of factors (gene) they carried. In other words, the separation/segregation of chromosomes consequently result in segregation of genes/factors therefore, it was concluded that both the chromosomes and genes behave similarly. Sutton related his knowledge of chromosomal segregation with that of Mendelian principles and he called it the chromosomal theory of inheritance.

3. Later on, Thomas Hunt Morgan worked on Drosophila melanogaster ( a type of fruit flies) to explain the influence of sexual reproduction on the variations of their offsprings. He carried out a dihybrid crossing between white-eyed and yellow-bodied females against red-eyed and brown-bodied males. Later on, he conducted a Self-crossing of the F1 generation, surprisingly, the resulting  F2 generation was without being the ratio of 9:3:3:1. The outcome exhibited a divergence from Mendel’s dihybrid cross in peas.

 

Concept of Linkage and Recombination-key points

Linkage is the physical association between different genes on a chromosome whereas the term recombination describes the generation of non-parental gene combinations. Morgan and his group found that even when genes were grouped on the same chromosome, some genes are tightly linked, i.e., linkage is stronger between two genes. Linkage is what made humans unique. There cannot be a person exactly the copy of other, this was rationalized by the fact that genes of the 2 persons recombine differently, as result, the resulting combination between the 2 people might slightly change the phenotype. A human somatic cell holds 23 pairs of chromosomes that were transmitted by the parents during reproduction. We can't even speculate on which chromosome has the genes/ alleles that control a particular trait and it is impossible to say which of the 23 homologous pairs of both parents has these alleles. There are approximately  8,324,608 possible genetic combinations within the 23 chromosome pairs which can cause huge variations between the individuals; this is what we all are unique and. Similarly, there are  70,368,744,177,664 genes in humans. Such an enormous number of gene pairs are made possible because of the complex linkage and recombination. 

Experiments for Linkage and Recombination

Loosely arranged genes are more likely to result multiple recombinations and variations. For example, scientists have hybridized the yellow-bodied and white-eyed females drosophila, with brown-bodied and red-eyed males drosophila, the resulting F1-progeny was again inter-crossed. The resulting F2-generation had 98.7% similarity when compared to the parental combination and the recombinants were hardly 1, 3%. In another cross (cross-II) between white-bodied female fly with miniature wing and a yellow body male fly with the normal-wing, the resulting parental combinations were 62.8% and recombinants were 37.2% in F2-generation. Therefore, it was proved that the linkage between genes for yellow-body and white-eyes is stronger than the linkage between the white body and miniature wing. Alfred Sturtevant (Morgan’s student) used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and he ‘mapped’ their position on the chromosome. Genetic maps are now used as a starting point in the sequencing of whole genomes and this is the principle on which human genome sequencing project worked.

 

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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