Chromosomal Theory of Inheritance

Introduction

Chromosomes are the coloured bodies( chromo refers to colour) found inside the nuclear material of each cell. Many biologists have proposed theories to explain the phenomenon of inheritance, but Mendel has made a significant impact. 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 reopened Mendel`s work to prove the facts.

 

Table of Content

1. Introduction

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

3. Similarities between genes and chromosomes

4. Boveri–Sutton`s chromosome theory

5. 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 showing or communicating on what he was doing. 

2. The second reason was that his concept of genes or factors, as a stable and discrete unit enabling 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 technology, genetic science has seen a lot of inventions during the 19th and 20th centuries. By 1900, scientists like de Vries, Correns and von Tschermak have reworked on the Mendelian results. In the year 1902, they have clearly noted that, during mitosis, there is chromosomal movement, 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 certain way on a similar basis( table below explains the relative differences between genes and chromosomes). 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 in pairs in the form of alleles

Occur in pairs, rarely in triplets or singles indicating chromosomal disorders.

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 become genes/DNA.

 

Boveri–Sutton`s chromosome theory

The theory was proposed by Walter Sutton and Theodor Boveri hence the name Boveri–Sutton`s chromosome theory. According to this theory, there are similarities in the structural, numerical and behavioural patterns between the genes and the chromosomes. Some of the similarities between genes and chromosomes are:

1. 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 will result in the segregation of genes/factors proved the fact that there is a similarity between the movements of genes and chromosomes. Sutton related the 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. He again dod 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

1. Linkage is the physical association of genes on a chromosome and 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.

2. Linkage is what made humans unique. There is no one person exactly the copy of others, this is because of the variation resulted from the transfer of recombination of factors from parents to the progeny. 

3. An individual human somatic cell has 23 pairs of chromosomes and the same has happened when the gamete has formed where each of the 23 pairs came from the mother and father through meiosis. In a meiotic division, the paired homologous chromosomes segregate to ensure that each gamete has only one of the pair of alleles for an individual trait.

4. The probability and choice decide which chromosome has the genes/ alleles that are controlling a particular trait and it is impossible to say which of the 23 homologous pairs of both parent has these alleles. There are around 8,324,608 possible genetic combinations of 23 chromosome pairs and this will lead to a state where 2 gametes virtually never have exactly the same combination of chromosomes.  An individual chromosome also has thousands of different genes as we already discussed genes that follow similar characteristics of chromosomes.

5. There are about 70,368,744,177,664 genes in humans .Such a vast number of genes was made possible because of linkage and recombination.

6. It’ is a normal rule that the frequency of recombination is greater if the genes are loosely linked, for example, in the case of Drosophila, scientists have hybridized the yellow-bodied and white-eyed females with brown-bodied and red-eyed males and the resulting F1-progeny has been inter-crossed again. The resulting F2-generation had parental combination of 98.7% 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, 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.

7. 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.

Linkage and recombination





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