Introduction:

Define: This theory states that chromosomes are linear series of genes. This unifies theory stating that inheritance patterns might be usually described by supposing that genes are positioned in particular sites on chromosomes.

The chromosomal theory of inheritance is the idea that genes, that is, the units of heredity, are physical in nature and are found in the chromosomes. The theory occurs at the turn of the 20th century and became one of the foundations of the modern comprehending of genetics.

Assumption of Chromosome theory of Inheritance:

Hertwiig working in the company of sea urchins and some other investigators working with other organisms proposed that the two equivalent-sized nuclei, one from the sperm and the other from the egg fuse at fertilization. This is despite of the fact that the egg is much bigger than the sperm.  In another words the difference is in the quantity of cytoplasm not the nuclear content. Based partially on this fact and the outcomes of crossing (mating) various kinds, Hertvig and Strasburger as well in the year 1885 advanced the theory that the cell nucleus should include the hereditary materials.

Earlier in the year 1883, Eduoard van Beneden (1846-1910) had discovered in the Parascaris equorum (previously Ascaris megalocephala, such names seems to be still favored) that the fertilized egg of this nematode includes only four chromosomes. Moreover, at the time of fertilization, the sperm and the egg nuclei include two chromosomes each. In the light of this information one could be more particular regarding the equivalent nuclear contribution by both the female and male parent to the zygote. The components of the nucleus which are visibly distributed throughout cell division are the chromosomes. It is thus, quite logical to wind up that because the parents share equivalent numbers of chromosomes, the chromosomes should be the carriers of the hereditary material.

Interpretation devoid of the advantage of knowledge of van Beneden's discovery, Wilhelm Roux (1850-1924), as well in the year 1883, in a purely hypothetical conversation of the importance of the mitotic procedure strongly involved that the chromosomes are the carriers of hereditary materials.

The approach of Roux's was teleological that is, he began from the point of view that there should be a cause for the elaborate mitotic procedure. (For instance, it is teleological to state that we developed eyes as we required seeing). The question in quintessence was 'why must the division of a simple structure similar to nucleus be so complex?'

In accordance to Roux, if one supposed that there are in the nucleus, lots of submicroscopic units that control the life procedures of cell, then it would be comprehensible that great care must be taken in separating the nuclear content.

On other hand, just restriction of the cell would be adequate for dividing the cytoplasm. Roux reasoned that an appropriate technique for making sure a similar distribution of the lots of submicroscopic units into each daughter cell would be for each and every unit to be first divided, and then the sister units would be divided. The jobs of separation and division would though be very much facilitated if the units were arranged similar to beads on a string. There would be some such assemblies, carrying various units, in the cell. Throughout cell division each 'string of beads' would then split longitudinally and the halves would move into split daughter cells. Roux then went on to state that as the mitotic procedure is so complex it should serve a rationale in the organism. The main purpose is the equivalent distribution of the nuclear material significant for the physiological and developmental methods of the cell. We know these days that Roux's units are the genes, the hereditary material, and they are taken on the chromosomes.

In preparing his theory of the Germplasm in the year 1885, Weismann particularly stated that the chromosomes function as carriers of the hereditary units, however the chromosome theory was still to be evidently stated.

Subsequent to the rediscovery of Mendelian Laws in the year 1900, it didn't take long before the genes and chromosomes were recognized. The fact that the observable kind of transmission of chromosomes (that is, the cytological evidence) corresponds to the deduced kind of transmission of genes (that is, the Mendelian Laws of inheritance) was pointed out separately by Sutton and by Boveri in the year 1903.  Their conclusions comprise the Chromosomes Theory of Inheritance. The major points of the theory are as follows:

A) The genes are positioned on chromosomes in such a way that one member of a pair of genes is on one chromosome and the other member is on the partner chromosome, that is, the homologous chromosome with which it synapses in the meiosis.

B) Various pairs of genes are positioned on different chromosomes. This is not to state that there is just one gene on each chromosome. Instead, the point is that non-homologous chromosomes take dissimilar genes. There is more than one gene on each and every chromosome.

The parallels among the genetic and cytological information which form the foundation for the theory are:

a) In diploid organisms, genes take place in pairs and so do chromosomes.

b) Members of the gene pair divide at the time of gamete formation in such a way that each gamete gets only one member of the pair. The similar is true for chromosomes.

c) The members of dissimilar gene pairs recombine at random at the time of segregation throughout the gamete formation.

Sutton and Boveri didn't have equivalent proof for chromosomes however they also didn't have proof to the contrary. Recall the fact that the metaphase-I orientation of one bivalent didn't affect the orientation of the other bivalent. This piece of proof was given later and it confirmed the supposition that the above (c) was also valid to chromosomes.

The most credible confirmation of the theory that genes are on chromosomes was given by Theodor Boveri in his experiments by the sea urchin. Boveri worked with a species in which 2n = 36.  In another words at fertilization each and every gamete adds a haploid number of chromosomes of n = 18. Generally, only one sperm fertilizes an egg however there are rare exemptions in which more than one sperm fertilizes the egg. This condition is termed as polyspermy. It is termed as dispermy if only two sperm are comprised. Polyspermic embryos die early in growth. Boveri discovered that there was huge variability in the time of death and as well in the kind of organ whose abnormal growth led to death.

Other Evidence in Support of the chromosome Theory:

In our assumption of cell division, we discover that the chromosomes in a cell could be considered as sets, in such a way that a diploid cell would encompass two sets of chromosomes. The general terms employed to explain the number of whole sets of chromosomes is 'ploidy'. Ongoing the similar theme, there are euploid aneuploidy conditions. The word euploidy is employed to explain variations in the numbers of whole sets of chromosomes haploid = n; diploid = 2n; triploid = 3n. Such variations that engage whole sets of chromosomes usually outcomes in normal development. Aneuploidy on the other hand signifies to variations in the numbers of individual chromosomes.

These variations provide unbalanced sets of chromosomes. From the statement of Boveri's sea urchin experiments above, it is evident that aneuploidy gives a lot of information in support of the theory that genes are positioned on chromosomes. The similar is true for the declaration that many chromosomes carry dissimilar genes.

In the explanation of chromosomes one frequently talks of karyotype and idiogram. A karyotype is an individual's chromosomes balance in terms of number and size of chromosomes and also the position of the centromere in the dissimilar chromosomes. The idiogram on the other hand is a diagrammatic illustration of an individual's karyotype with the dissimilar chromosomes arranged in order of reducing size.

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