Chromosome Theory of Inheritance, Biology tutorial

Deduction of Chromosome Theory of Inheritance:

Hertervig working with sea urchins and few other investigators working with other organisms found that two equal-sized nuclei, one from sperm and other from egg fuse at fertilization. This is despite the fact that egg is much larger than sperm. In other words difference in amount of cytoplasm not nuclear content. Based partially on this fact and results of crossing (mating) different kinds Hertervig, and Stransburger also in 1885 advanced theory that cell nucleus should contain hereditary material.

Earlier in 1883, Eduoard Beneden had found in Parascaris Equorum (formerly Ascaris megalocephala - these names appears to be still preferred) that fertilized egg of this nematode has only four chromosomes. In addition, at the time of fertilization, sperm and egg nuclei contain two chromosomes each. In the light of this fact one could be more specific about equal nuclear contribution by both male and female parent to zygote.

Reasoning without advantage of knowledge of van Beneden's discovery, Roux, also in 1883, in a merely hypothetical discussion of importance of mitotic process strongly implied that chromosomes are bearers of hereditary material. Roux's approach was teleological that is he began from the point of view that there should be a reason for detailed mitotic process.

According to Roux, if one supposed that there are in nucleus, very many submicroscopic units that control life processes of cell, then it would be comprehensible that great care must be taken in dividing nuclear content.

On the other hand, simple constriction of cell would be enough for dividing cytoplasm. Roux reasoned that appropriate method for ensuring identical distribution of very several submicroscopic units in each daughter cell would be for each unit to be divided first, and then sister units would be separated. During cell division each string of beads would then divide longitudinally, and halves would move in separate daughter cells. Roux then went on to say that because mitotic process is so complicated it should serve purpose in organism. Purpose is equal distribution of nuclear material significant for physiological and development procedures of cell.

In formulating theory of Germplasm in 1885, Weismann particularly said that chromosomes function as carriers of hereditory units, but chromosome theory was still to be evidently declared. After rediscovery of Medilian Laws in 1900, it didn't take long before parallel becharious of Mendelian foctors (genes) and chromosomes were recognized. The fact that observable kind of transmission of chromosomes (that is cytological evidence) corresponds to deduced kind of transmission of genes (Mendelian Laws of inheritance) was indicated independently by Sutton and by Boveri in 1903. Their conclusions constitute Chromosomes Theory of Inheritance. The main points of theory are:

1. That genes are situated on chromosomes such that one member of pair of genes is on one chromosome and other member is on partner chromosome that is homologous chromosome with which it synapses in meiosis.

2. Different pairs of genes are situated on different chromosomes. This is not to say that there is only one gene on every chromosomes. This is not to say that there is only one gene on each chromosome. Parallels between genetic and cytological facts that form basis for the theory are:

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

b) Members of the gene pair separate at time of gamete formation so that each gamete receives only one member of pair. Same is true for chromosomes.

c) Members of different gene pairs recombine at random at time of segregation in gamete formation.

Sutton and Boveri didn't have corresponding proof for chromosomes but they also didn't have proof to contrary. The most convincing evidence of theory that genes are on chromosomes was given by Boveri in his experiments with sea urchin. Boveri worked with the species in which 2n = 36. Usually, only one sperm fertolizes an egg but there are uncommon exceptions in which more than one sperm fertilizes egg. This condition is known as polyspermy. It is known as dispermy when only two sperm are engaged. Polyspermic embryos die early in development. Boveri found that there was great variability in time of death and also in kind of organ whose abnormal growth led to death. Sea urchin embryo can be divided in four guardrants, each of which arose from one of first four cleavage blastomeres are cells. Boveri seen that four quadrants frequently grow differently, therefore one guardant may be normal and other three abnormal but in various ways and to different degrees. This variability in development of different parts of same embryo was very significant observation by Boveri.

At fertilization in sea urchin sperm contributes the centriole that divides to form two poles i.e. asters of itotic spindle that is formed as asters move apart. Each of the 18 chromosomes contributed by each gamete in normal fertilization becomes duplicated and comes to lie at metaphase plate (equatorial plate). This is normal mitosis. Zygote has 36 chromosomes and to blastomeres are formed as a result of the first cleavage. Following seocnd cleavage total of for blastomeres gives rise to cells that will form one quadrant of embryo. When there is dispermy, two centriole are introduced in egg. Each divides giving rise to two asters. Effect of dispermy is production of four asters in zygote. Four asters are arranged like corners of the square. As there is spindles are fund in zygote such that they are correspond to sides of square. When such a zygote divides, for blastomerses are formed at one in first division. The nucleus of each of these gametes has 18 chromosomes, thus, there will be 54 chromosomes. This is 3n number of chromosomes and it is said to be triploid number.

Boveri was able to illustrate that abnormal development of dispermic embryo was result of erratic chromosme distribution rather than dispermy per se. In other words, dispermy doesn't always lead to abnormal development. Bovery examined his results as follows: He found that the size of a nucleus is dependent on the number of chromosomes present in it. Thus, he compared sizes of nuclei with degree of developmental success (that is degree of normal development) in each quadrant of embryo and with degree of developmental success in quadrants having similar-sized nuclei in other embryo.

Comparison of Development in Two Dispermic Embryos:

 

Embryo A

Embryo B

Number Size

Quadrants

Quadrants

 

I

II

III

IV

I

II

III

IV

1

 

+

 

 

 

 

111

 

2

1111

 

+

 

 

11

 

 

3

 

 

 

 

 

 

 

+

4

 

 

 

11

+

 

 

 

From table it is seen that similar-sized nuclei may result in various abnormalities, therefore different degrees of developmental success. Boveri thus concluded that variability in development is the reflection of qualitative rather than quantitative differences between nuclei in different quadrants. For example if growth were dependent on nuclear size only, quadrants I and III containing similar-sized nuclei must have had similar degrees of developmental success.

Other proof in Support of chromosome Theory:

Chromosomes in cell could be considered as sets, such that diploid cell would have 2 sets of chromosmes. The general terms utilized to explain number of whole sets of chromosomes is ploidy. Continuing on same theme, there are euploid aneuploidy conditions. Term euploidy is utilized to explain variations in numbers of complete sets of chromosomes haploid = n; diploid = 2n; traploid = 3n. These variations that involve complete sets of chromosomes usually result in normal development. Aneuploidy in contrast refers to variations in numbers of individual chromosomes. Such variations provide unbalanced sets of chromosomes. From discussion of Boveri's sea urchin experiments above, it is obvious that aneuploidy gives lot of information in support of theory that genes are situated on chromosomes. The same is true for assertion that different chromosomes carry different genes.

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