Chromosome theory of heredity
In the last quarter of the nineteenth century, Van Beneden, Strassburger, and Fleming carefully observed chromosomes under the microscope. It was seen that chromosomes occur in pairs and that members of a pair separate from each other at the reduction division.
The chromosome theory was arrived soon after the rediscovery of Mendel's principle in 1900. Boveri and Sutton, were among the first to point out parallelism in the behavior of chromosomes and genes and to suggest that the chromosomes continued the genes. E.B Wilson also at the same time advocated the chromosome theory. In 1911 and later, Morgan in collaboration with Bridges, Muller and Strutevant presented more strong experimental evidences in support of the chromosome theory, leading to its final acceptance by geneticists in general.
It was observed that a) both chromosomes and genes occur in pairs, b) the members of a pair, chromosomes or genes, separate at the reduction division, and c) the members of different pairs assort independently of one another. This parallelism led to the conclusion that the chromosomes contain the gene.
chromosome theory
The foregoing considerations clearly demonstrate that genes are located in chromosomes. But before the chromosome theory of inheritance can be accepted, more prerequisites of the theory must be verified. They are fallowing
I) Continuity of the chromosomes:
At interphase the individual chromosomes are not visible but look long interweaved and attenuated chromosome material or chromatin of the cell. The German zoologist Rabl noted that during cell division the configurations of the chromosomes were again visible in the succeeding prophase, all had their apices oriented in the same manner as in the proceeding anaphase. This observation is in consistent with the hypothesis that the chromosomes are individual and continues from one cell division to the next.
Theodor Boveri in 1889 observed that the nuclei of Ascaris megalocephala, a parasitic roundworm, commonly show a number of finger-shaped lobes formed at telophase by the free ends of the V-shaped chromosomes. There were several arrangements among the chromosomes but sister cells are identical in the number and arrangements among the chromosomes but sister cells are identical in the number and shape of nuclear pockets formed. When the chromosomes separated at anaphase, the ends of the chromosomes tend to stay attached to each other and were the last parts of the chromosome to separate while advancing towards the respective poles. In the following prophase the chromosomes always reappears with their free ends lying in lobes in the same way as observed in the preceding telophase. From this evidence Boveri concluded that it was unlikely that a continues chromosome thread would break up during interphase and redistributed as separate chromosomes.
Bridges first presented the most convincing evidence in favour of this hypothesis in 1917 who showed a deletion of the X chromosome during cell division. From such cells where a chromosome was lost or broken, the deficiency is always repeated exactly in subsequent cell generations.
II) Pairing between homologous chromosomes
In 1900, Winiwater found in the ovary of a half day old rabbit that during meiosis homologous chromosomes paired side by side for a part of their length. Montogomery, after a study of fourty two species of insects in 1901, concluded that chromosomes in somatic cells occured in pairs and each association at zygotene involved one paternal and one maternal chromosome. During meiosis he saw that the largest chromosomes always paired with the largest chromosome, the smallest with the smallest, and so on but it always involved identical homologous chromosomes. If diploid chromosome number was fourteen, only seven bivalents were observed. This same conclusion was reached by Sutton in grasshoppers. The male grasshopper has twenty three chromosomes in somatic cells, and these chromosomes differed very much in size. Sutton observed that apart from one X-chromosome, 11 paired associations were present in each nucleus and showed same size difference as at mitosis. Although due to degree of contraction, absolute length of paired chromosomes differed at different stages of meosis, but their relative lengths were constant. These confirmed Montogomery's conclusion that associations were bought about only between paternal and maternal chromosomes.
III) Qualitative difference between the chromosomes:
Montogomery also observed size and shape difference in the chromosomes of Hemipterans, or true bugs. Experiments by Boveri on sea urchin embryonic devellopment led him to conclude that each chromosome of a haploid set is qualitatively different and possess different qualities. He also showed that presence of a complete set of chromosome was important for survival. These proved that somatic chromosome number is made up of two identical haploid sets, and that each chromosome within a haploid set is different from the rest.
IV) Chromosome pairs recombine independently:
E. E Carothers observed in an insect orthopteran, Brachystola, a homologous chromosome pair that synapsed during meiosis but were of unequal size. Each member of the pair of chromosomes could be visibly recognized in male spermatids, as well as the presence or absence of the X chromosome. Thus a male segregating independently for both kinds of chromosomes should form four types of sperm in equal numbers: XH, Xh,OH, and Oh. Indeed, Carothers found there were one fifty four each of XH, Oh, and one fourty six of Xh, Oh, in a particular testis, thereby furnishing cytological evidence for independent assortment.
The foregoing considerations clearly demonstrate that chromosome are the carriers of the genetic material. This conclusion is one of the fundamental principle of genetics.