Lakhasly

Chromosomal theory of inheritance Although the relationship between chromosomes and genes was suspected at the time of Mendel's work, it was rediscovered when Weisman (1900) put forward the view that an individual's heredity endowment, carried on the chromosomes, is halved in each gamete. It took further three years to establish the interrelationship between genes and chromosomes which has persisted to the present time. During that period Montgomery (1901) showed that chromosomes occur in distinct pairs and that the synapsis involves the union of maternal and paternal chromosomes. Boveri (1902) showed by his studies on polyspermy in the fertilization of sea urchin eggs that the chromosomes of an individual were not equivalent to one another, and a full complement is necessary for the normal development of the cell. Correns and Cannon both in 1902 pointed out the close parallelism between Mendelian segregation and chromosome number reduction during meiosis concluding that the genes are on the chromosomes. In the same year, Guyer showed an understanding that a random assortment of different pairs of chromosomes would give the independent assortment of genes required by Mendel. It was, however, Sutton (1903) who brought together the data from cytology and genetics to clearly show the role of the chromosomes in heredity. Boveri in the same year published a paper in which he advanced many of the same ideas to that hypothesis correlating genes and chromosome transmission. The hypothesis is therefore known as the Sutton-Boveri hypothesis and is summarized in the following points: 1. In somatic cells there are two similar groups of chromosomes; one of maternal and one of paternal origin. This occurrence of chromosomes in homologous pairs parallels the occurrence of genes in pairs. 2. The chromosomes retain a morphological individuality throughout the various cell division stages that is transmitted across generations and genes show a similar continuity. 3. During meiosis homologous pairs of chromosomes are brought together and then the members of each pair segregate into different gametes independently of the members of other pairs, Mendelian genes segregate independently at sometimes prior to gamete formation. 4. Each chromosome or chromosomes pair has a definite role in life and development of the individual, so has each gene. In addition to establishing the relationship between genes and chromosomes, Sutton recognized that there must be non-independent assortment of some genes, otherwise, as he noted, the number of distinct characters could not exceed the number of chromosomes. The association of a particular inherited character with a particular chromosome was made between 1906 and 1910 by McClung, who showed that Hemiptera and Orthoptera females have one more chromosome than the males. This so-called X chromosome occur in all eggs of a female but in only 50% of male sperms, so that half of the resultant zygote are XX and female, while the other half are XO and males. The presence of a small Y chromosome, partially homologous with the X chromosomes in males of beetles,