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| Chromosomal Basis of Inheritance |
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| Heredity depends upon the protoplasmic continuity between the parent and the offspring. It is the gametes that establish the continuity between parent and offspring and hence, the mechanism of inheritance operates across this exceedingly slender protoplasmic bridge. The machinery of inheritance lies mainly in the nucleus or more particularly the chromosomes of the cells which form the gametes. The chromosomes in fact are the only entities that are always passed on in equal quantities from parents to offspring, during sexual reproduction. The rediscovery of Mendelism in 1900 and the subsequent developments in the field of genetics, very clearly established the importance of chromosomes in the inheritance of characters. This led to the establishment of the chromosomal theory of inheritance. |
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| With the rediscovery of Mendelism, several scientists focussed on identifying the basis for inheritance. William Sutton recognised a close parallelism between the transmission of 'factors' identified by Mendel and the behaviour of chromosomes during meiosis and fertilization. These ideas can be summarised as follows. |
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 Mendel assumed that hereditary factors exist in pairs. It is now established that chromosomes occur in pairs in the body cells. |
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 Mendel proposed the idea of separation of paired factors during the formation of gametes, in the law of segregation (law of purity of gametes). It is now established that the paired homologous chromosomes separate during meiosis in such a way that each gamete receives one chromosome of each homologous pair. |
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 Mendel proposed the idea that the factors responsible for traits of each pair are independent of every other pair in the process of their distribution into the gametes (law of independent assortment). It is now known that during meiosis, the chromosomes of various homologous pairs assort at random so that the chromosomes of each pair segregate independently of the chromosomes of every other pair. |
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 Mendel firmly believed that the parental characters mix and express in the offspring. It is now known that the homologous chromosomes from the two parents come together in the zygote as result of fusion of male and female gametes. |
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 Mendel held the view that characters are not lost completely, even when they do not express. The chromosomes retain their structure and identity throughout the life of an organism, irrespective of whether they are noticeable or not. |
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| Thus, the behaviour of chromosomes particularly during meiosis and fertilisation provides a firm basis for explaining Mendel's laws of inheritance. |
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