This classic article describes
chromosomes as unique carriers of heredity. Specific characters transmitted from parent to offspring develop in
germ cells of an individual. The developmental process in germ cells takes place in accordance to two defined theories (i) the particulate theory which believes the egg/sperm contain particles of all characters. (ii) the reaction theory, which suggests that
egg or sperms undergo development in various stages.
Theory (i) seems to be more practically relevant in context of development or heredity. All germ cells posses a fundamental property to undergo various chemical changes that lead to the formation of an adult. The “egg” is a major entity in which developmental changes take place. In terms of transmission of characters, the male germ cells i.e., sperms contribute equally as the egg. The “chromosomes” have survived as crucial carriers of hereditary traits and their transmission through successive generations. A faithful segregation and transmission of chromosomes helps the
expression of characters. Dynamic changes in the cytoplasm of a fertilized embryo are the real players that bring about transformation of egg and the chromosomes remain constant thereafter (defined as “genetic continuity of chromosomes” or “individuality of chromosomes”). The process of development depends on cells, which bear characteristically sampled chromosomes. In fact, the idea to term chromosomes as “assimilating centers” capable of accumulating characters and setting them free in time is ideal as it helps us understand with simplicity what chromosomes really do in a cellular microenvironment. Important clues of how chromosomes really function i.e. segregate/assort have come from studies on embryology in different animal model systems. The quick synapsis of chromosomes and their separation referred to as “touch and go” are very important processes in this cellular event. But there are some unique cases, which do pose a question on this phenomenon and consequently the evolution of various complexities.
Restating the principles of genetics (Mendelism), in this article, Morgan explains “segregation” and “independent assortment” in numerical proportion/s and relevant capacity in any given genetic cross. In short, explaining the process of reduction in number of chromosomes to half from somatic to germ cells (realized by different theories of Sutton & Boveri, Bateson and Weismann), Morgan has also interestingly mentioned a “presence and absence” theory. This is basically suggestive of the fact that it is either the
presence or absence of character/s, which is the final determinant for origin of a new race, after all. On the other hand, the idea of quantitative concept suggests that a sufficient amount of genetic material should be present in any resultant product for its genuine expression. Single dose of a dominating
character is enough to
Express and a double dose of recessive trait is required to express in the subsequent generation. This has been very nicely explained by Morgan illustrating several examples (for details, see the article), however, exceptionally, the dominating characters do not express fully but may need another dose to express in the next generation. Thus, it is the presence/absence of characters or quantity of genetic information, which determines the expression of a specific character. The failure of expression of a dominating character may result from inadequate quantity of the relevant chromatin rather than due to the presence of a blocking or inhibitory factor, and; it is the blending of characters from two parental contributions that adds up to form the hybrid character. The local factors, however may also affect the outcome in some cases. An earlier view that the sex of an individual is determined by external factors is completely disregarded and that the sex determination is an absolutely intrinsic in character is widely accepted (but with some exceptions viz., some parthenogeneetic animals which depend on food and other environmental factors). However, there are other internal mechanisms that regulate this effect, pointing to the fact that the process of sex determination resides and takes place in chromosomes of germ cells. Important revelations made through different experimentations and interpretations on germ cells have now conclusively proved that a very complicated problem has been finally solved.
More abstracts about the Chromosomes and Heredity