BIOLOGY IN HUMAN AFFAIRS 



by the experimental data. It was found in the behavior of 

 the chromosomes during sexual reproduction. 



The essential feature of sexual reproduction is the for- 

 mation of a new organism by the union of two cells, the 

 egg and the sperm. Now the egg is usually very much 

 larger than the sperm, yet the tiny sperm is just as effective 

 in transmitting genes to the offspring as the huge egg. The 

 physical basis of inheritance, therefore, must lie in some 

 cell component present in equal quantity in both germ 

 cells. That component is the chromatin. The chromatin 

 undergoes numerous changes during the life history of a 

 cell, but at some point in the process it always fragments 

 into bodies known as chromosomes, having a definite 

 number, shape, and size for each species. There are twenty 

 in corn, forty-two in certain wheats, forty-four in rabbits, 

 forty-eight in man. 



The chromosomes are the chief performers in ordinary 

 cell division (mitosis). They initiate the process; and they 

 alone, among the constituents of the cell, go through an 

 exact dichotomy. Probably they also have a good deal to 

 do with directing growth and differentiation; though 

 what their precise duties are in this connection, is still 

 questionable. But their really star role is associated with 

 reproduction. During the preparation of the germ cells for 

 fertilization (meiosis), their behavior is precisely what 

 is required by the data from all crossing experiments for 

 carriers of the genes. 



As the cells of every organism are characterized by a 

 definite number of these bodies, and as every new individual 

 is ordinarily produced by the union of two cells, it follows 

 that some provision must be made for reducing chromosome 

 numbers prior to fertilization. What happens is virtually 

 this: when either type of germ cell matures, the chromo- 

 somes line up in pairs, and one member of each pair passes 

 to a daughter cell. Thus, if there are two pairs of chromo- 



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