Chapter 4 



REPRODUCTION AND MEIOSIS 



It is frequently seen that the same character may be pos- 

 sessed by a number of individuals in different generations of a 

 family, and it is reasonable to assume that these individuals 

 must possess the same gene. One of the important problems of 

 genetics is the way these related individuals came to possess the 

 same gene. It happens that this problem of the distribution of 

 genes is probably the best-understood problem in genetics. 



Since the genes are located in the chromosomes, the problem 

 of gene distribution becomes a problem of chromosome distribu- 

 tion, and since chromosomes are found only in cells the whole 

 problem comes down to a study of how cells are transmitted 

 from one generation to another. Long years of study have 

 shown that at no time during the life of an individual does it 

 receive any cells from its parents except at the moment of 

 reproduction. It is therefore important to understand the various 

 methods by which living organisms reproduce before delving 

 into the manner in which genes are distributed. 



The fundamental processes of reproduction are the same in 

 plants and in animals although the details and accessory proc- 

 esses may vary considerably. In the simplest process an entire 

 individual divides into two, but this method is necessarily re- 

 stricted to the very lowest forms of life. In many organisms a 

 piece of an individual consisting of several cells may develop 

 into a new individual, as in the fragmentation of filamentous 

 algae and of certain coelenterates and flatworms, in the formation 

 of buds in Hydra and some sponges, in the gemmules of certain 

 sponges and the gemmae of liverworts, and in various types of 

 vegetative reproduction in the higher plants. None of these 

 methods involves the union of any cells; they are examples of 

 asexual reproduction. 



In contrast to asexual reproduction is sexual reproduction, 

 which involves typically the union of two cells known as germ 



47 . 



