492 THE BRIDGE OF LIFE 



know that genes may change at rare intervals through mutation and 

 thus something new may appear, but mutations are random things 

 which bear no relation to the activities of the organism carrying the 

 genes. 



We could not close a historical account of the development of the 

 science of genetics without mentioning the one man who did more than 

 any other to start modern research in the field of heredity. This man 

 was Gregor Mendel, a monk who lived in a monastery in Brunn in 

 central Europe during the latter half of the nineteenth century. Through 

 an extensive series of breeding experiments with garden peas, Mendel 

 worked out the method of gene transmission which we now know applies 

 to most forms of life. 



With this background on the development of thinking on the physi- 

 cal mechanism of heredity we can now turn our attention to a more 

 detailed study of the bridge of heredity. 



Meiosis 



In Chapter 2 the method of cell duplication through mitosis was 

 described in some detail. In that chapter it was pointed out that a 

 special type of mitosis, known as meiosis, was necessary when gametes 

 were produced or there would be a doubling of the chromosome number 

 each generation when sperms and eggs united. We will now consider 

 this process of meiosis more fully and learn how it is connected with 

 the mechanism of heredity. It will be well to review Chapter 2 care- 

 fully before preceding with this study, because a thorough knowledge 

 of mitosis is necessary for an understanding of meiosis. 



It should be kept in mind that the primary accomplishment of meiosis 

 is the production of cells with only one half of the chromosome number 

 which is normally found in the body cells. This is done by a series of 

 two cell divisions. The process is essentially the same for all animals, 

 but let us take man as a specific example. Man has 48 chromosomes in 

 his body cells. These are of assorted sizes and shapes, but it can be 

 noted under the microscope that there are two of each kind. In other 

 words, each chromosome will have a homologous mate like itself in 

 size and shape which contains similar genes. (In the male there is one 

 exception to this — one pair of chromosomes are somewhat dissimilar. 

 This pair is connected with sex determination.) 



In the prophase of ordinary mitosis, each of these chromosomes be- 

 comes doubled ; then they line up at the metaphase plate and are pulled 

 apart so that a full 48 chromosomes goes to each of the daughter cells 

 which is formed. In meiosis, on the other hand, there exists some at- 



