28 



GENETIC VARIATIONS 



When the individual himself forms germ cells, the two 

 chromosomes of any pair separate into different germ cells; 

 the maternal member of the pair going to one germ cell, the 

 paternal one to another germ cell (Figure 4). Thus each germ 

 cell receives but one chromosome of each pair (and conse- 

 quently but one gene of each pair). Later tvi^o germ cells 

 (sperm and ovum) unite, and as each brings a member of 

 each pair of chromosomes, the new individual or fertilized 

 egg again has its chromosomes (and genes) in pairs, as shown 

 in Figure 4. 



aren 



is 



Germ Cells Fertilized Egg 



Figure 4. Diagram of formation of germ cells and their union to 

 form the fertilized egg, to show the separation of the chromosomes of 

 each pair into different germ cells, and the re-formation of the pairs in 

 the fertilized egg. 



We come now to the accidents and aberrations that give 

 rise to genetic variations. The complex and delicate operations 

 that we have just described do not always take place accu- 



