Essentials of Animal Breeding. 7 



directly, but indirectly it can be most valuable to him, because it 

 explains many things that otherwise are difficult to understand. 



Suppose we are breeding animals in which the number of chromo- 

 somes is only four (two pairs). By the time both egg and sperm 

 cells had gone through the maturing process, and the egg had been 

 fertilized, 16 different combinations would be possible, some of them 



/ 



A 



f 



' 



What Chromosomes Look Like. 

 (Courtesy of Dr. E. E. Carothers.) 



FIG. 1. A remarkable photograph of a group of maturing male germ cells taken 

 through a high-power microscope. The pairs of darkly stained chromosomes may 

 easily be seen in two of the cells in the act of separating to form the nuclei of two 

 new cells. Each of these resulting cells will have only half the original number of 

 chromosomes and will thus be ready for union with a similarly reduced egg cell. 

 Hereditary characteristics are transmitted in the chromosomes. 



The factors which make up the identity of the chromosomes bear very much the 

 same relation to heredity that atoms bear to chemistry. 



no doubt differing very little from each other, but others having 

 little mutual resemblance. In animals such as hogs, however, with 

 20 pairs of chromosomes in each cell, the possible combinations are 

 almost infinite in number. 3 



These combinations are the results of chance. Here we have an 

 egg with 20 pairs of chromosomes, about to mature. Twenty of 

 these chromosomes only will be left when the egg is ready to unite 



8 The number of possible combinations in this particular case is 1,099,511,627,776. 



