2 MENDELIAN SEGREGATION 



carries a definite number of bodies called chromo- 

 somes. The egg carries the same number. Conse- 

 quently, when the sperm unites with the egg, the 

 fertilized egg will contain the double number of 

 chromosomes. For each chromosome contributed by 

 the sperm there is a corresponding chromosome con- 

 tributed by the egg, i.e., there are two chromosomes 

 of each kind, which together constitute a pair. 



When the egg divides (Fig. 1, a-d), every chromo- 

 some splits into two chromosomes, and these two 

 daughter chromosomes then move apart, going to 

 opposite poles of the dividing cell (Fig. 1, c). Thus 

 each daughter cell (Fig. 1, d) receives one of the 

 daughter chromosomes formed from each original 

 chromosome. The same process occurs in all cell 

 divisions, so that all the cells of the animal or plant 

 come to contain the double set of chromosomes. 



The germ cells also have at first the double set of 

 chromosomes, but when they are ready to go through 

 the last stages of their transformation into sperm 

 or eggs the chromosomes unite in pairs (Fig. 1, e). 

 Then follows a different kind of division (Fig. 1 , /) 

 at which the chromosomes do not split but the 

 members of each pair of chromosomes separate and 

 each member goes into one of the daughter cells 

 (Fig. 1 , g, h) . As a result each mature germ cell 

 receives one or the other member of every pair of 

 chromosomes and the number is reduced to half. 

 Thus the behavior of the chromosomes parallels the 

 behavior of the Mendelian units, for in the germ cells 

 each unit derived from the father separates from the 



