Union of the Hdploid Groups, rertillzalm 



Zygote 



Division of the Diploid Group Mitosis 





:s^-~_'^/} 



Reduction of the Diploid Groups to Hdploid. Melosls. 



Synapsis Disjunction Haploid Groups. Qametes. 





yia.Bb.Cc.Dd. ABCd, 



Recombinations in FertlUz<ation. 



AbcD. etc 



Summary of Chromosome Behavior and Relationship 



In the germ cells, egg and sperm, the chromosomes are not paired. The four 

 shown for illustration are distinguished by relative lengths. The maternal elements 

 are shown In outline and identified as A, B, C, D; and the paternal in black, as a, b, 

 c, d. On fertilization the zygote acquires the double or diploid number of chromo- 

 somes — that is, It now has four pairs, one of each pair from the rriother (egg) 

 and one from the father (sperm). The constitution of the new individual is, Aa,. 

 Bb, Cc, Dd. In the course of normal cell division (Mitosis) each member of each 

 pair divides into two; and each daughter cell has a full complement of chromosomes, 

 continuing: the composition of the zygote. When new germ cells are being formed 

 the diploid number is again reduced (Meiosis), the members of each pair going to two 

 different cells — but the separation is random. Thus, the two new gametes illustrated 

 show the combinations aBCd and AbcD, sixteen different combinations being possi- 

 ble for four chromosomes (4-) When fertilization takes place, 

 with 16 types of sperms and 16 types of eggs, mating Is random and there are 256 

 possible combinations (16^), of which seven are shown. From Wilson, The Cell in 

 Heredity and Development, published by The Macmillan Company." 



Fig. 261. 



