256 Experimental Transformation of Species 



finally revealed that there are four distinct groups. To show 

 that these four groups of characters correspond to the four 

 pairs of chromosomes was not so simple a matter. A clue 



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Fig. 68. Recombinations with Independent 



Assortment 



Let us assume four chromosomes in the germ cells of a plant or ani- 

 mal — A, B, C, D in the egg and a, b, c, d in the sperm. On fertilization 

 the zygote is formed with four pairs of chromosomes, Aa, Bb, Cc, Dd. 

 This represents the constitution of the new individual. If the capital and 

 small letters of each pair stand for alternative characters in the parents, the 

 symbols for the zygote represent the constitution of the hybrid. When 

 new gametes are being formed the chromosomes of each pair become segre- 

 gated in the course of the reduction division — that is, each germ cell has 

 only four chromosomes instead of four pairs of chromosomes. There are 

 thus sixteen combinations possible (4^). Only one of the sixteen derives 

 all of its germinal material from the mother (ABCD) and only one from 

 the father (abed). These theoretically possible recombinations are in per- 

 fect agreement with the results of Mendelian segregation. From Wilson, 

 The Cell in Heredity and Development, published by The Macmillan 

 Company. 



which led to the unraveling of this complicated problem 

 was found after it was learned that the sex of the indi- 

 vidual is determined by the chromosomes and that certain 



