30 



reducing division is purely a matter of 

 chance— that is, that any chromosome 

 pair may lie with maternal or paternal 

 chromatid indifferently toward either 

 pole irrespective of the positions of 

 other pairs— and hence that a large 

 number of different combinations of 

 maternal and paternal chromosomes 

 are possible in the mature germ-prod- 

 ucts of an individual. To illustrate this, 

 we may consider a form having eight 

 chromosomes in the somatic and pre- 

 synaptic germ-cells and consequently 

 four in the ripe germ-products. The 

 germ-cell series of the species in gen- 

 eral may be designated by the letters 

 A, B, C, D, and any cleavage nucleus 

 may be considered as containing chro- 

 mosomes A, B, C, D from the father 

 and a, b, c, d, from the mother. Synap- 

 sis being the union of homologues 

 would result in the formation of the 

 bivalent chromosomes Aa, Bb, Cc, Dd, 

 which would again be resolved into 

 their components by the reducing 

 division. Each of the ripe germ-cells 

 arising from the reduction divisions 

 must receive one member from each 



SUTTON 



of the synaptic pairs, but there are 

 sixteen possible combinations of ma- 

 ternal and paternal chromosomes that 

 will form a complete series, to wit: a, 



B, C, D; A, b, C, D; A, B, c, D; A, B, 



C, d; a, b, C, D; a, B, c, D; a, B, C, d; 

 a, b, c, d; and their conjugates A, b, c, 

 d; a, B, c, d; a, b, C, d; a, b, c, D; A, B, 

 c, d; A, b, C, d; A, b, c, D; A, B, C, D. 

 Hence instead of two kinds of gametes 

 an organism with four chromosomes in 

 its reduced series may give rise to 16 

 different kinds; and the offspring of 

 two unrelated individuals may present 

 16 X 16 or 256 combinations, instead 

 of the four to which it would be 

 limited by a hypothesis of parental 

 purity of gametes. Few organisms, 

 moreover, have so few as 8 chromo- 

 somes, and since each additional pair 

 doubles the number of possible com- 

 binations in the germ-products ^ and 

 quadruples that of the zygotes it is 



^ The number of possible combinations in 

 the germ-products of a single individual of 

 any species is represented by the simple 

 formula 2" in which n represents the number 

 of chromosomes in the reduced series. 



Table 1 



