76 PHYSICAL BASIS OF HEREDITY 



producing sperm). The iirst double chromosome (No. 1) 

 has a different mode of fibre attachment to each half, 

 but the halves are here not different in size. In 

 five cases the chromosome with terminal attachment is 

 going to the cell that will get the sex-chromosome (the 

 upper one here), while in three cases it goes to the pole 

 that will not get the sex-chromosome. Chromosomes 7 

 and 8 are slightly different in size, but this is not distin- 

 guishable in these figures. In the first four cells {viz., 

 h, c, d, e) the halves of 7 and 8 with subterminal attach- 

 ments are going to opposite poles ; in the remaining four 

 cells {f, g, h, i) they are going to the same pole. Again, if 

 we compare Xos. 7 and 8 with No. 1 it is found that in four 

 cells (/, g, h, i) the half with terminal attachment passes 

 into the cell with the same attachment (/ and i) (for 7 

 and 8), and the other half into the cell w^ith the other 

 attachment (g and h). In other words, the distribution 

 for four chromosomes pairs (1, 4, 7, 8) is here a random 

 assortment. Let A, B, C represent the chromosomes with 

 one kind of attachment, and a, h, c their mates with the 

 other kind of attachment. D is the sex-chromosome and 

 d its absence. There will then be sixteen possible assort- 

 ments of these four, all equally probable. Thus : 



ABCB aBCD ahCB ahcB ahcd 



AhCB AhcB ahCd 



ABcB ABcd ciBcd 



ABCd aBcB Ahcd 

 aBCd 

 AhCd 



There were 100 spermatocytes recorded by Miss Carothers 

 as to the distribution of their chromosomes to the two 

 poles, giving data for 200 cells. Their distribution as 

 well as the expectation for free assortment is as follows : 



Expected Realized 



Only one chromosome with sub-terminal attachment. }4- X 200=50 48 

 Any two chromosomes with sub-terminal attachment. H X 200=75 84 

 Any three clnomosomes with sub- terminal attachment. j<( X 200 =50 48 

 All four chromosomes with sub-terminal attachment, y^ X 200 = 123^ 8 



