76 PHYSICAL BASIS OF HEREDITY 



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

 has a different m.ode 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 (/, g, h, i) they are going to the same pole. Again, if 

 we compare Nos. 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 with 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, aU equally probable. Thus : 



ABCD aBCD ahCD abcD abed 



AbCD AhcD abCd 



ABcD ABcd aBcd 



ABCd aBcD Abed 



aBCd 



AbCd 



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. X X 200=50 48 

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

 Any three chromosomes with sub-terminal attachment. X X 200 =50 48 

 Any four chromosomes with sub-terminal attachment. j\ X 200 = 12 J^ 8 



