1 62 GENETICS 



proportions of descendants that will show certain combina- 

 tions of characteristics, when the combinations occurring in 

 the ancestors are known, as will be brought out in later 

 pages. 



Maps of the chromosomes have been made for only a 

 few organisms thus far, particularly for two or three spe- 

 cies of Drosophila. Beginnings have been made for the 

 chromosomes of maize, the pea plant, the sweet pea, the 

 rat and mouse. For most organisms the experimental data 

 are as yet too scanty to permit even the beginnings of 

 chromosome maps to be made. This is notably the case for 

 man. His 24 pairs of chromosomes, together with the fact 

 that experimental breeding cannot be carried on in man, 

 will make the preparation of maps of his chromosomes a 

 difficult if not impossible task. 



Place and Frequency of Breaks in the Chromosomes. — 

 Certain important points have been discovered as to the 

 frequency of breaks, the number of breaks, and their loca- 

 tion, in the exchanges that occur between the two mem- 

 bers of a pair of chromosomes. The evidence for these 

 cannot be given here, as it is detailed and voluminous, but 

 the main facts may be summarized. It will be recalled that 

 the breaks and exchange occur in the germ cells at the time 

 when the two chromosomes of a pair are side by side as 

 long threads (figure 37, A, B, C). 



1. In the different cells of even the same individual, the 

 chromosomes of any type, as X, or a particular autosome, 

 show breaks in different numbers and locations. 



2. In some of the cells the chromosome does not break, 

 so that there is no exchange. This is as a rule the case in a 

 rather large proportion of the different cells. 



3. In some of the cells the chromosome breaks in but 

 one place, so that, if we designate one of the chromosomes 

 p, the other 7n, a single continuous piece of p including one 



