THE PHYSICAL BASIS OF MENDELISM 



367 





I 



Fig. 6j. — Diagram of chromatin inter- 

 change between homologous members of a 

 pair of chromosomes. {From Babcock and 

 Clausen, after Midler.) 



organization of the chromosomes with respect to the chromomeres 

 which they contain, this interchange of material must involve exactly 

 equivalent portions of the two chromosomes. The chromosomes of 



the reduction division shown at 

 F may not, therefore, be identi- 

 cal with the four originally 

 present in /I, but may represent 

 various combinations of portions 

 of both members of a particular 

 pair of chromosomes. The re- 

 sults of such interchange between 

 members of homologous pairs of 

 chromosomes is shown in Fig. 63. 

 At the left is shown a pair of 

 chromosomes, one in outline, the 

 other in full black. In the middle the steps in chromatin interchange 

 are diagrammed and finally at the right this interchange results in 

 a pair of chromosomes each of which is made up of parts of both 

 members of the original pair of chromosomes. Various combinations 

 may result depending on the points at which interchange takes place, 

 but in every case the exchange involves corresponding portions of 

 the two chromosomes. 



Independent distribution of chromosomes. — ^In Fig. 64 are illus- 

 trated diagranmiatically the chromosomes of Drosophila, with particu- 

 lar reference to their size and form relations and to their character- 

 istic pairing in the cell. One member of each of these pairs of chro- 

 mosomes was contributed by the female parent and one member by 

 the male parent. In the reduction divisions these chromosomes are 

 separated so that each germ cell contains one member of each pair of 

 chromosomes. The simplest condition which could obtain is that of 

 independent distribution in each pair of chromosomes such that the 

 particular member of one pair which went to a given pole of the reduc- 

 tion spindle would have no influence on the distribution of the mem- 

 bers of any other pair. Such independent distribution of chromo- 

 somes appears to be actually the type followed in reduction. As a 

 consequence the germ cells contain various combinations of chromo- 

 somes with respect to their original parental derivation. In Fig. 64 

 the types of combinations of maternal and paternal chromosomes and 

 their mode of derivation in Drosophila are shown diagrammatically. 

 Two germ cells, one from the female with the chromosomes in outline, 



