104 



AN INTRODUCTION TO MODERN GENETICS 



gation occurs in the second division can be recognized, as we have seen, 

 by the occurrence of 4-type tetrads from a double heterozygote ; 

 but clearly the occurrence of 4-type tetrads in itself does not indicate 

 which of the two pairs of factors is the one xiaving second division 

 reduction. In Neurospora, however, the arrangement of the ascospores 



I' = 



J- 



ROSSINJG OVER 



} 4 



TELO 



A 



B 



A 

 B 



b 



GAMETES 



■l). 



PHASE 



A 

 B 



A 

 b 



Fig. 49. Diagram of Crossing-Over and First and Second Division Segrega- 

 tion. — On the left, above, are tv^o pairs of chromosomes, both heterozygous, the 

 upper with Aa and the lower with 6b. The chiasmata leave these factors attached 

 to their original centromeres. In the first division telophase A is therefore separated 

 from and 6 from b; the gametes then form a 2-type tetrad, with two >A8's and 

 two ob's. On the right, crossing-over takes place between B and the centromere; 

 at first division telophase A is separated from a, but both daughter nuclei contain 

 6 and b; the gametes form a 4-type tetrad, with AB, Ab, 06, and ab. 



in a row makes it possible to decide this ; if a pair of factors is reduced 

 in the first division one allelomorph must be entirely at one end of 

 the row and the other at the other end. 



The proportion of second division reductions is equal to the propor- 

 tion of cases in which crossing-over has occurred. Now if we consider 

 ordinary crossing-over between linked factors, in each case in which one 

 cross-over occurs, two of the daughter chromatids will show new 

 combinations and the other two the original combinations, so that the 

 proportions of cases of crossing-over is twice the proportion of new 

 combinations ; we therefore have only to divide the proportion of second 

 division reductions by 2 to obtain the cross-over distance between the 



