23-4 



THE YEAST CELL 



The following analysis shows that this is due to the location of 

 the genes N/n on a different chromosome from the genes L/l, but 

 both genes are probably sufficiently near the centromeres of their 

 respective chromosomes so that crossing over between the locus 

 and the centromere is very rare. 



KnLl 



arm) 



>I1 TiL TiL Kl 



Nli 



NL 



Fig. 23-5 Two Kinds of Asci Discovered by Winge and 

 Laustsen in S. Ludwigii. 



Since the centromeres always separate reductionally at the firsi 

 meiotic division, a gene near the centromere usually undergoes re- 

 duction at the first division. The zygote is heterozygous for N/n and 

 L/1, both of which are near the centromeres of two different chromo- 

 somes (fig. 23-6). Just before the first reduction division, the entire 

 chromosome (except the centromere) is duplicated, and the dyads 

 carried to the poles of the first spindle produce two nuclei. It is ob- 

 vious that at the two -nucleate stage only two kinds of asci are possi- 

 ble: (1) asci with an NNLL and an nnll nucleus, or (2) asci with an 

 NNU and an nnLL nucleus (fig. 23-5). Both nuclei are homozygous 

 for either N or n and either L or 1. The first ascus can produce 

 only NL and nl spores. The second ascus can produce only Nl and 

 nL spores. No asci will be found in which all four types of spores 

 are produced. 



The spindles for the second division are oriented so that spores 

 carrying complementary alleles are placed near each other and the 

 fusion of adjacent spores produces heterozygous diploids. 



In a subsequent paper, Winge (1947) extended his genetical anal- 

 ysis and indicated his agreement with the interpretation which I have 

 outlined above. 



