258 



CARL C. LINDEGREN 



CHROMOSOMAL INHERITANCE 



In our selected breeding stocks of Saccharomyces, irregular segregations 

 do not occur very frequently. In maize or Drosophila a similar frequency of 

 irregularity would not be detectable since tetrad analysis is not possible in 

 these forms. Using regularly segregating stocks of Saccharomyces we have 

 mapped four and possibly five chromosomes for genes controlling the fer- 

 mentation of carbohydrates and the synthesis of various nutrilites (Fig. 

 16.1). 



HI 



AN 



PN 



AD I 



IN 



PY 



TH 



24 



23 



22 



10 



30 



24 

 AD 2 



26 



ME 



l_ 



40 



45 



I 



22 



PB 



22 



UR 



Fig. 16.1 Chromosome maps of Saccharomyces. 



Chromosome I, PN (pantothenate), centromere, ADl (adenine), IN 

 (inositol), PY (pyridoxine), and TH (thiamin). 



Chromosome II, centromere, G (galactose), AD2 (adenine), ME (meli- 

 biose). 



Chromosome III, centromere, a (mating type). 



Chromosome IV, centromere, PB (paraminobenzoic acid). 



Chromosome V, centromere, UR (uracil). 



Chromosomes IV and V may or may not be different; UR and PB have 

 not been used in the same hybrid. 



HI (histidine) and AN (anthranilic acid) are linked to each other (24 

 morgans) but have not yet been located on a chromosome. 



DIRECT TETRAD ANALYSIS 



The focal point in the life cycle is the reduction division, at which the 

 chromosomes of a diploid cell are sorted out, and the haploid sex cells (such as 

 sperm, eggs, pollen, or yeast spores) are produced. Each diploid parent cell 

 divides twice to produce a tetrad of four haploid sexual nuclei. This process 

 is substantially the same whether a single yeast cell produces four spores or a 

 cell in the testis produces four sperm. In yeast, however, each of the four 

 spores of a single tetrad can produce clones which are available for individual 

 study, and the reduction division can be analyzed directly instead of by in- 

 ference. 



