Hapioid cells 



Diploid zygote 



Formation of tetrads 



t gal 



t gal 



t gal 



Fig. 3. Genetic recombination through sexual fusion. The gene for tenn- 

 perature sensitivity is indicated by the symbol t^; f" is its allele and de- 

 notes temperature resistance. We use gal* to denote ability to utilize galac- 

 tose, gal" to denote inability. Thus, cell A is heat sensitive and can utilize 

 galactose; cell B is heat resistant but cannot utilize galactose. The chromo- 

 somes bearing these genes are shuffled together when ceil A and cell B 

 fuse to form the zygote. Each pair of chromosomes then duplicates to 

 yield a quartet or tetrad. The nucleus then divides tv/ice (meiosis I and 11), 

 and in the process reduces the quartets to pairs and the pairs to singles. 

 The random assortment of chromosomes during meiosis I can produce a 

 situation in which the same cell will receive both the gene for temperature 

 resistance and the gene for galactose utilization; that is, a recombination 

 of the parental traits will take place. 

 12 



