Chapter 25 



PROOF OF INTERCHROMOSOMAL TRANSFER OF HEREDITARY 

 MATERIAL BY TETRAD ANALYSIS 



Sixty asci from a hybrid heterozygous for a/ a , AD/ad, G/g, 

 IN/in, PN/pn, PY/py were analyzed and in 57 all eight alleles seg- 

 regated regularly 2:2. The first 4 asci in fig. 25-1 are typical of 

 the 57 and are characterized by 2 : 2 segregations at all eight loci. 

 Each of these asci produced 2 pink and 2 white cultures. Pink cul- 

 tures are adenine dependent. Pink mutations have not appeared 

 spontaneously, our first pink culture was one obtained by treatment 

 of haplophase No. 4 (fig. 18-1). Ascus Nos. 5918-22 and ascus Nos. 

 5979-82 both contained 3 pink cultures. Mutation of white to pink 

 does not occur in this stock without treatment. The ad gene controls 

 the production of pigment and it is in the well-marked chromosome 

 shown in fig. 24-2. Four other loci in this chromosome segregated 

 in a perfectly regular manner so the possibility of chromosomal ir- 

 regularities causing the phenomena is eliminated. Adenine indepen- 

 dent cultures ordinarily attain full growth in adenine deficient me- 

 dium in two days, while adenine dependent (pink) cultures require 2 

 weeks. Two of the pink cultures in each ascus attained full growth 

 in two weeks, while one in each grew up in 5 days. This pink cul- 

 ture is designated by ad circled . However, it is phenotypically 

 pink in culture and this type is never encountered in our inbred 

 white stocks. 



This is an example of the type of irregular ratio which can only 

 be detected by tetrad analysis (fig. 25-2). The simplest explanation 

 is that in the AD/ad heterozygote, one of the AD loci loses some of 

 its essential substance to the two ad ioci thus degrading the AD locus 

 and reducing its ability to contribute as effectively to the synthesis 

 of adenine. The Konversion Hypothesis of Winkler suggested that 

 exchanges of this type occurred and used this idea to explain crossing 

 over. However, it is only in tetrad analysis that conversion and 

 crossing over can be distinguished. 



CONVERSION OF RECESSIVES TO DOMINANTS 

 IN THE HETEROZYGOTE 



One of the 60 asci (No. 6040-43) produced 4 white cultures in- 

 stead of the expected 2 white and 2 pink. When tested in defined me- 

 dium lacking adenine, they all attained full growth in 2 days. Proof 

 that an illegitimate hybridization has not occurred is the fact that 



25-1 



