364 



CELL HEREDITY 



Parents 



paba y + 



(B) 



ad 



Heterokaryon 

 prototrophic with 

 yellow and white 



asexual spores 



paba 



ad 



Heterozygous 



diploid* 



prototrophic with 



green asexual spores 



paba 



ad 



A green heterozygous Yellow haploid (y) or 



diploid (v/+ +/w) sector diploid (y/y) sectors from 



from a heterokaryon of a green heterozygous 



constitution y + diploid colony (>>/ + ) 



*When the two chromosomes P"^° -^ and 



are in different nuclei in 



paba y 

 a heterokaryon, the genotype is written - ; when the two chromosomes 



+ + , 



paba y 



are m the same diploid nucleus, the genotype is written — 



FIGURE 12.5. Sectored colonies of Aspergillus. 



The presence of both parental chromosome sets in the diploid nucleus 

 can also be demonstrated by somatic recombination and a process of 

 chromosome reduction which cause heterozygous diploid colonies to 

 show sectors for color. For example, heterozygotes from a heterokaryon 

 between yellow ( y) and white (w) auxotrophs of this composition, 



2 ^ "*" "*" form yellow and white sectors that are no longer hetero- 



+ + wad 



zygous for color markers and sometimes not for auxotrophic markers 



either (Figure 12.5). About 15 per cent of these sectors give rise to 



haploid conidia. When white haploid patches are selected, half are of 



genotype paba y w ad and half are + + w ad , showing an independent 



behavior of the two chromosomes in the reduction process. When yellow 



haploid patches are selected, they are all paba t/ + + . The alternate 



