HEREDITY IN SOMATIC CELLS 369 



mitotic crossing over does not seem associated with haploidization. Con- 

 sideration of the other two chromosomes, for which no selection was 

 made, shows that the segregation of white, nonhomologous chromosomes 

 is random. 



Parental Associations Recombinant Associations 



These haploids seem to arise as aneuploids (2n — a, where a = 1 at the 

 onset of the process, and a = n at the end). At first some chromosomes 

 are in a disomic condition, but their eventual loss may be selected for 

 by the advantage possessed by a fully balanced haploid. Loss is prob- 

 ably achieved by nondisjunction, which would also give rise to the ca. 5 

 per cent of diploids that are homozygous for all markers in both arms of 

 one chromosome. 



The high rate at which chromosome reduction occurs accounts for the 

 fact that Aspergillus is not a diploid organism. A rough estimate of 

 the balance of processes can be made as follows: 



1. Proportion of diploids in a newly synthesized heterokaryon — 10~^. 



2. Proportion of haploid nuclei produced by diploid nuclei — 10~^. 



3. Proportion of crossover nuclei produced by diploid nuclei — 10~^. 



Without selection, the equilibrium between diploid and haploid nuclei 



Chromosome I Chromosome II 



paba y + + 



+ + 



paba y 



+ y 



paba y 



paba y 



paba y 



paba y 



paba y 



paba y w ad 



Heterozygous diploid 



i 



Ist-order segregant 



i 



2nd-order segregant 



3rd-order segregant 



i 



4th-order segregant 



FIGURE 12.7. Sequential development of homozygosity through repeated somatic 

 crossing over. 



